February’s column focused on potential errors in orthometric heights using a digital barcode leveling system with multi-piece leveling rods. As stated in the column, businesses need to make decisions based on expenses and ultimately on the profit margin; but making a business decision that results in a bad technical outcome is never the right decision. This newsletter column is going to highlight a new feature in the National Geodetic Survey (NGS) Beta OPUS Projects 5.1 routine permitting the use of RTN vectors to support the development of the 2022 Transformation model.
On Jan. 12, NGS held a webinar titled “Using RTN Data in OPUS Projects 5 for GPSonBM.” Users can download the video and PowerPoint slides here.
I’ve been highlighting NGS’s GPS on Bench Mark program that supports the 2022 Transformation Tool in my columns since 2018. NGS delayed the completion date for the new modernized NSRS until 2025, so they have extended the cut-off date for submitting GPS on Bench Mark data for use in the 2022 Transformation Tool until Sept. 30.
NGS GPS on BenchMarks Program (Image: NGS website)
NGS has been developing tools that facilitate submitting data to the NGS GPS on BM campaign such as OPUS Share. The latest tool is the OPUS Project 5.1 routine that allows the use of RTN vectors. OPUS Projects 5.1 is a beta product, but NGS is now allowing users to use the routine to submit data for the GPS on BM campaign. My October 2021 column highlighted NGS’s Beta OPUS Projects 5.1.
The 2023 requirements for using OPUS Projects in the GPS on BM program (Image: NGS website)
I’d like to note that OPUS has been updated to support the newly released ITRF2020 (IGS20) orbits. My October 2022column discussed the latest International Terrestrial Reference Frame of 2020 (ITRF2020) released by the International Earth Rotation and Reference System Service (IERS). A previous NGS news bulletin provided a statement about the new reference system and products.
Clicking on the link titled “NEW: 2023 Requirements for Use in the GPSonBM Campaign” on the OPUS Projects 5.1 webpage provides the requirements for using OPUS Projects 5.1 and Real-Time Network (RTN) data to support the 2022 Transformation Tool; that is the 2023 GPS on BM campaign. There are five sections in the writeup: Introduction, Project Planning, Equipment and Configuration, Field Requirements and Office Requirements. The Introduction section states that the requirements are limited to the GPS on BM Campaign and will be replaced, or superseded, when NGS finishes its new GNSS surveying specifications.
Introduction Section from Requirement Write Up (Image: NGS website)
The project planning section of the announcement states that RTN vectors of 5-minute occupations can be used instead of the 4-hour occupations required for OPUS Share.
However, the Field Requirement section states that the mark must be occupied three different times.
“During the RTN survey, measure each mark in your project (including the RTN Validation Station) for a minimum of 5 minutes for three independent occupations. All three measurements must agree by 3 cm horizontal and 5 cm ellipsoid height. They also must be separated by at least 3 hours (even if occupied on different days). Plan to occupy a mark, go occupy a few more in the area, then circle back. Or rotate day-by-day,” the section states.
As stated in the section on office requirements for using OPUS-Projects 5 in the 2023 GPS on BM Campaign writeup,“The OPUS-Projects User Guide provides instructions on how to run the software and submit a project to NGS. The User Guide states to follow the steps in the order listed below, and it explains steps 1 – 7 and 9 – 11 in detail. For step 8 and when including GVX data in OPUS-Projects 5, refer to those portions of the User Guide’s Quick Start which are highlighted in yellow. NGS is working on fully updating the User Guide to include more details; for now, use the Quick Start Guide for assistance with GVX.”
OPUS Projects User Guide (Image: NGS website)Quick start guide. (Image: NGS website)
I recently used OPUS Projects to analyze some GNSS results using Harris-Galveston Subsidence District CORS and PAMS GNSS data. I want to emphasize that it may seem like a lot of work the first time you use the routine, but NGS makes it fairly simple to complete each task. The manual is very complete and does a good job of describing every step. The manual can be downloaded here. In my experience, the most time-consuming task is creating the descriptions. There are several items that must be correctly entered because the answer to some entries affect the answers to other entries. That said, NGS supports a description entry software called WinDesc that facilitates entering the appropriate information. The OPUS Projects User Guide provides an appendix that describes using the WinDesc module to enter description metadata.
For marks that are in the NGS database, known as the NGS Integrated Data Base (NGSIDB), WinDesc will import information from NGSIDB, thereby decreasing the number of entries users need to address. In other words, if the mark has a PID then it should be in the NGSIDB. If you are occupying a mark that is part of NGS GPS on Bench Marks website then it probably has a PID and a description in NGSIDB.
I’ve included three slides from the Jan. 12 webinar that summarize the basic requirements.
This slide is a depiction of how a CORS station must be connected to the RTN vectors. (Image: NGS website)This slide provides the occupation and precision requirements. (Image: NGS website)This slide provides a list of the required metadata for the project. (Image: NGS website)
As for the requirement of at least three independent RTN occupations on different times, in my opinion at least one occupation should be on a different day. My October 2021 column addressed a study that reported on using RTN solutions to estimate accurate horizontal and vertical coordinates.
The report stated, “When differenced with coordinates from a static GNSS survey campaign, the horizontal and vertical RMSE of the NRTK-derived coordinates was 2.3 cm horizontally and 4.5 cm vertically at 95% confidence. Repetitive NRTK vectors on each baseline differed between ± 2.4 cm horizontally and ± 3.4 cm vertically at 95% confidence.”
The report also stated, “Adjustment of hybrid survey networks with four repeat NRTK vectors per bench mark produced network accuracies at 95% confidence for the adjusted coordinates at all bench marks less than 1 cm horizontally and 2 cm vertically (ellipsoid height).”
The requirements are limited to the GPS on BM Campaign and will be replaced, or superseded, when NGS finishes its new GNSS surveying specifications.
(Image: Screenshot of Accuracy of GNSS Observation from Tree Real-Time Networks in Maryland, USA)
The paper by Gillins, et. al was presented at the 2019 FIG Working Week held in Hanoi, Vietnam, on April 22–26, 2019. The International Federation of Surveyors (FIG), involves a wide range of professional fields within the international surveying community; this includes surveying, cadastre, valuation, mapping, geodesy, hydrography, and geospatial and provides an international forum for discussion and development to promote professional practice and standards. FIG meetings are held all over the world. I’d like to highlight that the 2023 FIG Working Week is going to be held in Orlando, Florida, on May 28 – June 1, 2023.
NGS will be presenting a full-day worth of content on NSRS Modernization during the FIG Working Week 2023. For the first time in more than 20 years, this annual FIG gathering will take place in the United States, hosted by the National Society of Professional Surveyors (NSPS).
I’ve participated in several FIG meetings. I’ve learned a lot from presentations as well as holding hallway meetings with experts from the international surveying and mapping community. All geospatial users should plan on attending this event. I have provided information about the FIG commissions in my August 2021 newsletter. I would encourage everyone to visit the FIG website and review the information about the 2023 FIG Working Week. The a list of the FIG Commissions can be found here. More information can be obtained on each commission by clicking on its title.
Future columns will highlight the FIG Working Week as the agenda is developed. I would encourage everyone to check NGS’s Website for updates on Beta products and new surveying specifications. Geospatial users should also subscribe to NGS’s News Services at the following here. Check out the NGS News Services site for what’s available.
The National Geodetic Survey (NGS) has revised an important technical document on the modernized National Spatial Reference System (NSRS). Zilkoski explores a use case on flood mapping, discussing an Elevation Certificate example, Flood Insurance Rate Map and Flood Insurance Study. NGS has scheduled a webinar for April 8 to discuss the four use case examples.
In February 2021, the National Geodetic Survey (NGS) revised NOAA Technical Report NOS NGS 67 Blueprint for the Modernized NSRS, Part 3: Working in the Modernized NSRS. Users can download the publication. See the box titled “NOAA Technical Report NOS NGS 67.”
This column will highlight one of the four use cases: “Use Case 1: Flood Mapping.” The case study discusses the Elevation Certificate (CE) example, Flood Insurance Rate Map (FIRM), and Flood Insurance Study (FIS).
The following is the scenario that NGS considered in this use case:
“This use case’s examples are set in an imaginary flood-prone coastal community experiencing non-uniform ground subsidence at the watershed scale (see Figure 10). Although many areas are not subject to this level of vertical motion, the full benefits of NSRS modernization are most apparent in this context. We illustrate differences in the use of the NSRS of today and the modernized NSRS with two common NFIP workflows. First, we consider steps anticipated in the certification of NAPGD2022 elevations for a NFIP Elevation Certificate. Second, we step into the shoes of a FEMA Mapping Partner to examine the ways future NSRS tools support more accurate mapping in Flood Insurance Rate Map (FIRM) and Flood Information Study (FIS) updates.”
I think this is a good scenario to use to demonstrate the full benefits of the NSRS modernization in areas of subsidence, but I believe there are important issues that will need to be addressed before the implementation of NAPGD2022 in flood mapping projects. I will highlight some of these issues later in the newsletter. First, let’s look at NGS example.
As depicted in figure 10 in NOS NGS 67 technical document, the area has three difference subsidence rates (<0.1 cm/yr., 2 cm/yr., and 5 cm/yr.). See the box titled “Diagram of fictional case study location for Use Case 1.” As NGS stated in the document, “Although many areas are not subject to this level of vertical motion, the full benefits of NSRS modernization are most apparent in this context.”
This may not be the typical situation of a flood mapping project but it should be noted that this type of high individual rates and large relative rate differences has occurred in the Houston-Galveston, Texas, region (see the following publications):
NGS’s example illustrates differences in the use of the NSRS today and the future NSRS with two common National Flood Insurance Program (NFIP) workflows. The example addresses surveyors performing a FEMA Elevation Certification using NAPGD2022 elevations, and the ways future NSRS tools support more accurate mapping in Flood Insurance Rate Map (FIRM) and Flood Information Study (FIS) updates.
It should be noted that according to the September 27, 2017, Office of Inspector General Department of Homeland Security OIG-17-110 report, FEMA’s goal is to review flood maps every five years.
“According to the National Flood Insurance Reform Act of 1994, FEMA must assess the need to revise and update all floodplain areas and flood risk zones identified once during each 5-year period. Thus, valid miles will expire every five years if not assessed. Failure to assess an NVUE compliant mile within the 5-year window will result in the mile being re-categorized as “Unknown” in the Needs Database. Unknown miles have not been subjected to the validation process to determine whether they reflect the current flood risk or are in need of restudy. In 2009, FEMA set a goal to attain 80 percent NVUE by the end of fiscal year 2014.” — Excerpt from Department of Homeland Security OIG-17-110 report
The modernized NSRS will help facilitate meeting this goal. This is described in NGS’s use case example:
NFIP products will primarily utilize the official NSRS reference epochs
“As the NFIP is structured today, NFIP products will primarily utilize the official NSRS reference epochs. Additionally, some NFIP products such as the EC form itself, as well as guidance, and technical references for FIRM and FIS preparation would benefit from updates that reflect changes to the NSRS. While the time-dependency and incorporation of a gravimetric geoid model will manifest as improved risk assessment reliability in inundation map products, we notably anticipate that NSRS modernization will have a limited impact on the basic structure of most recommended workflows associated with the NFIP of today. The most significant development is therefore the opportunity for FEMA’s National Flood Mapping Program (NFMP) to increasingly leverage the new capabilities of the NSRS to ensure that current, accurate ground elevation data is used, and to better incorporate relevant flood control structure and future conditions mapping data to support decision-making beyond the NFIP. Details of how the modernized NSRS can help FEMA achieve broader NFMP objectives and opportunities for data-driven case studies to explore this are described at the end of the use case.”
So, what does this really mean? The document uses two diagrams to explain how the new NSRS would be used to estimate a height for a FEMA Elevation Certificate (see box titled “Figure 11 from Use Case 1”). The top cartoon labeled “Tie to Passive Control” describes the process being performed today. That is, a surveyor locates the two closest marks that have published orthometric heights, follows the appropriate surveying procedures to ensure that the marks have not moved since the last time they were leveled to, and then performs the appropriate procedures to obtain the height for the Elevation Certificate. Depending on the location of the published orthometric heights in the area of the structure, this process could be very expensive. The lower cartoon labeled “Tie to Active Control” describes the process that will be used in the modernized NSRS using NADGP2022 heights. The user would occupy a temporary mark near the structure with GNSS to obtain a NAPGD2022 orthometric height computed using the appropriate ellipsoid height and geoid height value, and then perform the appropriate leveling procedures to obtain the height for the Elevation Certificate. This process will provide the most up-to-date height in the area.
