GPS World

Tag: NSRS

  • NGS Discusses the New NSRS at the International 2023 FIG Working Week

    NGS Discusses the New NSRS at the International 2023 FIG Working Week

    Anyone reading my previous columns knows that I have been highlighting the new, modernized, National Spatial Reference System (NSRS) of the National Geodetic Survey (NGS). During the 2023 Fédération Internationale des Géomètres (FIG) Working Week held on May 28 – June 30, in Orlando, Florida, NGS held an all-day session addressing various topics related to the NSRS modernization project.

    More than 30 NGS staff members supported two days of sessions that included a day on the NSRS modernization, sessions for the Young Surveyors Network, and FIG Commission 5 meetings, which focused on meeting the highest accuracy levels for positioning and measurement.

    Juliana Blackwell, director of NGS, kicked off the third plenary session tackling the global challenges, with a presentation titled “The Modernized U.S. National Spatial Reference System — Aligning National Geospatial Data to the Globe.”

    Blackwell highlighted the importance of geospatial data from many different sources being interoperable and defined within a modern reference frame. She noted that NGS is part of the National Oceanic and Atmospheric Administration (NOAA), the mission of which is to understand and predict changes in climate, weather, ocean, and coasts. This includes a mandate to define, maintain and provide access to the NSRS.

    NGS’s NSRS modernization project has been underway for a decade and is nearly complete. Blackwell explained that the new NSRS will align critical U.S. geospatial data assets within global data inventories and enable improved analysis and modeling of climate changes and impacts to society and the environment. The modernized NSRS will enable data integration of new and old technologies, adopts modern standards, and empowers growth in new fields and applications.

    The remainder of the presentations during the all-day event covered three themes: the practical implications of NSRS modernization — changing survey methodology; an update on the NOAA CORS Network and the Online Positioning User Service (OPUS); and case studies of surveys — what NGS does now and how they will change.

    Many of these topics have been discussed by NGS during their webinar series, but during these presentations NGS provided the latest information on many of the activities associated with the modernization of the NSRS. This venue allowed for participants to ask questions as opposed to typing them in a box. Also, the NGS employees that participated in the FIG working week were available for discussions before and after the session. I enjoyed my discussions with old colleagues as well as meeting some new NGS employees.

    The session titled “Practical Implications of NSRS Modernization — Changing Survey Methodology” addressed the following topics:

    practical impacts of the modernized NSRS

    Canada’s implementation of the modernized frames

    changes afoot: State Plane 2022 and Retirement of the U.S. Survey Foot and

    preparing for the modernization of the NSRS.

    Dru Smith, NSRS modernization manager, started by explaining the practical impact of the modernized NSRS and why it is needed. He mentioned that the current NSRS was defined in the pre-GNSS era and that it has failed to keep up with emerging requirements, such as accurately measuring sea level rise.

    (Image: NGS Website)
    (Image: NGS Website)

    He highlighted the practical implications of the modernized NSRS, such as that every latitude, longitude, and ellipsoid height will change from its NAD 83 values in the +/-2-meter range, and every orthometric height will change from its NAVD 88 values in the +/-2-meters median range.

    He mentioned that the published coordinate functions of the NOAA CORS Network (NCN) will be the primary geodetic control of the NSRS. He noted that NGS is working on the integration of web-based tools to improve consistency and reduce confusion, such as enhancements to NGS’s OPUS to ingest digital data from surveying instruments directly into OPUS 6 via a Geodetic Data Exchange (GDX) format. This would include raw measurements from GNSS receivers, levels, total stations, and gravimeters. The talk titled “Augmenting Data Exchange Formats for OPUS of the Future” by Ryan Hardy, discussed the GDX format in more detail.

    As with the International Terrestrial Reference Frame, the modernized NSRS will have a time-dependency component. It will be built into the new NSRS, but users will have the ability to disengage from it.

    Smith provided a timeline of the project for the next couple of years, which can be referenced in the image below. NGS plans to release data and support tools on their BETA website during 2024 and 2025.

    (Image: NGS Website)
    (Image: NGS Website)

    Smith discussed how some products will be released early for users to test and evaluate how the new NSRS products will affect their products and services, and to be ready for their customers after the new NSRS is released for publication. Products scheduled for early releases (Alpha Release), include SPCS2022, EPP2022, and GEOID2022.

    He emphasized that Alpha products, by definition, can be one or more of the following:

    • incomplete
    • inaccurate
    • buggy
    • subject to change without notice

    As such, their early release is primarily for users to see the “big picture” such as formats of data and the general direction that NGS is taking.

    He provided a list of new products that will be announced soon, and some alpha products tentatively planned for release in 2023.

    (Image: NGS Website)
    (Image: NGS Website)

    Michael Dennis did a nice job of discussing the State Plane Coordinate System of 2022 (SPCS 2022) and the retirement of the U.S. Survey Foot. He mentioned that the U.S. Survey Foot was superseded by the international foot on December 31, 2022. His presentation gave a brief overview on the status and rollout plans for SPCS2022, along with how and why NGS will continue to support the U.S. Survey Foot in the existing NSRS (but not in the modernized NSRS).   

    See the image below for the number of zones for each state.  

    (Image: NGS Website)
    (Image: NGS Website)

    The SPCS2022 will be an alpha product released soon. Part of the alpha product will have options to view maps depicting the different zones in each state.  

    Example of Florida Multizone Complete Zones. (Image: NGS Website)
    Example of Florida Multizone Complete Zones. (Image: NGS Website)

    When NCAT2022 is released in alpha product it will contain the SPCS2022.   

    Example of NCAT2022 (Alpha). (Image: NGS Website)
    Example of NCAT2022 (Alpha). (Image: NGS Website)

    Representatives from the Canadian Geodetic Survey presented and participated in the discussions. 

    The session titled “Update on the NOAA CORS Network and OPUS” addressed the following topics: 

    • the NOAA CORS Network (NCN) services 
    • updating OPUS-S to support multi-GNSS 
    • OPUS -Projects 5: supporting RTK for establishment of geodetic control 
    • OPUS projects for manager’s training – transitioning from instructor-led to online, self-paced instruction and 
    • augmenting data exchange formats for OPUS of the future. 

    Dan Gillins gave a presentation on the advantages of using NGS’s OPUS-Projects 5 web routine. OPUS-Projects make it easier for users to submit a GNSS project to NGS for publication. I discussed OPUS-Project 5.1, when it was released as a Beta product, in my October 2021 column.  

    (Image: NGS Website)
    (Image: NGS Website)
    (Image: NGS Website)
    (Image: NGS Website)

    Gillins mentioned that a new publication providing guidance to meet standards for GNSS surveying is being reviewed and will be available soon. I discussed these new standards in my May 2023 column.

    (Image: NGS Website)
    (Image: NGS Website)

    Another presentation titled “OPUS-Projects for Manager’s Training – Transitioning from Instructor-Led to Online, Self-Paced Instruction” by Erika Little, described how NGS is transitioning to providing OPUS projects training on an online, self-paced instruction site. NGS has training material available for OPUS-Projects.

    (Image: NGS Website)
    (Image: NGS Website)

    Ryan A. Hardy gave a talk describing the new Geodetic Data Exchange (GDX). As previously mentioned, GDX is an XML-based data format that will be the input format for OPUS. GDX will be the successor to the GNSS Vector Exchange (GVX) format. GDX currently supports GNSS, classical, and leveling measurements.

    The GDX structure will have the following fields[[these are clearly the names of database fields; if it were a list of different types of information, we would not put them in all caps and would not use the underscores]]:

    • SOURCE_DATA
    • PROJECT_INFORMATION
    • PERSONNEL
    • UNITS
    • EQUIPMENT
    • POINTS
    • MEASUREMENT_SETTINGS
    • MEASUREMENTS
    • REDUCTIONS
    • OBSERVATIONS

    NGS is planning to release an alpha version of GDX soon.