Figure 11 from Use Case 1. Cartoon of Elevation Certificate field surveys based on establishing a tie to the NSRS via passive control leveling (top panel) and via active control with GNSS (lower panel). (Image: NGS)
There is an issue that should be noted here: the temporary mark determined using active control may provide the most up-to-date height at a particular location but that height may not be consistent with the heights used to establish the Base Flood Elevation (BFE). At first, someone would say, that’s good because it’s indicating that the flood hydraulics have changed on the floodplain map. However, without performing a detailed height analysis in the region, the user won’t really know whether the BFE value should be updated based on the current changes in topography in the floodplain region. In other words, if the entire region has subsidence at the same rate then the relative height difference hasn’t changed, and the new starting height may not be consistent with the published BFE on the FEMA Floodplain Map. In most floodplain mapping regions, the changes in heights are probably less than the accuracy of the maps but using the height of a mark that is not consistent with the BFE could place a homeowner’s house incorrectly in a flood zone. A good surveying practice would include occupying several marks with GNSS (or leveling between marks) that were involved in the creation of the flood insurance study and the generation of the floodplain map to ensure that the height used on the Elevation Certificate is consistent with the BFE. This is a good procedure to use for the current NSRS as well as the modernized NSRS. However, this is not economically practical using the current NSRS but could be in the new NSRS which is a major benefit of the modernized NSRS.
So, let’s look at the Houston-Galveston region using the latest information available.
The box titled “Snapshot of Vertical Control from Harris County Floodplain Reference Marks Website” depicts the location of one of the reference marks, denoted as 050190.
Clicking on the datasheets link, provides the information about the floodplain reference mark in the Harris County Flood Control District’s system (see the box titled “Harris County Floodplain Reference Mark Datasheet”).
It should be noted that the GNSS-derived orthometric heights were based on GEOID99 and the official hybrid geoid model published by NGS today is GEOID18. A GNSS-derived orthometric height computed using NGS’ webtool OPUS will use GEOID18 not GEOID99. The difference between GEOID99 and GEOID18 at this location is approximately 0.45 feet (0.138 meters). Users must ensure that they are using heights that are consistent with the BFE on the FIRM. The new NAPGD2022 will help to reduce issues associated with effects due to changes in geoid models.
Page 113 from the November 15, 2019 Flood Insurance Study 48201CV001G addresses the issues associated with riverine flood in the region. (See the box titled “Page 113 from November 15, 2019 Flood Insurance Study 48201CV001G.”) The highlighted sections basically state that subsidence within inland watersheds has little or no effect on flood depths when the entire watershed subsides at the same rate. However, it also states that differential subsidence can cause changes in flood depths. The report goes on to say that the “Harris County and Incorporated Areas are affected by wide-scale, uniform subsidence with minor differential subsidence within individual watersheds.” It also states that “The local effects of subsidence may be adequately addressed, in the short term, by assuming that BFEs subside by the same amount the ground subsides.” The Houston-Galveston, Texas, region is a very complicated area due to the differential subsidence and numerous individual watersheds.
That said, let’s look some of the latest subsidence data in the region. The Harris-Galveston Subsidence District’s 2018 Annual Groundwater Report By Robert Thompson, William M. Chrismer, and Christina Petersen, PhD, P.E. provide some of the latest estimates of subsidence in the region. The box titled “HGSD Exhibit 18” depicts the locations of the GNSS sites used in the study. The plot provides the average compaction in centimeters over the past five years. The values range from 0.0 cm/year to greater than 2.5 cm/year.
HGSD Exhibit 18. This map shows the locations of the GPS sites throughout the area. The colored dots represent the average compaction over the past five years for each site, in centimeters. They range from 0.0 cm/year to greater than 2.5 cm/year. (Image: Harris-Galveston Subsidence District)
I used the information from Appendix B provided in the report to generate a few plots that show the estimate of subsidence in feet over 5 years. I’ve highlighted some marks that have large relative height changes. (Note: The units of the previous figure are centimeters; the units of the next several plots are feet.)
The relative height change between the two marks PA01 and CFHS, which are about 1.5 kilometers (approximately 1 mile) apart, is 0.197 feet in only 5 years. (See the box titled “Estimate of Amount of Subsidence in 5 Years at Pam 1– Units Feet.”)
The estimated relative height change between mark PA46 and ROD1, which are about 8 kilometers (approximately 5 miles) apart, is 0.277 feet in five years. (See the box titled “Estimate of Amount of Subsidence in 5 Years at Pam 46 – Units: Feet.”)
The effect of these large relative differences may not have any effect on the BFE on a particular watershed. These subsidence estimates are at a specific mark so they only provide information at a particular location. The new NAPGD2022 along with NGS’s webtools will enable users to economically obtain current, accurate heights in the entire region. Leveraging the capabilities of the new NSRS will help facilitate the implementation of FEMA’s goal of assessing the need to revise and update all floodplain areas and flood risk zones identified once during each five-year period.
This column highlighted the potential effects of subsidence on published heights in the Houston, Texas, region, which implies that most of the published heights based on older surveys in the region are not current or accurate.
According to the announcement, only 28 marks will have publicly available orthometric heights on NGS datasheets in Southeast Texas. This NOAA website provides more information. See the box titled “NGS Southeast Texas Orthometric Heights.”
I would encourage everyone to check out the website to obtain a better understanding of what this suppression of published heights means to their operations. Future newsletters will address the suppression of the orthometric heights in Southeast Texas, and how users can help densify the network and prepare for the new, modernized NAPGD2022. Again, a benefit of the new modernized NSRS will facilitate the establishment of consistent, accurate NAPGD2022 GNSS-derived orthometric heights.
Lastly, NGS is convening the 2021 Geospatial Summit on May 4 and 5. The 2021 Geospatial Summit will provide updated information about the planned modernization of the National Spatial Reference System (NSRS). Register here.
My last column highlighted an ArcGIS web application that incorporates various datasets and data layers to assist surveyors planning vertical control surveys. On Jan, 29, the National Geodetic Survey (NGS) released the latest experimental geoid model, xGeoid20, and a new gravity interpolation tool (see box below, “NGS Releases Annual e& Gravity Interpolation Tools”).
This newsletter will highlight some attributes of these two new products. First, why am I writing about another experimental geoid model. I discussed xGeoid18 in my December 2018 column and xGeoid16 in my June 2017 column. What’s important here is that this will be the last experimental geoid model until 2022, and the dynamic geoid model has also been updated this year in the form of xDGEOID20.
xDGEOID20 is produced by NGS within the Geoid Monitoring Sƒervice (GeMS) and is part of the new NAPGD2022. Therefore, users only have a few more years to understand the differences between the hybrid geoid model that is being used today to estimate GNSS-derived orthometric heights and the gravimetric geoid model which will be used to estimate North American-Pacific Geopotential Datum of 2022 (NAPGD2022) GNSS-derived orthometric heights.
NGS also announced a new gravity tool, denoted as “The Experimental Gravity Model 2020 (xGRAV20).” xGRAV20 is designed to provide a full-field gravity value and a digital elevation model height at a-specified location. The xGRAV20 model will be important to users that are computing leveling-derived orthometric heights consistent with NAPGD2022.
It is important to note that the xGEOIDs provide a preliminary but increasingly-accurate view of the changes expected from the upcoming NAPGD2022. Also, the xGEOID20 geoid model is the first combination of the geoid models computed by scientists at NGS and Canadian Geodetic Survey (CGS). One unique element to xGEOID20 is that the differences between the A and the B model are due to the contribution of the GRAV-D airborne gravity and differences in methodology.
The National Geodetic Survey (NGS) has published annual experimental geoid (xGEOID) models since 2014. Each of these experimental geoids demonstrate the improvements provided by the addition of airborne gravity data (GRAV-D data) and by the refinement of geoid computation methods.
As the image above indicates, the xGEOID20 is available over a very large area. The box below lists the latitude and longitude boundaries of the areas where xGeoid20 is available.
Areas Where xGeoid20 Model Is Available. (Image: NGS)
To use the xGeoid20 Interactive Computation Page, the user can click on the “ACCESS TOOL” button below the map or the Interactive Computation button on the left side of the webpage (see the image above, “Experimental Geoid Models 2020 (xGEOID20)”). I’d like to highlight a statement that NGS added as a note on the computation page:
Coordinates will be processed as IGS14.
The epoch should be in decimal year format and reflect the user-specified output epoch. If no epoch is entered, the tool will use a default epoch equal to the epoch of the static geoid model, which is currently 2020.00.
The user needs to know that the epoch is used to compute the xDGEOID20 value. I will demonstrate how this works later in this column.
As in past xGeoid interactive computations web applications, the user can submit data in various formats. The box titled “Input Formats Permitted for xGeoid20 Webtool” provides a list of the permitted formats. It should be noted that inputting an ellipsoidal height, epoch and name are optional. However, the default epoch is 2020.00, so if you want a different epoch, you need to enter the date. Also. the program will only compute an orthometric height if the user provides an ellipsoidal height.
Input Formats Permitted for xGeoid20 Webtool. (Image: NGS)
Users have the option of getting the output from the xGeoid20 tool on their computer screen or in the CSV format. The box below is an example of inputting data using the screen option. Once you enter your data, the user clicks on the submit button.
Example of Input Format for Screen Option. (Image: NGS)
The next image shows an example of the output using the screen option. I have highlighted a few numbers that I’d like to address.
Your input in NAD83 (2011) epoch 2010.00 (red). I entered my coordinates as NAD 83 (2011), and it assumed that these coordinates are epoch 2010.0.
Your Result in IGS14 epoch 2020.00 (blue). The routine provides your output coordinates in IGS14, epoch 2020.00. This is the epoch of the static geoid model.
The geoid height of GEOID18 (with respect to NAD83) and the orthometric height in NAVD88 (based on GEOID18) (green). This NAVD 88 value is for comparison purposes only. It is using GEOID18 and provides an estimate of the differences between the future NAPGD2022 and the current NAVD 88. The orthometric height is computed using the following formula: NAD 83 (2011) ellipsoid height (epoch 2010.0} minus GEOID18.
Ortho Height (brown). This is the estimation of the orthometric height using the following formula: IGS14 ellipsoid height (epoch 2020.0} minus xGEOID20A (or B).
Ortho(model)-NAVD88(GEOID18) (purple). These differences are the estimates of the differences between the future NAPGD2022 and the current NAVD 88. It provides the differences for both the xGeoid20A and xGeoid20B model. I look at the B model because it used the GRAV-D data in the development of the model.
Accuracy (yellow). This is the estimated 95% confidence interval for geoid height.
Example of Output Format from Screen Option
xGEOID20 Interactive Computation Output
Note: The GRS80 ellipsoid is used for both NAD83 and IGS14.
N: The geoid height at epoch t0 = 2020.0, which is geocentric and relative to the GRS80 reference ellipsoid.
Accuracy: Estimated 95% confidence interval for geoid height.
DN: The time-dependent geoid change computed between user inputted epoch (t) and t0. To obtain the dynamic geoid height at user inputted epoch (t), add N + DN.
Either Model A or Model B N values may be used for this depending on user preference.
Example of Output Format from Screen Option. (Image: NGS)
The box below shows an example of inputting data using the CSV option.
Example of Output Format from CSV Option
Note: The GRS80 ellipsoid is used for both NAD83 and IGS14.
N: the geoid height at epoch t0 = 2020.0, which is geocentric and relative to the GRS80 reference ellipsoid.
Accuracy: Estimated 95% confidence interval for geoid height.
DN: the time-dependent geoid change computed between user inputted epoch (t) and t0. To obtain the dynamic geoid height at user inputted epoch (t), add N + DN. Either Model A or Model B N values may be used for this depending on user preference.
Example of Input Format for CSV Option. (Image: NGS)
The printed output from the CSV option looks very confusing, but it can be imported into an excel spreadsheet. The headings and values are all separated by a comma so everything falls into the appropriate columns after importing the data (see image below.)
Example of CSV Output Format Imported into Excel. (Screenshot: David Zilkoski)Example of CSV Output Format Imported into Excel. (Screenshot: David Zilkoski)
I stated in the xGeoid20 write up that the dynamic geoid model has also been updated this year in the form of xDGEOID20. This model is produced by NGS within the Geoid Monitoring Service (GeMS) and is part of the new NAPGD2022. For a thorough discussion on GeMS and the time-dependent geoid, view the webinar from NGS’ presentation library. See the box titled “GeMS Webinar by Kevin Ahlgren.”
Also, one of my previous columns described NGS’ GeMS program. The images titled “Examples of the Time-Dependent Geoid Change in Alaska EPOCH 2020.0” and “Examples of the Time-Dependent Geoid Change in Alaska EPOCH 2025.0” show the change in geoid value from Epoch 2020 to Epoch 2025 for two stations in Alaska.
Examples of the Time-Dependent Geoid Change in Alaska EPOCH 2020.0. (Image: NGS)Examples of the Time-Dependent Geoid Change in Alaska, EPOCH 2025.0. (Image: NGS)
First, looking at the box titled “Examples of the Time-Dependent Geoid Change in Alaska EPOCH 2020.0,” the change between NAPGD2022 and NAVD 88 is approximately 1 meter. Users should note that the GEOID12B is used to establish the NAVD 88 height. Alaska was not included in GEOID18. Comparing the two Alaska labeled boxes, the xDGEOID2022 change between 2020.0 and 2025.0 is –4 mm. I will address this topic in more detail in future newsletters.
As stated by NGS news announcement, “The xGEOID models provide a preliminary but increasingly-accurate view of the changes expected from the upcoming North American-Pacific Geopotential Datum of 2022 (NAPGD2022).” NGS has produced many figures that describe the bias and trend between the future NADGP2022 and NAVD 88. In my June 2017 column I provided a plot that depicted the difference between NAPGD2022 and NAVD 88 based on the GPS on Bench Mark dataset. See the image below.
Figure from June 2017 Survey Scene column. Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). (Image: NGS)
These figures provide a broad picture of the change but to better understand the changes across the Nation, I used the GPS on Bench Mark dataset, that was involved in the creation of Geoid18 model, to compute an average latitude, longitude, and ellipsoid height for every State. Obviously, this is a fictitious mark but it provides an idea of the average change based on marks that have both a GNSS-derived ellipsoid and a leveling-derived orthometric height. The plot titled “Difference Between the Future NAPGD2022 and NAVD 88” depicts the average difference for each state based on the GPS on Bench Mark data file. These differences were generated using the xGeoid20B values from the output of the xGeoid20 website.
Difference Between the Future NAPGD2022 and NAVD 88. (Image: NGS)
I would encourage everyone to select a couple of marks and compute the differences to understand the change in their particular region. I was the NAVD 88 Project Manager and I informed users of the potential changes between the NGVD 29 and NAVD 88 for about a decade, and I still had surveyors tell me that they didn’t know it was coming. Please take a few minutes to read NGS’ write up on xGEOID20, estimate the differences in your area of interest, and spread the word to your colleagues, friends, and clients.
The last item that I’d like to highlight is that NGS has released a beta version of a surface gravity model consistent with xGEOID20. See the box titled “Experimental Surface Gravity Model 2020 (xGRAV20).” Users can access the beta webtool here.
The box below provides the output using the tools sample data.
Output from Screen Output Format from xGRAV20 Tool. (Image: NGS)
This gravity tool will be important when users want to incorporate leveling-derived orthometric heights into NAPGD2022. We will address this tool in more detail in future newsletters. I want to emphasis that these two web tools are beta sites. As a beta site, users should verify all information from the site. I encourage everyone to access the tool and check out a few of their favorite marks, and then send an email to NGS informing them of what you like, what you would like to change, and what you would like to see added to the tool.
NGS is releasing this tool as a beta product to get feedback from users. They are interested in your feedback concerning its function and usability as well as how users would like to interact with NGS web tools in the future. Email NGS at [email protected].
In conclusion, I want to leave you with a thought about change. When I give presentations and seminars, I usually include a slide that probably expresses the thoughts of many individuals.
My brother once told me:
“If you geodesists did it correctly the first time you wouldn’t have to keep performing adjustments and changing the values. Just do it right the first time.”
He’s a doctor and said he must do it right the first time.
My response to my brother and to everyone else is the following:
If you want to improve you have to be willing to change, and if you want to continue to meet future positioning requirements you need to continually change.
Winston Churchill said it better “To improve is to change; to be perfect is to change often.”
My last column described a new National Geodetic Survey (NGS) webtool for obtaining geodetic information about a passive mark in their database. The column highlighted some features that may be of interest to GNSS users. It provides all of the information about a station in a more user-friendly format. This column highlights an ArcGIS web application that incorporates various California specific datasets and NGS data layers to assist surveyors planning vertical control surveys. The GNSS Leveling Web Application was provided to me by Jay Satalich, chief, Office of Surveys, Caltrans (see box titled “Linkedin Notification from Jay Satalich).
Linkedin Notification from Jay Satalich
Supervising Transportation Survey (Chief, Office of Surveys) at State of California, Department of Transportation:
“GNSS Leveling Web Application” [is] an Esri ArcGIS online web app created for my “GNSS Leveling” students at College of the Canyons. Designed as a practical tool when planning vertical control surveys using GNSS. National datasets include: National Spatial Reference System (layers: satellite visibility, stability, and vertical control source), geology, and GEOID18 (layers: GEOID18 height, difference between GEOID18 and GEOID12B, and GEOID18 uncertainty). California-specific datasets include: oil/gas/fracking/injection wells, fault lines, oil fields, groundwater basins, and landslide areas. The NOAA National Geodetic Survey data layers were created and published by Brian Shaw. People who influenced development of this app include Dave Zilkoski, Kevin M Kelly, Ken Hudnut, David D Jackson, Ross S. Stein, and Arthur Sylvester.
The box titled “GNSS Leveling Web Application” depicts a map of the Los Angeles area that provides the list of published marks in NGS’ database with an overlay of the uncertainty of NGS’ hybrid geoid model GEOID18. Plotting the published marks from NGS’ database is very useful for surveyors reconning marks for a GNSS survey project. The attributes allow users to quickly identify stations that have published heights from leveling adjustments projects (labeled as ADJUSTED) and those that have heights published from GNSS adjustments projects (labeled as GPS OBS). (See here for definition of attributes.)
Source: Esri ArcGIS GNSS Leveling Web ApplicationSource: Esri ArcGIS GNSS Leveling Web Application
Source: Esri ArcGIS GNSS Leveling Web Application
The list all of the layers of the web application are provided in the box titled “GNSS Leveling Web Application Layers.” (Note: After you open up the web application, click on the Layers icon to obtain the list of available layers.)
GNSS Leveling Web Application Layers
Source: Esri ArcGIS GNSS Leveling Web ApplicationSource: Esri ArcGIS GNSS Leveling Web ApplicationSource: Esri ArcGIS GNSS Leveling Web Application
Source: Esri ArcGIS GNSS Leveling Web Application
As you can see from the list of layers, the web application enables users to select the layers that are pertinent to their survey project requirements. The application is designed for California surveyors but the concept is transferable to other States. For example, the following layers are not just for California surveyors: Arizona water wells, Louisiana oil and gas well, U.S. oil and natural gas wells, Principal Aquifers of the United States, and, of course, all of the NOAA NGS data layers.
One layer that is very important to California users is the layer that provides the fault activity in their region. The box titled “Fault Activity Map of California: Pre-Quaternary and Quaternary Faults – Quaternary Faults” depicts the list of published marks in NGS’ database with an overlay of the fault activity map.
Fault Activity Map of California: Pre-Quaternary and Quaternary Faults — Quaternary Faults
Source: Esri ArcGIS GNSS Leveling Web Application
Another great feature of the application is that it has a layer providing the satellite visibility code for published NSRS marks (see the box titled “Published NSRS Stations (by satellite visibility”). Once again, a great feature for field personnel performing reconnaissance.
Published NSRS Stations (by satellite visibility)
Source: Esri ArcGIS GNSS Leveling Web Application
The application also has a feature that lists the marks that were involved in the development of NGS’ hybrid geoid model GEOID18. (see the box titled “GNSS Leveling Web Application GEOID18 GPS on Bench Mark Layer”). Clicking on a mark’s icon provides information and statistics about the mark (see boxes titled “GEOID18 GPS on Bench Mark Layer — PID EW6989” and “Information for GPS on Bench Mark for PID EW6989”). This is one of the layers that provides information for the entire CONUS region. All this information is available from NGS’ website but this application incorporates all of NGS’s data as well as the local information in one application. This web application is very useful to a surveyor planning a survey project and/or providing information to a field reconnaissance team.
GNSS Leveling Web Application GEOID18 GPS on Bench Mark Layer
Source: Esri ArcGIS GNSS Leveling Web Application
GEOID18 GPS on Bench Mark Layer — PID EW6989
Source: Esri ArcGIS GNSS Leveling Web Application
Information for GPS on Bench Mark for PID EW6989
Source: Esri ArcGIS GNSS Leveling Web Application
Source: Esri ArcGIS GNSS Leveling Web Application
Users that are participating in NGS’ GPS on Bench Mark program can click on the layer for “NGS GPS on Bench Marks Transformation Service Tool, priority 10 km hex” to determine marks that need to be occupied by GNSS to improve a transformation tool being developed by NGS. See boxes titled “NGS GPS on Bench Marks Transformation Service Tool, priority 10 km hex” and “Information for GPS on Bench Mark Priority List for PID EW6989.” There’s also layers that depict the priority mark list for the GPS on Bench Marks program (“NGS GPS on Bench Marks Transformation Tool Service — priority mark list”) and the 2 km hexagon priority grid (“NGS GPS on Bench Marks Transformation Tool Service — priority 2km hex”).
NGS GPS on Bench Marks Transformation Service Tool, priority 10 km hex
Source: Esri ArcGIS GNSS Leveling Web Application
Information for GPS on Bench Mark Priority List for PID EW6989
Source: Esri ArcGIS GNSS Leveling Web ApplicationSource: Esri ArcGIS GNSS Leveling Web ApplicationSource: Esri ArcGIS GNSS Leveling Web Application
Source: Esri ArcGIS GNSS Leveling Web Application
Individuals interested in participating in NGS’ GPS on Bench Mark program should register for NGS’ Dec. 10 webinar, which will discuss the status of the program. See the box titled “GPSonBM Transformation Tool Campaign Update — 12 months remaining” for the information on the webinar. Users can register for the webinar here. I would encourage all users to access the web application tool developed by Jay and/or NGS’ website before participating in the next NGS GPS on Bench Mark webinar.
Almost all of my columns have focused on establishing accurate GNSS heights. Most of my 45 years of working in the field of geodesy has been focused on heights; that is, leveling-derived orthometric heights, GNSS-derived orthometric heights, and geoid heights. Gravity is very important to estimating all of these types of heights. Recently, a colleague sent me a video proving Galileo’s famous gravity experiment. It’s an older video (November 2014), but it’s really fascinating. You can see the entire video here. Another individual pointed me toward the same experiment performed on the Moon during the Apollo 15 mission. What’s amazing to me is that over 400 years ago an individual spent time studying the effects of gravity and developing the concept of acceleration due to gravity. I wonder what the world would look like today if Galileo would have just accepted Aristotle’s theory of gravity (which states that objects fall at speed proportional to their mass) and decided to focus on other tasks. Saying that, I am amazed that most geospatial users do not realize the importance of gravity (and physical geodesy) in the development of the geospatial products and services that they use daily; and, how critical it is that more research is required to meet future geospatial needs. The advancements in satellites and computers have enabled geodesy to expand into many different disciplines. Geodetic science and technology now underpin many sciences, large areas of engineering (such as driverless vehicles and drones), navigation, precision agriculture, smart cities, cellular telephones, and location-based services. (See the GPS World First Fix column about the shortage of American geodesists).
When I end one of my presentations, I always emphasize that Geodesy Provides the Foundation for all Geospatial Products and Services, and Integrated and Collaborative Organizations Create Geospatial Solutions. Geodesy is just as important today as it was 400 years ago.
I hope everyone stays safe during this COVID-19 pandemic and enjoys the holidays.
NGS has developed a new beta tool for obtaining geodetic information about a passive mark in their database. This column will highlight some features (available as of Oct. 5, 2020) that may be of interest to GNSS users. It provides all of the information about a station in a more user-friendly format. The box titled “Passive Mark Lookup Tool” is an example of the webtool. The tool provides a lot of information so I have separated the output of the tool into several boxes titled “Passive Mark Lookup Tool — A through D.”
I will highlight several attributes that I believe will be very useful to users, especially users of leveling-derived and GNSS-derived orthometric heights. I’ve highlighted several attributes in the box titled “Passive Mark Lookup Tool — A” that are important to users such as published coordinates, their datum and source, Geoid18 value, GNSS Useable, and the date of last recovery. All of these values are available on a NGS datasheet but, in my opinion, this provides the information in a more user-friendly format.
One calculation that the user can easily compute for marks that have been leveled to and occupied by GNSS equipment, is the difference between the published leveling-derived orthometric height and the computed GNSS-derived orthometric height. This may indicate that the mark has moved since the last time it was leveled to or that its height coordinate has been readjusted since the creation of the published geoid model.
The table below provides the calculation using the data from the box titled “Passive Mark Lookup Tool — A.” The calculation [HGNSS = hGNSS — NGeoid18; Difference = HGNSS — HNAVD 88] has been described in several of my previous columns (this one, for example).