    (Image: NGS Website)
    (Image: NGS Website)

    The session titled “Case Studies of Surveys — NGS Does Now and How They will Change” addressed the following topics: 

    • implementing NGS OPUS-Projects’ GVX feature to align RTK vectors to the NSRS to establish geodetic control for FirstNet indoor mapping 
    • IGLD: a case study for leveraging digital tools to enhance QA/QC on large scale static GNSS observation campaigns 
    • geodetic leveling in the modernized NSRS and 
    • NGS field operations: modernizing in many ways. 

    Ben Erickson gave a good presentation on leveling in the new NSRS, a topic about which I am very interested in knowing more[[Please avoid dangling participles, prepositions, conjunctions, and modifiers.]]. I discussed the new procedures in my June 2020 column.  

    One major change is that leveling surveys will require GNSS occupations to ensure that orthometric heights computed in leveling surveys are up to date and are connected to the NSRS through the NOAA CORS Network. The network accuracy is obtained through GNSS data and a high-accuracy geoid model, and the local accuracy is provided through the leveling data. Specific procedures will be required to incorporate leveling data in the North American Pacific Geopotential Datum (NAPGD2022).  

    Basic Procedures for NAPGD2022 Orthometric Heights. (Image: NGS Website)
    Basic Procedures for NAPGD2022 Orthometric Heights. (Image: NGS Website)

    I discussed these procedures in more detail in my June 2020 column. The image below provides a conceptual diagram that illustrates what this means to a typical leveling project.

    GNSS + Leveling 2022 Procedures at the Start and End of the Leveling Project”. (Image: Diagram based on information from Dan Gillins, NGS, and modified by David B. Zilkoski)
    GNSS + Leveling 2022 Procedures at the Start and End of the Leveling Project”. (Image: Diagram based on information from Dan Gillins, NGS, and modified by David B. Zilkoski)

    Erickson provided a diagram of a level network that contained a loop, which can be referenced below.

    (Image: NGS Website)
    (Image: NGS Website)

    I have worked with leveling data for most of my career and I am pleased to know that NGS is going to provide tools to incorporate leveling data into the new, modernized NSRS. When performing leveling projects, there is a requirement to level to previously established benchmarks that were within a certain distance from the project. This helped to ensure that different leveling projects were consistent with each other. NGS stated that making adjacent projects at different epoch consistent is under development, and their plans include updating leveling documentation to explain the leveling methodologies and GNSS control.

    (Image: NGS Website)
    (Image: NGS Website)

    I have only highlighted a few of the presentations. It was an all-day session, and a lot of information was presented on the new, modernized NSRS. The presentations can be downloaded from the NGS website at https://geodesy.noaa.gov/datums/newdatums/fig-2023.shtml. I would encourage everyone to download the presentations to obtain the latest information on NGS’s modernization of the NSRS. See the image below for the list of presentations and the links to download specific presentations.

    NGS Presentations at FIG 2023 Working Week. (Image: NGS Website)
    NGS Presentations at FIG 2023 Working Week. (Image: NGS Website)
  • Science of geodesy and surveying: support progress report

    Science of geodesy and surveying: support progress report

    Image: Avalon_Studio/E+/Getty Images
    Image: Avalon_Studio/E+/Getty Images

    On March 20, 2023, I wrote a short announcement about a funding opportunity by the National Geodetic Survey (NGS) to support the science of geodesy.

    As mentioned in previous columns, Everett Hinkley wrote about the geodesy crisis in an ION article. Hinkley’s article summarized several action items that could help improve the lack of trained geodesists in the United States. One action was to encourage U.S. government support in the form of grants, professional development of staff, and research collaborations/affiliations. A pilot PhD geodesy educational program with three National Geospatial-Intelligence Agency (NGA) and one NGS employee is in place. He stated that the NGA expects to continue growing this program. Click here for more information on NGA’s academic research program.

    NGS’ geospatial modeling grant is another example of this action item. There needs to be more funds added to this task, but it is a start. The program priorities under NGS’ grant program include: research and develop new methodologies for defining and applications for working with the NSRS; develop and evaluate tools, models, and guidelines to access, analyze, and manipulate geodetic data; enhance infrastructure of geodetic control, coastal remote sensing data, survey measurements, and other physical datasets that comprise the NSRS; support education, capacity building, and technology transfer for the future of geodesy; coordinate through partnerships with local, state, and regional users such as state and local governments, universities, and/or the public sector.