Data: National Geodetic Survey
In this example, the difference between the GNSS-derived orthometric height and the Published NAVD 88 height is 6.1 cm. NGS is looking for comments on this beta webtool so if users would like this computation added to the tool, they should send a comment to NGS using the link provided on the site (This is a beta product. NGS is interested in your feedback concerning its function and usability as well as how users would like to interact with NGS datasheet information in the future. Email us at [email protected].) So, the user should ask the question, did the station move since the last time it was leveled?
Another attribute that would be nice to be part of this tool is which station was used to create the hybrid geoid model. As of Oct. 5, 2020, users have to go to the Geoid18 webpage to get the information. The Excel file and shapefiles provide whether the station was used to create the Geoid18 model. In the case of this example, KK1531, CHAMBERS, the mark was not used in the creation of Geoid18 so NGS felt that the station may have moved and/or the GPS on Bench Mark residual was large relative to its neighbors. See NGS’s technical report on Geoid18 for more information on the creation of Geoid18. The GPS on Bench Mark residual analysis was described in several of my previous columns (see “The differences between Geoid18 values and NAD 83, NAVD 88 values” and “NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 6” for examples).
The webtool provides a map depicting the location of the station, photos (if available), and previously published, superceded values of the mark. See the box titled “Passive Mark Lookup Tool — B.”
In the example of Chambers, KK1531, no photos were available. It would be helpful if a user would provide photos to NGS when visiting this station. (Note: NGS has a webtool for users to submit recovery information about a mark as well as to provide current photos of the station.) The new Passive Mark webtool also provides information about the survey projects that the mark has been involved in such as leveling and GNSS projects.
In this example, mark CHAMBERS was leveled to in a 1984 first-order, class 2 leveling project (Leveling Line number L24838/6) and, in 1995, the mark was part of a GNSS project (GNSS Project GPS1010). It also provides all the descriptive text and recovery information (See boxes titled “Passive Mark Lookup Tool – C” and “Passive Mark Lookup Tool – D”).
I want to highlight a few other attributes of this webtool. The station, PID AA3862, has an interesting attribute that users should take note of; that is, the NAD 83 (2011) position source is NO CHECK. See box titled “Passive Mark Page for PID AA3862.”
This means that the mark’s NAD 83 (2011) coordinates were determined without redundant observations. This is not a good survey practice but there are times that a project may contain check observations for some purpose or, more likely, the mark did contain other GNSS vector but they were rejected in the final adjustment. Either way, a good survey practice would be for users to verify the coordinates of these marks before using them.
Passive Mark Page for PID AA3862
Data: National Geodetic Survey
As previously mentioned, the tool provides the location of the station on a map and photos if they are available. This is a really nice feature for anyone searching for the mark. The map can be enlarged as well reduced by clicking on the box. See boxes titled “Passive Mark Page for PID AA3862” and “Photos of Mark PID AA3862.” The box titled “Photos of Mark PID AA3862” provides all three photos of mark PID AA3862.
Photos of Mark PID AA3862
Photo: National Geodetic SurveyPhoto: National Geodetic Survey
Photo: National Geodetic Survey
It should be noted, according to the Geoid18 GPS on BMs dataset that users can download, this station, AA3862, was not used in the creation of Geoid18. The table below provides the difference between the GNSS-derived orthometric height and the published NAVD 88 height.
In this example, the difference between the GNSS-derived orthometric height and the published NAVD 88 height is 9.9 cm. Also, the webtool provides the network accuracy values for the station. In this example, the horizontal network accuracy is 20.65 cm and the vertical network accuracy value is 14.50 cm (see highlighted values in box titled “Passive Mark Page for PID AA3862”). These are very large network accuracy values. This should be a flag to anyone that is using this station as control.
Data: National Geodetic Survey
As I previously mentioned, as a beta site, users should verify all information from the site. NGS is requesting feedback on this tool so they can improve it and make it an operational webtool. I encourage everyone to access the tool and check out a few of their favorite marks, and then send an email to NGS informing them of what you like, what you would like to change, and what you would like to see added to the tool.
NGS is releasing this tool as a beta product to get feedback from users. As NGS states in the heading of the tool, they are interested in your feedback concerning its function and usability as well as how users would like to interact with NGS datasheet information in the future. Email NGS at [email protected].
One last item that may be of interest to GNSS users is that NGS, working with the University Corporation for Atmospheric Research (UCAR), developed another online GNSS lesson (see box titled “New GNSS Lesson by NGS and UCAR”). These lessons are free but users must sign up to access the website and lesson.
Auto Mining: A driverless Cat 793F CMD truck leaves an iron ore pit. (Photo: Caterpillar)
Individuals who use GNSS today may not know the significant advancements that have been accomplished over the past 30 years to obtain accurate GNSS-derived coordinates, especially GNSS-derived orthometric heights.
Thirty years ago, there were two limiting factors for estimating GNSS-derived heights — estimation of accurate ellipsoid heights in a timely manner and the availability of an accurate geoid model. The geoid model was only good to the decimeter level, between two stations relatively close together. A significant improvement of the measurement of the Earth’s gravity field (such as from the GRACE mission) and digital elevation data (from the Space Shuttle Radar Topography Mission) facilitated the creation of more accurate geoid models. Geoid models went from decimeter values to centimeter, and then sub-centimeter values between closely spaced marks.
A new national network
During the past three decades, the U.S. National Geodetic Survey (NGS) has developed a national network of Continuously Operating Reference Stations (CORS). These CORS, along with the states’ real-time networks (RTNs), have provided the ability to compute accurate GNSS-derived coordinates in an efficient and effective manner. The modeling of antenna phase patterns was a critical development for combining different types of antennas.
Today’s GNSS processing software is basically a “hands-off black-box” system. But 30 years ago, the analyst had to identify cycle slips and ensure that all unknown cycle ambiguities of the carrier-phase data (integers) were determined correctly. It was a time-consuming task, and analysts needed to understand the data. So many things can go wrong when someone relies on an answer from a black box. That said, federal agencies such as NGS and GNSS software companies have produced hands-off software that provides statistics and warning messages, as well as guidelines for ensuring results are consistent and accurate.
The advancements in estimating GNSS-derived coordinates (including orthometric heights) have changed the way many industries do business. Farmers use it to drive their tractors and combines, mining companies control driverless vehicles, construction companies use automated machine guidance to build roads, and, of course, it has improved how individuals navigate from one location to the next.
Hands-off farming and mining
Thirty years ago, few farmers thought they would be able to sit in their cab and let their combine harvester drive itself. Geodesist, surveyors, and engineers had a vision of using GNSS to automate the use of farming and construction equipment, which became a reality.
What will it be like in another 30 years? Will it be routine for individuals to program their car for a destination, and then sit back and read a book?
Positioning with GNSS will be critical for the safety factor of driverless vehicles and the use of drones for delivery. Geodesists, surveyors and engineers, once again, need to lead the way to meet the positioning requirements of the future.
The National Geodetic Survey (NGS) recently announced two new items related to the modernized National Spatial Reference System (NSRS). First, it announced that there will be a delayed release of the modernized National Spatial Reference System (NSRS). See the box titled “Updates notices from NGS Homepage” for the link to the notice.
The box titled “Delayed Release of the Modernized NSRS” provides a summary of the notice. The announcement stated they are performing a thorough review of all tasks and will provide regular updates on their progress. What this means is that the modernized NSRS will not be completed by 2022. Even if it’s delayed a couple of years, it’s never too early to obtain an understanding of the new, modernized NSRS, and start preparing for the transition to the new NSRS.
NOAA’s National Geodetic Survey (NGS) is announcing a delay in the release of the modernized National Spatial Reference System (NSRS).
In 2007, NGS began planning for the modernized NSRS, acquiring its first airborne gravimeter, creating and initiating the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project and by 2008 had codified its modernization plans into a Ten Year Plan. At that time, the target completion date was 2018. By 2013, that date seemed unlikely, due to both the broadening of the GRAV-D coverage area and the experience of five years of operational planning and execution.
In 2013, NGS revised its 2008 Plan, and targeted 2022 as the date of the release of the modernized NSRS. This date was reinforced with a 2018 Strategic Plan revision. By 2017, confidence in hitting the 2022 target was high enough to reach final agreement with Canada and Mexico on a naming convention for certain components, to include “2022” in their names.
Since 2017, operational, workforce, and other issues have arisen and compounded, causing NGS to recently re-evaluate whether a successful roll-out by 2022 is possible. The most significant impacts have been in workforce hiring and retention, and in meeting GRAV-D data collection milestones, which underpin the NSRS modernization efforts.
NGS is currently conducting a comprehensive analysis of ongoing projects, programs and resources required to complete NSRS modernization and will continue to provide regular updates on our progress. To get the latest news on NSRS modernization and track our progress, subscribe to NGS News or visit our “New Datums” web pages.
The second important announcement by NGS was that two Federal Register Notices related to the modernized NSRS were published on July 24. See the box titled “NGS News.”
Image: National Geodetic Survey
The first Federal notice was titled “Upcoming Changes to the National Spatial Reference System.” See the box titled “Federal Register Notice titled Upcoming Changes to the National Spatial Reference System” for the summary. This announcement provides a statement about the new, modernized NSRS and that it’s going to be published between 2022 and 2025. The information about the modernized NSRS shouldn’t be new to anyone that’s been reading my newsletters, but the Federal Notice makes it official and NGS provides dates of when the modernization will be rolled out.
Federal Register Notice titled “Upcoming Changes to the National Spatial Reference System”
The second Federal Notice was titled “Consideration of Potential Age Limiting Observations To Be Used To Compute 2020.00 Reference Epoch Coordinates in the National Spatial Reference System.” This is a very important notice that users of NGS published coordinates should read and understand. NGS is considering imposing data age limits that will be part of the new, modernized NSRS. See the box titled “Imposing Age Limits of Data in 2022” for a summary of the Federal Register Notice announcement.
My last column highlighted that in the modernized NSRS the only way to get “into the datum” will be through a GNSS survey. It noted that leveling projects generate relative height differences not absolute heights. It emphasized that in the new modernized, time-dependent NSRS, the absolute height will be provided by up-to-date GNSS data; and the relative height differences between leveling marks will be provided by the leveling data. Many of my previous newsletters have explained different aspects of the new NSRS and how it may affect the surveying and mapping community products and services. As the Federal Register Notice implied, at this moment, NGS expects large uncertainties in the vertical component of the Intra-Frame Velocity Model (IFVM) which will translate into the GNSS-derived height Limiting the age of data will help to reduce the amount of uncertainty in the vertical component based on older data. Saying that, this could have an impact on users that rely on coordinates established using data acquired prior to 2010. NGS is requesting that users take new GNSS observations on all stations of interest that haven’t been occupied since the year 2010. The supplementary information in the Federal Register notice contains some very important statements. I have highlighted several statements in the box titled “Supplementary Information from Imposing Age Limits of Data in 2022.”
NGS hasn’t decided on the date of the age limit but the notice states that “For instance, it is unlikely that such an age-limit will be fewer than 10 years.” This is why NGS recommends the following “that users take new GNSS observations on geodetic control marks of interest that have not been surveyed since January 1, 2010, and asks the users to submit the observations to NGS before December 31, 2021.” Another important item in the supplemental information section is that NGS is enhancing the OPUS-Projects tool to include real-time kinematic and real-time network (RTK/RTN) observations. This should help to facilitate users submitting data on marks of interest so that they will have 2020.0 Reference Epoch Coordinates (REC).
Supplementary Information from Imposing Age Limits of Data in 2022
SUPPLEMENTARY INFORMATION:
In 2017, the National Geodetic Survey (NGS) announced its plans to estimate RECs on a five-year cycle in NOAA Technical Report NOS NGS 67, 2019, starting with the first reference epoch at 2020.00, as part of the modernization of the NSRS. In the Technical Report, the exact observations to be used for this estimation were listed as “To Be Determined.” NGS is considering imposing age limits upon the observations that will be used, particularly because of expected uncertainties in the vertical component of the IFVM. These age limits cannot be determined until additional well-structured, data-driven experiments are conducted. Such experiments are expected to occur during the 2020 reference epoch adjustment projects (geometric, orthometric, and gravimetric), which are scheduled for calendar year 2022.
However, since the cut-off for new observations to enter those adjustment projects is December 31, 2021, any decision to age-limit input observations will come too late for submissions to impact the 2020 RECs. While the cut-off for age-limited observations is unknown, certain assumptions are safe to make. For instance, it is unlikely that such an age-limit will be fewer than 10 years. Older observations may be used in the estimation of 2020 RECs, but this cannot be guaranteed. As such, NGS requests that users take new GNSS observations on geodetic control marks of interest that have not been surveyed since January 1, 2010, and asks the users to submit the observations to NGS before December 31, 2021. Users may either (a) submit existing unsubmitted observations through the OPUS-Share tool or (b) conduct new GNSS observations and submit the data to NGS via the OPUS-Share tool.
In order to increase the submission of GNSS observations on marks, NGS is prioritizing the finalization of an expanded OPUS-Projects tool, which will allow real-time kinematic and real time network (RTK/RTN) observations to be submitted, rather than the standard four-hour observations required in OPUS-Share. Initial roll-out of this new tool is expected to occur during calendar year 2020.
This action is designed to increase both the number and the coordinate accuracy of geodetic control points, which in the modernized NSRS will have an estimated 2020.00 REC. Historically, NGS has combined data across multiple decades to estimate geodetic coordinates, yet such efforts have not fully accounted for the lack of information about vertical motion of geodetic control points throughout the years. Since height information is critical to the understanding of floods, failure to compute heights accurately can have negative impacts on property and lives. NGS views periodic re-surveys of geodetic control points, rather than the estimation of coordinates from observations that are years (or even decades) old, as the most effective way to maintain accurate and up-to-date knowledge of geodetic coordinates, including heights. As such, this announcement provides users of the NSRS with advance notice that geodetic control points of interest to them should be re-surveyed for the most accurate representation of geodetic coordinates, including heights.
NGS has scheduled a webinar for August 27, 2020, to discuss the delayed release of the modernized NSRS. See the box titled “Webinar on Delayed Release of the Modernized NSRS” for the announcement and web link to register for the webinar. I would encourage all users of the NSRS to register for this webinar.
Many users are probably wondering if the delay in the new, modernized NSRS will change the dates of other deadlines. The FAQs webpage addresses some of these questions. I have highlighted a few FAQs in the box titled “Questions from NGS FAQ Website.”
How will the delay affect the GPS on Benchmarks Phase II deadlines?
The deadline for submittal of GPSonBM data for the 2022 Transformation tool will remain December 31, 2021
If SPCS2022 zone designs are completed before other parts of NSRS modernization, will SPCS2022 be released sooner?
No. SPCS2022 is explicitly defined with respect the four 2022 terrestrial reference frames (not NAD 83), and SPCS2022 will be released along with the roll-out of those frames. If the frames are rolled out prior to other parts of the NSRS modernization, the frames will be accompanied by SPCS2022 (see the previous FAQ about phased roll-outs).
However, complete definitions of all SPCS2022 zones will be made available as soon as they are finalized. NGS expects that to occur by the end of 2021. Providing zone definitions early will give software vendors, database administrators, and others ample time to adopt and test them in their systems. Doing so will ensure SPCS2022 is available for immediate use upon roll-out of the 2022 terrestrial reference frames.
My projected height change seems to return me to NGVD 29 heights. Is this a coincidence?
This is coincidental. It so happens that, in some areas of the country the actual orthometric height in a region happens to be numerically closer to NGVD 29 than NAVD 88. NGVD 29 itself has biases and tilts which make it as inappropriate of an estimate of true orthometric heights as NAVD 88
[NOTE: I have heard this question from many of my readers so I provided an approximate estimate of the differences between NAPGD2022 orthometric heights and NGVD 29 height values in my June 2017 Survey Scene column. See figure below labeled “Figure 2 from June 2017 Survey Scene Newsletter.”]
Image: National Geodetic Survey
Figure 2 from June 2017 Survey Scene Newsletter
Future newsletters will address updates on the modernized NSRS as they become available to the user community.
By David Zilkoski, contributing editor, survey scene
David B. Zilkoski
I attended The Ohio State University (OSU) to obtain my graduate degree in Geodetic Science in 1979. Therefore, I will admit that I am a little biased — once a geodesist, always a geodesist. The basic definition of geodesy is the applied science for determining the size and shape of the Earth, designing and realizing reference frames, and determining where you (and anything else) is on the Earth.
In OSU’s geodesy heyday (1960–1990s), many Americans trained were sent by federal agencies: National Geospatial-Intelligence Agency (NGA), NOAA/National Geodetic Survey (NGS), USGS, Army, Navy and Air Force. During the 1970s, NGS was sending two employees back to school every year. These agencies needed geodesists because they were undertaking major projects such as NGS’ to readjust the U.S. national horizontal (NAD83) and vertical geodetic (NAVD88) networks.
I was one of the employees that NGS sent to OSU to be trained to support the NAD83 and NAVD88.
The advancements in satellites and computers have enabled geodesy to expand into many different disciplines. Geodetic science and technology now underpin many sciences, large areas of engineering (such as driverless vehicles and drones), navigation, precision agriculture, smart cities and location-based services. Geodesy is actually more important than ever.
Today, the environment is different. U.S. federal agencies still need geodesists for developing enhanced and refined geodetic models and tools. However, major U.S. companies, such as Google and FedEx, as well as the automobile industry, precision farming companies and mining companies also need more accurate geodetic models, tools and algorithms. Therefore, these companies also need trained geodesists to perform important research on topics that address their specific geodetic requirements.
Today, OSU’s Geodesy Department is training very few American citizens. As the U.S. moves toward achieving geodetic-grade positioning in real-time in support of new applications such as driverless vehicles and drones, the number of trained geodesists should be increasing, not decreasing [Note: In 1990, there were 92 geodetic science graduate students. In 2019, there were 25; only three were U.S. citizens]. OSU and other universities need to educate and train the next generation of the nation’s scientific workforce of highly skilled research geodetic scientists that will expand industry’s research expertise.
The shortage of American geodesists poses a significant economic risk for the U.S. Europe and China train many more geodesists than the US. There are very few geodetic science programs in the U.S. today, and education in geodetic proficiencies has been fragmented. The OSU graduate program is one of few surviving geodetic science programs.
Users of geodetic products and services need to support geodetic departments in universities so that U.S. geodesy programs can grow to meet the geospatial demands of the future. The geospatial component of the economy is worth about $500 billion/year. So why are we allowing its foundational discipline to shrink in this country?
This column will address why users will be required to perform GNSS occupations when submitting a leveling project to the National Geodetic Survey (NGS) after 2022. It will highlight a section of NGS Blueprint for 2022, Part 3, “Working in the Modernized NSRS,” that discusses the process of performing leveling projects after 2022. My October 2017 column briefly discussed NGS’ preliminary plans for incorporating geodetic leveling data into the North American-Pacific Geopotential Datum of 2022 (NAPGD2022) to establish orthometric heights consistent with GNSS-derived NAPGD2022 orthometric heights. It emphasized that after NAPGD2022 is established, the primary means for deriving orthometric heights on monuments will be using GNSS observations combined with the geoid model.
As a side note, NGS just released NOAA Technical Report NOS NGS 72–GEOID18, a report that provides a comprehensive explanation of the data and methods used to create the latest NGS hybrid geoid model. My February 2020 column provided an analysis of the differences between the latest published hybrid Geoid18 values provided on NGS’ Datasheet and the computed geoid height value using the published NAD 83 (2011) ellipsoid height and NAVD 88 orthometric height.
In support of the modernization of the National Spatial Reference System (NSRS), NGS has published three documents denoted as Blueprints for 2022 that describe the modernization of the NSRS (see the box titled “NSRS Modernization NGS Blueprint Documents”).
There are several sections in NGS Blueprint for 2022, Part 3, “Working in the Modernized NSRS,” that discuss the process of performing leveling projects after 2022. Something that will be new after 2022 is that NGS will require users to perform GNSS occupations in order to incorporate their leveling results into the new modernized NSRS.
NGS realizes that in the immediate future GNSS will not replace geodetic leveling for determining the most accurate local orthometric height differences. NGS’ plans include preparing a new leveling manual that will explicitly explain how to work in the modernized NSRS. Some of the new surveying procedures are described in Section 2.10 of Blueprint part 3. In section 2.10, NGS states that there will be substantial changes in how they process and serve up survey data, and that there will be some new ways of executing surveys. This column will focus on sections “2.10.2 Leveling” and “2.11.5 Leveling on Passive Marks” that discuss the new procedures for executing leveling surveys in the modernized NSRS. One major change is that leveling surveys will require Global Navigation Satellite System (GNSS) occupations to ensure orthometric heights computed in leveling surveys are up-to-date and are connected to the NSRS through the NOAA CORS Network. After the modernization of the NSRS in 2022, the NOAA CORS Network will be the primary access to the NSRS. This means leveling and classical surveys will require GNSS surveys to be part of the project. NGS’ plans include creating an OPUS option for processing all types of surveys. Users will be able, within OPUS, to adjust their projects using any mix of CORS data and passive control. Saying that, these same projects, on submission, will be deconstructed at NGS and reduced to the raw observations, then adjusted solely to the NOAA CORS Network to determine Final Discrete coordinates every GPS Month. The GPS Month concept may be new to some users. Blueprint Part 3 describes the concept in section “2.11.3 GNSS on Passive Marks.” The basic concept of a GPS Month is that it is four consecutive GPS weeks, with the first week in the GPS month having a GPS week number that is a multiple of four (see box titled “Definition of a GPS Month”).
Definition of a GPS Month
GPS month: Four consecutive GPS weeks, with the first week in the GPS month having a GPS week number that is a multiple of 4.
In this fashion, NGS defines:
GPS month 0 = GPS weeks 0, 1, 2, and 3 (1/6/1980 through 2/2/1980)
GPS month 1 = GPS weeks 4, 5, 6, and 7 (2/3/1980 through 3/1/1980)
GPS month 2 = GPS weeks 8, 9, 10, and 11 (3/2/1980 through 3/29/1980)
…
GPS month 513 = GPS weeks 2052, 2053, 2054, and 2055 (5/5/2019 through 6/1/2019)
etc.
So, what does this really mean to the user when performing a leveling project in 2022? For a leveling project to be processed using NGS software and/or submitted to NGS for inclusion into the NSRS database, the user must follow specific rules.
The following is from Blueprint, Part 3, section “2.10.2 Leveling:”
“As GNSS occupations are required for geodetic leveling, the rules for how many and how frequently will be:
For a leveling project to be processed using NGS software and/or submitted to NGS for inclusion into the NSRS database, its field observations should not span more than one year. Longer projects should be broken into sub-projects of less than one year.
A minimum of three “primary control marks” must be in the level network for every project.
More primary control marks should be added so there is never more than a 30-kilometer linear distance between marks in the entire network.
Each primary control mark must have the following GNSS occupations (details on using GNSS occupations to work in the NSRS will be found in the update to NGS 58):
A minimum of two occupations within +/- 14 days of the beginning of leveling, but also falling within the same GPS month and whose local start times are separated by between 3 and 21 hours.
It is preferable, but not required, that all occupations on any primary control mark occur within the same GPS month as those of all other primary control marks.
A minimum of two occupations within +/- 14 days of the end of leveling, but also falling within the same GPS month and whose local start times are separated by between 3 and 21 hours.
It is preferable, but not required, that all occupations on any primary control mark occur within the same GPS month as those of all other primary control marks.
All projects exceeding six months must have a third set of GNSS occupations on all primary control marks some time near the middle of the project, without a rigorous rule as to when. They must follow the “minimum of two occupations” rule as per above, and each mark’s occupation is required to fall in the same GPS month, with a preference that all primary control marks are occupied in the same GPS month.”
The box titled “GNSS Procedures for Leveling Projects” highlights the GNSS rules that need to be adhered to when performing leveling projects in 2022.
GNSS procedures for leveling projects
For the Immediate Years Following 2022, NGS Will Require That all Leveling Projects Turned in Have GNSS on Primary Control
Minimum of 3 Points with a Maximum Spacing of 30 km
At Least Two Occupations of Each GNSS Primary Control:
+/- 14 days of Beginning of Leveling
Within the Same GPS Month
+/- 14 days of Ending of Leveling
Within the Same GPS Month
Diagram: David B. Zilkoski
The boxes titled “GNSS + Leveling 2022 Procedures at the Start of the Leveling Project” and “GNSS + Leveling 2022 Procedures at the End of the Leveling Project” provide conceptual diagrams that illustrate what this means to a typical leveling project.
Diagram based on information from Dan Gillins, NGS, and modified by David B. ZilkoskiDiagram based on information from Dan Gillins, NGS, and modified by David B. Zilkoski
So, why is NGS requiring users to perform GNSS observations in support of leveling project. Leveling is a differential measurement technique; it generates relative height differences not absolute heights. In NGS’ modernized, time-dependent 2022 NSRS, the absolute height will be provided by up-to-date GNSS data; and the accurate relative height differences between leveling marks will be provided by the leveling data. (See box titled “Why NGS Requires GNSS Occupations on Primary Marks.”)
Why NGS requires GNSS occupations on primary marks
The Connection to NAPGD2022 is Obtained Through GNSS and a High-Accuracy Geoid Model
Network Accuracy
The Accuracy of the Height Differences are Provided Through the Leveling Data
Local Accuracy
Combining the leveling and GNSS increases the redundancy in a survey network
NGS is developing models and tools to facilitate the incorporation of leveling survey data and adjustment results into the new modernized NSRS in 2022. Blueprint, Part 3, section “2.13.3 OPUS for Leveling,” describes NGS plans to support leveling surveys through the use of the OPUS web tool. The box titled “OPUS for Leveling” outlines how NGS will modify the OPUS web tool to support leveling surveys.