    The geospatial modeling grant was included in the 2023 Omnibus Appropriations Bill. The agreement provides $8,000,000 for the program and states that all funding shall be distributed externally. Hopefully, the same amount or more will be in FY 24 appropriations. Additional information about NOAA’s appropriations can be found in the 2023 Omnibus Appropriation Bill under the explanatory statement for Commerce, Justice, Science and related agencies. The bill can be found here. To find the language in the bill click here, then search the document for “geospatial.” See the image below for the language in the bill.

    Image: Senate.gov website
    Image: Senate.gov website

    For those that are interested in the appropriation process, the image below provides a list of the senators that work on these agencies’ appropriations. If you are interested in learning more about the appropriation process and the geospatial modeling grants, contact your senator. The more congressional representatives know about the geodesy crisis — which includes the lack of trained geodesist as well as surveyors — the sooner they will support funds to help correct the problem. Click here for a list of senators on the Commerce, Justice, Science and Related Agencies Appropriation Committee.

    Advancing geodesy with conferences

    Another activity that promotes the advancement of geodesy and surveying are national and international surveying and mapping conferences. Before the American Congress on Surveying and Mapping (ACSM) disbanded, the four-member organization collaborated to convene annual surveying and mapping conferences in the United States. Topics like those presented at a FIG Working Week were presented at these conferences.

    Since these ACSM conferences are no longer being held, I encourage users of geospatial data and GNSS technology to attend conferences like FIG Working Week 2023. I have participated in several FIG meetings and learned a lot from presentations as well as holding hallway meetings with experts from the international surveying and mapping community. In the March column, I highlighted that FIG Working Week 2023 is going to be held in Orlando, Florida, on May 28 – June 1. NGS will be presenting a full-day worth of content on NSRS modernization during the conference. I want to highlight some presentations that may be of interest to readers. Register for FIG Working Week 2023 here.

    The image below provides a list of NGS presentations with scheduled times. There will be a panel session in the beginning of the day to set the context for the day.

    Agenda of NGS DAY at FIG Meeting (Image: FIG website)
    Agenda of NGS DAY at FIG Meeting (Image: FIG website)

    As in most conferences there are several ways participants can register, one day to the entire conference. This is a great opportunity to have discussions with the leadership of the National Geodetic Survey and individuals working on the development of the new, modernized NSRS.

    Image: FIG website
    Image: FIG website

    There are a lot of presentations on various topics so, I would encourage readers to look through the entire agenda. FIG’s technical work is led by ten commissions. The August 2021 column provided information about the FIG commissions. See the list of commission below:

    Commission 1 – Professional Standards and Practice
    Commission 2 – Professional Education
    Commission 3 – Spatial Information Management
    Commission 4 – Hydrography
    Commission 5 – Positioning and Measurement
    Commission 6 – Engineering Surveys
    Commission 7 – Cadastre and Land Management
    Commission 8 – Spatial Planning and Development
    Commission 9 – Valuation and the Management of Real Estate
    Commission 10 – Construction Economics and Management

    The full technical program lists the topics by date and time. I highlighted sessions by commission 5 and 6 that I think would be interested to the surveying and mapping community. See the image below.

    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website
    Image: FIG website

    Finally, I would like to highlight a NGS product that is now in production mode. That is, OPUS Project 5.1 is now a production product. *NGS did not make an official announcement about this change, but if you access OPUS Project the new version comes up. As described in the March column, OPUS Project 5.1 routine allows the use of RTN vectors and post-processed vectors from vender software.

    Clicking the “projects” icon on the OPUS page connects you to the latest version of OPUS Project 5.1. See image below. Please see the March column or NGS’ January webinar to learn more about OPUS Project 5.1.

    Image: NGS Website
    Image: NGS Website

    *Note: As of the writing of this column, March 29, it is still listed on the beta release section of NGS website. If you click on OPUS Project 5.1 in the Beta Release section, it will link to the production version of the routine.  

  • Great funding opportunity to support geodesy: March 22 deadline!

    Great funding opportunity to support geodesy: March 22 deadline!

    In July 2020, my First Fix article discussed the Geodesy Crisis in the United States. In January 2022, Mike Bevis, collaborating with others, prepared a white paper titled “The Geodesy Crisis,” documenting the concern about the lack of trained geodesists in the United States. Since then, my November 2022 survey scene column highlighted that without investment in geodesy, the United States will not have the available skills and knowledge to develop new geodetic technologies and improve models to address challenges to society. In December 2022, Matteo Luccio discussed the urgent need for U.S. geodesists with Everett Hinkley, who works for the federal government, serves as a subject-matter expert on several high-level boards, and dubs himself a “concerned citizen geodesist.”

    Well, things are starting to happen. NGS is soliciting grant proposals from eligible organizations to implement activities that modernize and improve the National Spatial Reference System (NSRS) and advance the science of geodesy in the United States. See the image below.

    I realize that this is very short notice, but all Letters of Intent (LOIs) must be received no later than Wednesday, March 22, 2023. Full proposals do not have to be completed until April 24, 2023. The grant information and related material can be found here.

    This is a great opportunity for institutions of higher education, state, local and Indian tribal governments to partner with industry and private consultants to advance the science of geodesy.

    (Image: NGS Website)
    Image: NGS Website

    NGS Geospatial Grant Opportunity: https://content.govdelivery.com.

  • The effects of geoid changes in NGS’s new, modernized 2022 NSRS

    The effects of geoid changes in NGS’s new, modernized 2022 NSRS

    My April column addressed the vertical movement at the NOAA CORS Network (NCN). The values at the sites indicate the potential movement of marks in the area of the CORS. The rates are based on GNSS data and have an estimate of error associated with them.

    As I mentioned in my previous column, I’m not sure how the National Geodetic Survey (NGS) will address the vertical movement effects in the new, modernized National Spatial Reference System (NSRS). That said, NGS will be monitoring the CORS and looking for trends to help describe the vertical movement at the CORS. These trends are an indication of what may be happening in that area.

    As stated in previous columns, orthometric heights in NAPGD2022 will be defined through ellipsoid heights and a geoid model, for example GEOID2022. In addition to the movement of individual marks due to crustal movement, there are geophysical reasons for changes in the geoid that affect the orthometric height of a mark. Therefore, changes in the geoid model will be very important to users estimating orthometric heights using GNSS.

    As stated in the NOS NGS 64 report, NGS has set a goal of maintaining geoid accuracy at 1 centimeter (1 standard deviation) in both absolute and differential geoid undulations. The box titled “Figure 13 from NOS NGS 64 Report” depicts an estimate of the secular change in the geoid. As indicated in the plot, the changes are very small, ranging from -1.25 mm/year to 1.5 mm/year.

    What I find interesting is the small negative change in the southeastern United States. There are other drivers for geoid changes. This column will address some of these changes and what they mean to users.

    Secular geoid change

    Photo: NGS
    Figure 13 from NOS NGS 64 Report (Image: NGS)

    As mentioned in many of my articles, the new, modernized NSRS has a time-dependent component. This includes the geoid model.  Table 5-1 from NOS NGS 64 report are examples of some of the physical processes being investigated by NGS to account for changes in the geoid.  (See the box titled “Some of the geophysical drivers of geoid change.”) As mentioned in the NOS NGS 64 report, the magnitudes in red have already been determined to be too small for NGS to model. The examples highlighted in yellow have magnitudes that are significant and NGS will attempt to account for these changes to the geoid. 

    Table 5-1: Some of the geophysical drivers of geoid change
    Table 5-1: Some of the geophysical drivers of geoid change

    NGS classifies the changes in the geoid in three different groups: Shape Change, Size Change, and W0 Change. The box titled “The Groups of Geoid Change” provides NGS’s definition and explanation of the terms.