OPUS for leveling
Support for leveling surveys will follow many of the best aspects of OPUS
Uploading and processing digital data files
Using a web-based graphical interface
Submitting data to NGS
Leveling is a differential measurement technique
It generates relative height differences not absolute heights
For users who need absolute heights in the NSRS
OPUS will support a mix of GNSS and leveling in a single project
NOTE: NGS will require a GNSS survey to be performed at specific times before and after leveling surveys in order for the data to be submitted for inclusion in the modernized NSRS after 2022.
NOTE: Leveling surveys longer than one year must be broken up into multiple projects. Leveling surveys between 6 and 12 months in duration require a third, intermediary GNSS data collection.
This column highlighted that in the modernized NSRS the only way to get “into the datum” will be through a GNSS survey. It noted that leveling projects generate relative height differences not absolute heights. In NGS’ new modernized, time-dependent NSRS, the absolute height will be provided by up-to-date GNSS data; and the relative height differences between leveling marks will be provided by the leveling data. A major requirement will be that users must collect GNSS data both at the beginning and at the end of a leveling survey project. Leveling survey projects that take longer than one year to complete must be broken up into multiple projects. NGS is developing model and tools to facilitate incorporating all types of survey data into the new NSRS. I would encourage all readers to read NGS’ Blueprint for 2022 documents to obtain a better understanding of the new, modernized NSRS.
My February 2020 column provided an analysis of the differences between the latest published hybrid Geoid18 values provided on NGS’ Datasheet and the computed geoid height value using the published NAD 83 (2011) ellipsoid height and NAVD 88 orthometric height. The column highlighted issues on differences due to published heights that have changed since the database pull for Geoid18. It mentioned that future columns will address differences in other portions of CONUS. This column will focus on differences between published Geoid18 values and Geoid12B values in Southern Louisiana. Why are users seeing large differences between the two models?
My last column mentioned that the technical report on Geoid18 provided a good explanation on the stations used in the United States Gulf Coast region. See box titled “GPS on Bench Marks for GEOID18 in the Gulf Coast Region.”
GPS on Bench Marks for GEOID18 in the Gulf Coast Region
Figure 1: GEOID18 Gulf Coast selected marks: There are areas of complex vertical crustal motion in the Texas/Louisiana Gulf Coast region of the United States which render many control station elevations in the region invalid. The selection of GPS on Bench Marks in this region was limited to the small number of marks where the leveling and GPS data agreed to minimize the influence of crustal motion in the hybrid geoid model. Figure 1 depicts the selection of stations used in the hybrid geoid model along the Texas/Louisiana Gulf Coast. (Image: National Geodetic Survey)
As highlighted in the last column, very few stations in Southern Louisiana were used in the creation of the Geoid18 hybrid geoid model. As provided in my last column the box titled “Differences on GPS on Bench Marks in the Gulf Coast Region” depicts the differences between the published Geoid18 value and the computed geoid value using the latest NAD 83 (2011) ellipsoid and NAVD 88 orthometric height.
Differences on GPS on Bench Marks in the Gulf Coast Region
Image: National Geodetic Survey
The plot indicates that there are many large differences. Many of these differences are to be expected because the Southern Louisiana is an area of known crustal movement. NGS recognizes this and includes the statement below on datasheets for stations published in Southern Louisiana (see box titled “Statement on NGS Datasheet for Stations in Southern Louisiana”).
Statement on NGS Datasheet for Stations in Southern Louisiana
This station is in an area of known vertical motion. Due to the variability of land subsidence, uplift, and crustal motion, NGS has, determined the orthometric heights for marks in these suspect subsidence areas should be considered valid only at the epoch date associated with the orthometric height. These heights must always be validated when used as control. All previously superseded orthometric heights are now considered suspect and are available in the superseded section. NGS does not recommend using suspect or superseded heights as control.
Looking at the figure indicates that there is a significant variation of subsidence occurring in coastal Louisiana. The legend indicates that the subsidence rates range between 0.6 to 1.2 cm/year.
Figure 1 from A New Subsidence Map for Coastal Louisiana
The box titled “Excerpt from Anthropogenic and Geologic Influences on Subsidence in the Vicinity of New Orleans, Louisiana” depicts estimates of crustal movement between 2009 and 2012 in the vicinity of New Orleans. Several of the areas in the plot indicate subsidence rates exceeding -1 cm/year. Once again, the figure shows the local variability of subsidence rates.
Excerpt from Anthropogenic and Geologic Influences on Subsidence in the Vicinity of New Orleans, Louisiana
Last year, NGS performed the Multi-Year CORS Solution 2 (MYCS2). This was described in previous columns, which can be viewed here and here. The MYCS2 process generated computed and modeled velocities for CORSs. The box titled “CORS NAD83 (2011) Vu Velocities” is a plot that depicts the velocities in the “upward” component in cm/year for NOAA CORS that are operational and have a computed velocity in Southern Louisiana. So, what does this mean to estimating a hybrid geoid model in Southern Louisiana?
CORS NAD83 (2011) Vu Velocities
Image: National Geodetic Survey
The plot indicates that the rates vary from -0.1 cm to -0.8 cm. It should be noted that these stations are CORS and they are typically installed on structures that may not capture the entire amount of subsidence at the land surface. The box titled “CORS Position and Velocity for Station GRIS” provides an example of a CORS sheet from NGS CORS website.
Now, let’s look at differences between Geoid12B and Geoid18 in Southern Louisiana. The box titled “GPS on Bench Marks Used in Geoid18 and Geoid12B” depicts the stations used in Geoid12 and those used in Geoid 18. As indicated in the plots, there were a lot more stations used in the generation of the Geoid12B model than those used to create the Geoid18 model.
GPS on Bench Marks Used in Geoid18 and Geoid12B
Photo: National Geodetic Survey
The box titled “Differences between Geoid12B and Geoid18 in Southern Louisiana” provides the values of Geoid12B minus Geoid18 in centimeters on the GPS in Bench Mark stations used in Geoid12B.
Differences between Geoid12B and Geoid18 in Southern Louisiana
Photo: National Geodetic Survey
As indicated in the plot, there are some large differences between Geoid12B and Geoid18 values; a few differences exceed 15 centimeters. Based on the previous discussion of crustal movement in Southern Louisiana, this probably shouldn’t come as a surprise. The box titled “Differences between Geoid12B and Geoid18 with Vu Velocity Values” depicts the differences in the hybrid geoid models and the NAD83 (2011) CORS Vu rate.
Differences between Geoid12B and Geoid18 with Vu Velocity Values
Photo: National Geodetic Survey
The box titled “Differences between Geoid12B and Geoid18 in Lafayette, Louisiana” depicts the differences in the two hybrid geoid models and the NAD83 (2011) CORS Vu rate values in the Lafayette, Louisiana, region. This region has some of the largest differences between Geoid12B and Geoid18 values in Southern Louisiana. As indicated in the plot, CORS station TONY has a Vu rate of -0.8 cm/year which is fairly large, and the differences between Geoid12B and Geoid18 values are fairly large at the -10 to -15 cm level. Once again, users should expect differences between the two hybrid geoid models because there has been movement in the area and because different GPS on Bench Mark stations were used in the generation of the hybrid geoid models. In the Lafayette region the two stations used in the generation of Geoid18 were not used in Geoid12B (see stations highlighted in a box).
Differences between Geoid12B and Geoid18 in Lafayette, Louisiana
Photo: National Geodetic Survey
The box titled “Differences between Geoid12B and Geoid18 in New Orleans, Louisiana” depicts the differences in the hybrid geoid models and the NAD83 (2011) CORS Vu rate values in the New Orleans, Louisiana, region. Two of the same stations that were used in the development of Geoid12B and Geoid18 are highlighted with a box. The difference between the two geoid model values are much less in this region compared with the Lafayette region. The CORS Vu velocities are also less than the CORS station (TONY) value in Lafayette. Saying that, the differences on stations not used in Geoid18 have differences ranging from -4 to -8 cm going southward toward the Gulf of Mexico. Once again, Southern Louisiana is subsiding so these differences are not surprising.
Differences between Geoid12B and Geoid18 in New Orleans, Louisiana
Photo: National Geodetic Survey
This means if someone uses NGS’ OPUS web tool to compute a GNSS-derived orthometric height, the NAVD 88 GNSS-derived orthometric height could be significantly different than the published stations in this region. Some of the difference could be due to the difference between the Geoid12B and Geoid18 published values, and some could be due to crustal movement in Southern Louisiana. Saying that, I mentioned in my last column that NGS performed a large GNSS network project in Southern Louisiana in 2016. The GNSS-derived ellipsoid heights were loaded in NGS’ database in March 2019, but the GNSS-derived orthometric height from the 2016 project are not yet finalized so they have not been loaded into NGS’ database. Once finalized and loaded into the database, the 2016 GNSS-derived orthometric heights should be more consistent with GNSS-derived orthometric heights estimated using the NGS’ OPUS web tool. This column focused on differences between published Geoid18 values and Geoid12B values in Southern Louisiana. It provided reasons why users may see large differences between the two models.
My last column, December 2019, highlighted the National Geodetic Survey’s (NGS) new Geoid Monitoring Service (GeMS); and, that NGS’ will be publishing a gridded geoid model GEOID2022 that will contain two components: (1) Static Geoid model of 2022 (SGEOID2022) and (2) Dynamic Geoid model of 2022 (DGEOID2022). That’s what going to happen in 2022, but what about today? Since GEOID18 has been officially released for public use, it’s time to look at differences between the Geoid18 published value and estimated geoid values computed using information from NGS’ datasheet. This column will provide an analysis of the differences between the latest published hybrid Geoid18 values provided on NGS’ Datasheet and the computed geoid height value using the published NAD 83 (2011) ellipsoid height and NAVD 88 orthometric height. This is what a user will see if they computed differences using NGS’ datasheets published values. The question will always be asked, why is there a difference between the published Geoid18 value and the computed geoid value. This column will explain some reasons for the differences.
It’s mostly good news but there are some issues that should be highlighted. This column will highlight issues on differences due to published heights that have changed since the database pull for Geoid18.
First, it should be noted that NGS’ hybrid geoid models are different than NGS’ experimental gravimetric geoid models. My December 2018 column explains these differences.
I would like to emphasize that, in my opinion, hybrid geoid models should be denoted as transformation models. Saying that, hybrid geoid models are related to “real” geoid models. Hybrid geoid model GEOID18 was computed based on NGS’ gravimetric geoid model xGeoid19b; therefore, GEOID18 is related to a gravimetric geoid model but its function is to estimate GNSS-derived orthometric heights consistent with NAVD 88 heights. As described in my previous columns, the GPS on Bench Marks (GPSBMs) data provide an estimate of the geoid height ‘N’ by differencing the ellipsoidal height ‘h’ from the orthometric height ‘H’: (N = h – H). These differences are then compared to the gravimetrically-derived geoid model. The box titled “Excerpt from Geoid18 Website Technical Details” provides a summary of the process from NGS Geoid18 web page technical details document.
The figure in the box titled “GEOID18 Conversion Surface in cm” is the surface that represents the difference between NAVD 88 as a datum and the geopotential (geoid) surface used in the gravimetric geoid. This is the difference between the hybrid geoid and the gravimetric geoid with respect to NAD83 (GEOID18 – xGEOID19B). This surface has three essential components: a bias, a continental tilt, and local warping from the bench marks.
Hybrid Geoid Model Construction
The residuals obtained in equation 1 are contaminated with a continential tilt and bias that is estimated and removed with a simple two-dimensional planar surface. The bias-free and tilt-free residuals are ultimately used to determine a mathematical model using least squares collocation (LSC) and multiple Gaussian functions to describe the behavior seen at the bench marks. Once the relationship between the points is modeled, the model is used to generate a 1 arcminute regular grid for interpolation purposes. Figure 2 shows the final conversion surface. This surface represents the difference between NAVD 88 as a datum and the geopotential (geoid) surface used in the gravimetric geoid. This is the difference between the hybrid geoid and the gravimetric geoid with respect to NAD83 (GEOID18 – xGEOID19B). This surface has three essential components: a bias, a continental tilt, and local warping from the bench marks.