    The groups of geoid change

    Photo: Dave Zilkoski

    NGS’s report on their Geoid Monitoring Service (GeMS) program provides figures that depict an estimate of the secular geoid rate trend based on the NASA GSFC mascon model. See the boxes titled “Estimate of Geoid Rate Over CONUS” and “Estimate of Geoid Rate Over Alaska.” For more details on GeMS, download the report NOAA Technical Report NOS NGS 69: A Preliminary Investigation of the NGS’s Geoid Monitoring Service (GeMS), and read my December 2019 Survey Scene column. The secular geoid rate trend is an example of the geoid changing its shape, but not the W0 value. What this means is that the local geoid undulations will change, but the overall size of the geoid will not.

    Estimate of geoid rate over CONUS

    Figure 32: Geoid rate over CONUS based on the GSFC mascon model [mm/yr] (Image: NOAA)
    Figure 32: Geoid rate over CONUS based on the GSFC mascon model [mm/yr] (Image: NOAA)
    Estimate of geoid rate over Alaska

    Figure 33: Geoid rate over Alaska from GSFC mascon model [mm/yr] (Image: NOAA)
    Figure 33: Geoid rate over Alaska from GSFC mascon model [mm/yr] (Image: NOAA)
    These changes in the geoid are fairly small values (+/- 1.3 mm/year), but they will accumulate over a decade. As previously stated, NGS’s goal is to maintain geoid accuracy at the centimeter level (1 standard deviation) in both absolute and differential geoid undulations. In my February 2022 column, I discussed how coordinates change because Earth’s surface is moving due to the movement of major tectonic plates. It’s fairly obvious how the tectonic shift affects horizontal coordinates, but earthquakes and volcanic eruptions can also cause large shifts in vertical coordinates.

    In recent history, on May 18, 1980, geologists watched in awe as Mount St. Helens erupted in a gigantic explosion. After the eruption, the volcanic cone of Mount St. Helens had been completely blasted away; the peak, which was at an elevation of 9,677 feet (2,950meters) was changed to a horseshoe-shaped crater with an elevation of 8,363 feet (2,549 meters). Extreme crustal movements such as the Mount St. Helens eruption can change the shape of the geoid. As explained in my April 2022 newsletter, NGS understands this and is attempting to manage the changing coordinates by providing a time-dependent component to a mark’s ellipsoid height, but there is also a time-dependent component to the geoid that affects the mark’s orthometric height.

    Ring of Fire

    Image: National Ocean Service
    Image: National Ocean Service

    The “Ring of Fire” map highlights earthquake activities around the world. As indicated in Table 5.1, earthquake or volcanic eruptions can change the shape of the geoid. Of course, they also can change the height of a mark due to crustal movement, which would typically be larger than the change in the geoid height. The amount of movement would be due to the size and magnitude of the event, but even small earthquakes could cause a change in the height of a mark located near the event. Earthquakes are occurring all over the world every day. 

    Earthquakes with large magnitudes are highlighted by news media outlets, but ones with smaller magnitude typically are not highlighted. The four figures below provide examples of earthquakes that have occurred over 30 days. This information can be obtained from the United States Geological Survey (USGS).

    Earthquakes during the past 30 Days
    Date: May 20, 2022

    Image: USGS
    Image: USGS

    Earthquakes in the lower 48 during the past 30 days
    Date: May 20, 2022

    Image: USGS
    Image: USGS

    Earthquakes in eastern United States in the past 30 days
    Date: May 20, 2022

    Image: USGS
    Image: USGS

    I found the large number of earthquakes that occurred in Oklahoma in just 30 days to be very interesting.  This isn’t something that I thought occurred in the eastern region of the United States. 

    Earthquakes in Oklahoma during the past 30 days
    Date: May 20, 2022

    Image: USGS
    Image: USGS

    The image below depicts earthquakes that have occurred in Oklahoma in the past five years. They are fairly small in magnitude, but what is the cumulative effect on the geoid in the region, as well as changes to the orthometric heights of marks due to crustal moment in the region?  This is why it is important for the new, modernized NSRS to implement time-dependent coordinates. 