GEOID18 Conversion Surface in cm
Image: National Geodetic Survey
Looking at the figure in the box, the bias and tilt between the hybrid geoid model (Geoid18) and the experimental gravimetric geoid model (xGeoid19b) are fairly obvious. It’s the local warping from the bench mark data that may cause some issues to surveyors or, at least at a minimum, raise some concerned by surveyors. The box titled “Plot of the GPS on Bench Marks Involved in Geoid18” provides a plot of the GPS on Bench Marks (GPSBMs) used in the generation of Geoid18. Users can download the list of GPSBMs stations from the NGS Geoid18 website. There were 32,357 stations used to generate the model. This was an increase of approximately 6,800 stations (26%) over the hybrid geoid model Geoid12B.
Plot of the GPS on Bench Marks Involved in Geoid18
Image: National Geodetic Survey
The boxes titled “Number of GPS on Bench Mark Stations by State” and “Number of GPS on Bench Mark Stations by State in Northeast U.S.” provide the number of data points per state.
Number of GPS on Bench Mark Stations by State
Image: National Geodetic Survey
Number of GPS on Bench Mark Stations by State in Northeast U.S.
Image: National Geodetic Survey
The box titled “Table of Number of Data Points per State” provides the number of stations per State in tabular form.
Table of Number of Data Points per State
Data: National Geodetic Survey
The box titled “Summary of Overall fit of Geoid18” provides a summary of the fit of residuals of Geoid18 from the NGS GEOID18 technical details document. Looking at the CONUS overall values, the standard deviation is very low 1.27 cm which is a little better than Geoid12B (1.7 cm). It should be noted that there are some large outliers (minimum value of -10.12 cm and maximum value of 8.17 cm).
For this column, the file of bench marks provided on the NGS Geoid18 web page were combined with the published ellipsoid, orthometric, and Geoid18 heights from NGS’ datasheet. The difference between the published geoid height (Geoid18) and the estimated geoid height [published NAD 83 (2011) ellipsoid height minus NAVD 88 orthometric height] was computed using the following formula:
Data: National Geodetic Survey
The box titled “Plot of Differences Based on GPS on Bench Marks Used in Geoid18” depicts these differences based on the stations used to generate Geoid18.
Plot of Differences Based on GPS on Bench Marks Used in Geoid18
Image: National Geodetic Survey
Most of the values depicted on the plot are within the +/- 2 cm which is what you’d expect because the standard deviation of the overall fit is 1.4 cm. One to two centimeters is a very reasonable difference between the modeled and computed values. The question someone may ask is, I thought the model should be good to 1.4 cm so why are there large residual values on the map? There are several reasons why some of these differences are large but each case needs to be investigated to determine why they are large. This column will address one region as an example and provide a method for others to investigate differences in their area of interest.
The box titled “Plot of GPS on Bench Mark Differences at the ND/MN Border” depicts a very large difference between the modeled geoid model and the estimated geoid height along the ND/MN border. As indicated in the box, the difference exceeds 6 cm.
Plot of GPS on Bench Mark Differences at the ND/MN Border
Image: National Geodetic Survey
The box titled “Plot of GPS on Bench Mark Stations in the ND/MN Border Region” depict the bench marks involved in the development of Geoid18. The green circles represent the GPSBMs stations used in the creation of Geoid18 and the red “x” denote the stations that were not used in the creation of the model. As indicated in the plot, there were a lot of GPSBMs stations in the State of Minnesota (11,011).
Plot of GPS on Bench Mark Stations in the ND/MN Border Region
Image: National Geodetic Survey
The box titled “Differences on GPS on Bench Marks in ND/MN Border — NOT Used in Model” depict the values of the rejected GPS on BMs stations. These stations were not used to create the hybrid geoid model Geoid18. As the plot indicates there are several large differences. This is not really surprising since these stations were not used in the model.
Differences on GPS on Bench Marks in ND/MN Border — NOT Used in Model
Image: National Geodetic Survey
The box titled “Differences on GPS on Bench Marks in ND/MN Border — USED in Model” depict the values of the GPS on BMs stations used to create the Geoid18 model. Some of these differences exceed 8 cm. You would expect these differences to be small since these stations were used to create the model. So, why are there large post-modeled residuals in the Fargo, ND, region of the United States?
Differences on GPS on Bench Marks in ND/MN Border – USED in Model
Image: National Geodetic Survey
In August 2019, NGS performed a large leveling network adjustment in the Minnesota. The adjustment was performed after the Geoid18 database pull. The adjustment resulted in a 7- to 9-cm bias between the published height values and the superseded values. The August 2019 Minnesota leveling network adjustment heights were not used in the creation of Geoid18. The post-modeled differences presented in this column were generated using the published NAD 83 (2011) ellipsoid heights and current NAVD 88 orthometric heights from the NGSIDB. It was determined by NGS that the differences in the Fargo region were mostly due to crustal movement. Therefore, since the differences were due to movement, secondary adjustments will need to be performed to feather the 7- to 9-cm differences to maintain consistency between published NAVD 88 heights in the region. The secondary adjustments have not been completed as of the publication of this column so the residuals west of Fargo in North Dakota are small. These values will change after the secondary adjustment is completed and loaded into NGS’ database.
As an example, I’ve highlighted the station Fargo 0009 (PID DF7623) in the area of Fargo, North Dakota (see box titled “Differences on GPS on Bench Marks Near Fargo, ND”). The difference (-8.3 cm) is between the published Geoid18 value and the computed geoid value using the published ellipsoid height and orthometric height from the NGS’ datasheet. The box titled “Excerpt from Datasheet for Station Fargo 0009 (DF7623)” provides the information from NGS datasheet for station Fargo 0009; the information used in the computations are highlighted in the box. The box titled “Computation of the Difference between the Modeled Geoid Value (Geoid18) and the Computed Geoid Value for Fargo 0009” provides the process used to compute all differences for this column.
Differences on GPS on Bench Marks Near Fargo, North Dakota
Image: National Geodetic Survey
Excerpt from Datasheet for Station Fargo 0009 (DF7623)
Data: National Geodetic SurveyData: National Geodetic SurveyData: National Geodetic Survey
Computation of the Difference between the Modeled Geoid Value (Geoid18) and the Computed Geoid Value for Fargo 0009
(Information from NGS Published Datasheet)
Data: National Geodetic Survey
So, why is this difference so large in this region? A stated above, NGS performed a readjustment in this region and superseded the heights that were used in the creation of the Geoid18 hybrid model. The Geoid18 hybrid model used the previously published orthometric heights, now provided in the superseded section of the NGS datasheet, because that was the current published height at the time of the data pull for the Geoid18 process. Therefore, if we substitute the superseded height from the datasheet into the equation the difference is reduced to 0.1 cm (1 mm). [See the box titled “Computation of the Difference between the modeled geoid value (Geoid18) and the computed geoid value for Fargo 0009 Using the Superseded NAVD 88 Value.”]
Computation of the Difference between the modeled geoid value (Geoid18) and the computed geoid value for Fargo 0009 Using the Superseded NAVD 88 Value
(Information from NGS Published Datasheet)
Data: National Geodetic Survey
This means if someone uses NGS’ OPUS web tool to compute a GNSS-derived orthometric height, the NAVD 88 GNSS-derived orthometric height will be about 8 cm different than the published stations in this region. This should not be an issue if the users follow published NGS Guidelines to estimate the NAVD 88 GNSS-derived orthometric height, and/or uses NGS Beta OPUS-Projects and NGS procedures to estimate the NAVD 88 GNSS-derived orthometric height. These processes will ensure that the height will be consistent with the current published NAVD 88 orthometric heights in the NGS database.
The technical report on Geoid18 provides a good explanation on the stations used in the United States Gulf Coast region. See box titled “GPS on Bench Marks for GEOID18 in the Gulf Coast Region.”
GPS on Bench Marks for GEOID18 in the Gulf Coast Region
There are areas of complex vertical crustal motion in the Texas/Louisiana Gulf Coast region of the United States which render many control station elevations in the region invalid. The selection of GPS on Bench Marks in this region was limited to the small number of marks where the leveling and GPS data agreed to minimize the influence of crustal motion in the hybrid geoid model. Figure 1 depicts the selection of stations used in the hybrid geoid model along the Texas/Louisiana Gulf Coast.
As indicated in the box titled “GPS on Bench Marks for GEOID18 in the Gulf Coast Region” very few stations in Southern Louisiana were used in the creation of the hybrid geoid model. The box titled “Differences on GPS on Bench Marks in the Gulf Coast Region” depict the differences between the published Geoid18 value and the computed geoid value using the latest NAD 83 (2011) ellipsoid and NAVD 88 orthometric height. The plot indicates that there are many large differences. This is to be expected because the orthometric heights used in the creation of the hybrid geoid model are all superseded heights. This is because the only published heights in Southern Louisiana are GNSS-derived orthometric heights and leveling-derived orthometric heights were used in the creation of GEOID18.
Differences on GPS on Bench Marks
in the Gulf Coast Region
Image: National Geodetic Survey
Saying that, NGS performed a large GNSS network project in Southern Louisiana in 2016. At the time of the writing of this column, the GNSS-derived orthometric height from the 2016 project were not yet finalized.
This column provided an analysis of the differences between the latest published hybrid Geoid18 values provided on NGS’ Datasheet and the computed geoid height value using the published NAD 83 (2011) ellipsoid height and NAVD 88 orthometric height. The column highlighted issues on differences due to published heights that have changed since the database pull for Geoid18. Future columns will address differences in other portions of CONUS.
The National Geodetic Survey (NGS) has published a technical report that describes options for how NGS can implement a time-dependent geopotential datum and thus a time-dependent geoid model. My last column described the latest version of NGS’ VERTCON model. As mentioned in the column, NGS is developing these models and tools to support the implementation of the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).
NAPGD2022 is going to be a time-dependent geopotential datum. In other words, the reference geopotential will change over time and therefore the geoid height value will change over time. NAPGD2022 was described in detail in NGS’ publication “Blueprint for 2022, Part 2: Geopotential Coordinates,” and my December 2017 column. Blueprint for 2022, Part 2 states that a gridded geoid model GEOID2022 will be created and it will contain two components:
The first component will be time independent, denoted as the Static Geoid model of 2022 (SGEOID2022).
The second component will be a time-dependent geoid undulation model, encompassing permanent geoid changes greater than or equal to 1 millimeter per year, denoted as Dynamic Geoid model of 2022 (DGEOID2022).
NGS will publish a GEOID2022 value that will be based on both SGEOID2022 and DGEOID2022. As stated in the document, GEOID2022 will be the official zero-height surface for orthometric heights within NAPGD2022, and thus within the NSRS. The box titled “Excerpt from Blueprint for 2022, Part 2, Figure 10-2” is a diagram that describes the process of creating the regional high resolution gridded GEOID2022 model. I have highlighted the GEOID2022 model and its two components, SGEOID2022 and DGEOID2022.
Excerpt from Blueprint for 2022, Part 2, Figure 10-2
Image: National Geodetic Survey
First, it’s important to note the role of the geoid in estimating GNSS-derived orthometric heights. As described in a previous column, GNSS-derived Orthometric Heights are computed using the following formula: orthometric height (H) = ellipsoid height (h) minus geoid height (N). See the box titled “NAPGD2022 GNSS-Derived Orthometric Height.”
NAPGD2022 GNSS-Derived Orthometric Height
Source: Slide 9 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
So, what does it take to compute a time-dependent geoid model and what is NGS’ plan to accomplish this project The technical report titled “ A Preliminary Investigation of the NGS’s Geoid Monitoring Service (GeMS)” describes options for how NGS can implement a time-dependent geopotential datum and thus a time-dependent geoid model (See box titled “NGS Publishes Report on GeMS”). The report contains too much information for a single column. This column will highlight some of the sections of the report. The document does contain a lot of technical information and I would encourage everyone to download the document.
The technical report describes the current state of knowledge and outlines next steps required to define a time-dependent geopotential datum for the Nation. NGS created a project called “The Geoid Monitoring Service,” or simply GeMS, to accomplish their long-term goal of establishing a time-dependent geopotential model.
The report addressed the following five topics:
A foundational introduction to the various types of geophysical phenomena that are causing both size and shape change to the geoid,
Geodetic observing techniques that are presently available to monitor geoid change,
An objective evaluation of NGS’s current ability to incorporate these techniques into a long-term monitoring service like GeMS,
Known barriers to accomplishing such a project, and
Potential observing techniques that might become available in the next 10-20 years, but are not currently mature enough for operational use.
The document presents a roadmap of options for how NGS could realize a time-dependent geopotential datum, and how NGS can support the dynamic datum into the future with independent validation surveys and datasets.
The report discusses the available geoid monitoring techniques that NGS has to support modeling the changes in the geoid. There are three existing NGS program areas and associated technical expertise that could be utilized in an operational GeMS:
NGS’s Gravity Program,
the NOAA CORS Network, and
GPS/geodetic leveling campaigns.
It is noted that individuals these techniques cannot provide 100% of what GeMS requires but combining various programs would be sufficient. The report does a great job of describing these three program areas. The box titled “Summary of Geoid Monitoring Techniques within NGS’ Current Expertise” is Table 3 from the Technical Report. The table list the affordability and accuracy attributes for each of the program areas. NGS’ Gravity Program provides high quality gravity data to internal and external stakeholders. The program provides gravity data required for NGS’s geoid modeling.
Summary of Geoid Monitoring Techniques within NGS’ Current Expertise
Source: National Geodetic Survey
The report provides a good overview of the expertise and instrumentation of NGS’ Gravity Program. The table titled “Summary of NGS’ Terrestrial Gravity Instruments” is a compilation of information on historical methods and instrumentation from the technical report.
Summary of NGS’ Terrestrial Gravity Instruments
FG5 Absolute Gravimeter. The FG5(X) absolute gravimeter is manufactured by Micro-g LaCoste Inc. in Lafayette, Colorado. It is currently the highest-accuracy, commercially-available absolute gravity meter, with an accuracy of about ±2 μGals. NGS owns and operates instrument number FG5X-102. (Source: National Geodetic Survey)
A10 Absolute Gravimeter. A field deployable version of the laboratory FG5 absolute gravimeter, was developed by Micro-g LaCoste in the early 2000’s. This instrument, now known as the A10, operates on principals nearly identical to the FG5 free fall gravimeter. However, the laser used in the A10 system is not a primary standard due to the low power and fragile nature of the Iodine based laser, and does need to be calibrated routinely. (Source: National Geodetic Survey)
Scintrex CG-6 Relative Gravimeter. The Scintrex CG-6 relative gravimeter is the newest generation of the CG line of quartz sensor relative gravity meters. The CG-6 (like its predecessors the CG-3 and CG-5) operates on the same fundamental theory as the LaCoste and Romberg G and D relative gravimeters, but uses a quartz sensor spring instead of a metal sensor. The primary advantage of a quartz sensor is its insensitivity to instrument shock or vibration that can cause offsets in the gravity measurements. (Source: National Geodetic Survey)
LaCoste and Romberg Relative Gravimeter. LaCoste and Romberg (L&R) relative land, air/sea, borehole, and tidal gravimeters have been manufactured by LaCoste and Romberg Gravity Meters, Inc. since 1959. The company later merged into Micro-g LaCoste, Inc., and the LaCoste land gravimeters were discontinued. Unlike the Scintrex gravimeters, these rely on metal zero-length-springs. (Source: National Geodetic Survey)
Superconducting Cryogenic Gravimeter. A superconducting cryogenic gravimeter is designed to be continuously monitor relative changes in the local gravity over time. It main applications include precise tidal analysis, ground water monitoring, and geodynamics. The precision of an SG is still unmatched by any other instrument at better than 0.1 μGals at short time scales. (Source: National Geodetic Survey)
gPhoneX Gravity Meter. The gPhoneX, manufactured by Micro-g LaCoste, is a low (linear) drift, metal spring-based gravimeter. Like the SG, it is designed to measure relative changes in gravity over time, at a fixed location. While not as precise as the SG at short time scales; at periods of longer than a few hours, the noise characteristics of the two instruments are quite similar. The advantages of a gPhoneX compared to a SG for long term monitoring include lower cost, lower power consumption, increased portability, and lack of a requirement for maintaining superconducting temperatures. (Source: National Geodetic Survey)
JILAg Absolute Gravimeter. The AFGL, JILA/IGPP and JILAg series of absolute gravimeters are the out-of-production predecessors to the FG5 gravity meter. These instruments were developed by James Faller and colleagues beginning in the mid-1970s, and were crucial for defining and providing the basis for the IGSN71, NGSGN, and other scientific projects. Agency (now NGA), the University of California at San Diego (IGPP), and NGS. (Source: National Geodetic Survey)
The document highlights something about the United States gravity data that most users don’t think about. That is, gravity values are referenced to a gravity network just like NGS’ published orthometric heights are referenced to the NAVD 88. In the mid-1950s, a coordinated effort was initiated by the International Association of Geodesy (IAG) to make gravimeter ties throughout collaborating parts of the world to support establishment of an International gravity datum.
It incorporated intercontinental, north-south, calibration lines and long-distance ties established by airplane. The majority of USA relative gravimeter work was done from 1965 – 1967, resulting in the network shown in the box titled “International Gravity Station Net of 1971 (IGSN71) in CONUS.” The report states that the calculations were completed by Urho A. Uotila of The Ohio State University around 1970.
The gravity network was constrained by a network of ballistic absolute gravimeters. Five of the eight absolute gravimeter sites were in CONUS. It was a world-wide, simultaneous adjustment and published as The International Gravity Standardization Net 1971 (I.G.S.N. 71). A
s of December 2019, the IGSN71 remains the official international gravity datum. Many of these stations have been destroyed over the decades, in particular those at passenger airport terminals.
International Gravity Station Net of 1971 (IGSN71) in CONUS
Figure 14: IGSN71 Gravity Stations. (Source: National Geodetic Survey)
In the mid-1970s, NGS was involved in two major readjustment projects, replacement of NAD27 with NAD 83 and the replacement of NGVD 29 with NAVD 88. At the same time, the NGS gravity group were evaluating the gravity data in NGS database and the gravity stations involved in the IGSN71. During the period 1975 and 1979, NGS and NGA (formally DMA) performed relative gravity surveys around CONUS to evaluate the stations.
A report by Robert Moose titled “The National Geodetic Survey Gravity Network” published by NGS in 1986 documents the results of the surveys. This network is denoted as the National Geodetic Survey Gravity Network (NGSGN) and depicted in the box titled “National Geodetic Survey Gravity Network (NGSGN) in CONUS.” The NGSGN was constrained by 8 absolute gravimeter stations and consisted of 232 stations. Differences between NGSGN values and IGSN71 values were computed to evaluate or detect change in gravity values.
The box titled “Gravity Differences between NGSGN and IGSN71 Common Stations” depict these differences. The report states “In summary, the gravity differences between NGSGN and IGSN are generally small and many of the larger differences may be due to vertical motion.
National Geodetic Survey Gravity Network (NGSGN) in CONUS
Figure 16: Difference between NGSGN and IGSN71 AG values [mgal] (Source: National Geodetic Survey)The basic rule of thumb for estimating land movement using gravity changes is: 1 meter of change equals 0.3086 mgals (1 cm of change equals 0.003086 mgals). It should be noted that a positive difference in gravity in the figure indicated apparent subsidence. As stated by the 1986 report by Moose, the large difference in Houston-Galveston region is most likely due to subsidence.
A report documenting the apparent movement in the Houston-Galveston region was published by NGS in 1980. The boxes titled “ Estimate of Subsidence in Houston-Galveston Area During 1963-78 Epoch” and “Estimate of Subsidence in Houston-Galveston Area During 1973-78 Epoch” provide estimates of the movement in the region that include the same epoch of the two gravity networks. These two plots agree with the summary statement in the 1986 report.
Estimate of Subsidence in Houston-Galveston Area During 1963-78 Epoch
NOTE: 30 cm approximately equals to 1 foot (Source: National Geodetic Survey)
What does all this mean to the geoid? Accurate and current gravity data are critical to the development of an accurate geoid model that includes estimating changes in the geoid model over time.
The technical report on NGS’ Geoid Monitoring Service (GeMS) describes geodetic and geophysical techniques that are currently known to NGS and show promise for GeMS (see the box titled “Summary of Known Geoid Monitoring Techniques that are currently outside of NGS’s Expertise). It should be noted that all of these techniques rely on a non-NGS entity to create a product (such as a model or dataset) that NGS can utilize in their products and services. This is nothing new; NGS leverages partnerships for other products such as the GOCO05S satellite gravity model produced by an ESA consortium led by the Technical University of Munich. This model is used by the NGS geoid team in static geoid modeling.
Summary of Known Geoid Monitoring Techniques that are currently outside of NGS’s Expertise
(Source: Table 7 from Technical Report NOS NGS 69)
Continuation of Summary of Known Geoid Monitoring Techniques that are currently outside of NGS’s Expertise
(Source: Table 7 from Technical Report NOS NGS 69)
As apparent by all of the types of data required to monitor the geoid, NGS has a challenging task to establish a Geoid Monitoring Service. Why is it important to invest resources to monitor the geoid? Analyzes of temporal satellite gravity missions provide changes in gravity values that can be use to create changes in the geoid. The GRACE (Gravity and Climate Experiment) satellite mission was designed to provide the temporal gravity field variations throughout its mission (duration 2002 – 2017). There are analysis centers that produce models using the GRACE data – University of Texas at Austin Center for Space Research (UTCSR), NASA Jet Propulsion Laboratory (JPLEM), and GFZ German Research Center for Geosciences (GFZOP). Release 6 denoted as RL06 is the most current GRACE data from these groups.
The data can be used to illustrate the magnitudes and resolutions that GRACE models provide to the seculargeoid rates for CONUS and Alaska. The boxes titled “GRACE Trend over CONUS from UTCSR RL06” and “GRACE Trend over Alaska from UTCSR RL06” are plots from Technical Report NOS NGS 69 that show these secular geoid trends from UTCSR-RL06. The plots indicate very small changes in the geoid but they are significant if the goal is to monitor the geoid model to the mm/year level.
GRACE Trend over CONUS from UTCSR RL06
Figure 27: GRACE Trend over CONUS from UTCSR RL06 Model [mm/yr] (Source: Figure 27 from Technical Report NOS NGS 69)
GRACE Trend over Alaska from UTCSR RL06
Figure 28: GRACE Trend over Alaska from UTCSR RL06 GRACE Model [mm/yr] (Source: Figure 28 from Technical Report NOS NGS 69)Another product available from various processing centers are surface mass concentrations (mascons) as observed by the GRACE satellites. Once again, these mascons can be used to generate a secular geoid rate. The boxes titled “Geoid rate over CONUS based on the GSFC mascon model” and “Geoid rate over Alaska from GSFC mascon model” are plots from Technical Report NOS NGS 69 that provide the secular geoid rate based on the NASA GSFC mascon model. Once again, the plots indicate very small changes in the geoid but there is a systematic change to the geoid based on the analysis of the data from the GRACE mission.
Geoid rate over CONUS based on the GSFC mascon model
Figure 32 From Technical Report NOS NGS 69: Geoid rate over CONUS based on the GSFC mascon model [mm/yr] (Source: Figure 32 From Technical Report NOS NGS 69)
Geoid rate over Alaska from GSFC mascon model
Figure 33 From Technical Report NOS NGS 69: Geoid rate over Alaska from GSFC mascon model [mm/yr] (Source: Figure 33 From Technical Report NOS NGS 69)The report stated that when considering monitoring the geoid, the greatest change to the geoid from glacial isostatic adjustment (GIA) processes is centered in northern Canada, but there is “still a significant geoid height trend in the Northern Plains, Great Lakes, and Northeast regions of CONUS.”
It was noted that if GIA processes are not considered, a 1 cm error in the geoid undulation would occur within 18 years. NADGPD2022 orthometric heights are going to be established using a NATRF2022 ellipsoid height and a GEOID2022 geoid height. This is why the geoid needs a time-dependent component.
This column highlighted NGS new Geoid Monitoring Service (GeMS); and, that NGS’ will be publishing a gridded geoid model GEOID2022 that will contain two components:
The first component will be time independent, denoted as the Static Geoid model of 2022 (SGEOID2022) and
The second component will be a time-dependent geoid undulation model, denoted as Dynamic Geoid model of 2022 (DGEOID2022).
NGS will publish a GEOID2022 value that will be based on both SGEOID2022 and DGEOID2022. The column provided examples of how GRACE data can be used to illustrate the magnitudes of secular geoid rates for CONUS and Alaska.
![Figure 16: Difference between NGSGN and IGSN71 AG values [mgal] (Source: National Geodetic Survey)](https://stage.globalpositioningnews.com/wp-content/uploads/zilkoski-december-2019-column-image-14.jpg)
![Figure 28: GRACE Trend over Alaska from UTCSR RL06 GRACE Model [mm/yr] (Source: Figure 28 from Technical Report NOS NGS 69)](https://stage.globalpositioningnews.com/wp-content/uploads/zilkoski-december-2019-column-image-20.jpg)
![Figure 32 From Technical Report NOS NGS 69: Geoid rate over CONUS based on the GSFC mascon model [mm/yr] (Source: Figure 32 From Technical Report NOS NGS 69)](https://stage.globalpositioningnews.com/wp-content/uploads/zilkoski-december-2019-column-image-21.jpg)
![Figure 33 From Technical Report NOS NGS 69: Geoid rate over Alaska from GSFC mascon model [mm/yr] (Source: Figure 33 From Technical Report NOS NGS 69)](https://stage.globalpositioningnews.com/wp-content/uploads/zilkoski-december-2019-column-image-22.jpg)