    Earthquakes in Oklahoma in the last 5 years
    Dates: 2017 to 2022

    Image: USGS
    Image: USGS

    To better understand the changes to the geoid, NGS performed a survey in Alaska to obtain geodetic data as part of its GeMS program.  On May 12, 2022, Kevin Ahlgren, a geodesist at NGS, described in a webinar the observations collected and some of the results. 

    The presentation provided an overview of a field campaign performed in support of the GeMS program and a time-dependent geoid model. The campaign included static GNSS, relative gravity, and deflection of the vertical techniques on 50 stations in Alaska. The webinar was can be downloaded.

    I encourage everyone to download the presentation.  The change in the geoid due to geophysical drivers is small, but if the new, modernized NSRS is going to include time-dependent coordinates, then changes in the geoid must be accounted for.  For demonstration purposes, NGS provides an example of the time-dependent geoid change in the xGEOID20 webtool.  The box below, “xGEOID20 interactive computation output,” is an example of using this tool. The two stations are located in Alaska. As indicated in the output from the tool, the change in the geoid is 8 mm in five years. Again, NGS’s goal is to maintain geoid accuracy at the centimeter level (1 standard deviation) in both absolute and differential geoid undulations.  These small changes can become significant over time. 

    xGEOID20 interactive computation output

    Note: DN is the time-dependent geoid change computed between user inputted epoch (t) and t.
    Note: DN is the time-dependent geoid change computed between user inputted epoch (t) and t. (Image: NGS)

    The last geoid change group that I’ll highlight has to do with the change in the gravity potential (W0) value that defines the model. The NOS NGS 64 Report states that the standing definition of the geoid, as adopted and used at NGS, is the following:  

    The geoid is the equipotential surface of the Earth’s gravity field which best fits, in a least squares sense, global mean sea level. 

    As stated in the NOS NGS 64 report, over a century of sea-level measurements imply that global mean sea level (GMSL) was rising at a rate of approximately 1.7 millimeters per year and was rising at a rate of 3.2 millimeters per year between 1993 and 2010 (IPCC, 2014). If NGS is going to define the geoid as the equipotential surface of the Earth’s gravity field that best fits, in a least squares sense, global mean sea level, then the geoid in the new, modernized NSRS must change when the GMSL exceeds a certain threshold. 

    Again, NGS’ goal is to maintain geoid accuracy at the centimeter level (1 standard deviation) in both absolute and differential geoid undulations.  What this means is that as GMSL rises, the value of gravity potential which best fits to GMSL (called W0) will also change.  In other words, the surface which was called “the geoid” and had W=W0 in 2022 will no longer be the geoid. A new value of W0 (W0new) is chosen, and “the geoid” would now be the surface W=W0new.   

    So, what does this really mean to users? The NOS NGS 64 Report states on page 37:

    “NGS and the Canadian Geodetic Survey have jointly adopted the value of 2.0 m^2/s^2 as the replacement threshold for a new geoid model (and new geopotential datum). This represents approximately 20 centimeters of GMSL (and thus geoid) rise. At the current rate of sea-level change of about +3 millimeters per year (IPCC, 2014), this means NGS expects to replace NAPGD2022 in approximately 60 to 70 years.” 

    Therefore, this should not be a major concern of users for a long time. 

    This column highlighted that orthometric heights in NAPGD2022 will be defined through ellipsoid heights and a geoid model, for instance GEOID2022; and therefore, changes in the geoid model will be very important to users estimating orthometric heights using GNSS.  It briefly described the geophysical reasons for changes in the geoid that affect the orthometric height of a mark. 

    If NGS is going to meet the goal of maintaining geoid accuracy at 1 centimeter (1 standard deviation) in both absolute and differential geoid undulations, they will have to address changes in the geoid. The secular changes in the geoid, as indicated in Figure 13 in the NOS NGS 64 report, are very small, ranging from -1.25 mm/year to 1.5 mm/year. Once again, these are small changes to the geoid, but they will accumulate over time, and that is why NGS is including time-dependent coordinates in the new, modernized NSRS.

  • Estimating heights with subsidence changes using NGS data and tools

    Estimating heights with subsidence changes using NGS data and tools

    This column details the potential effects of crustal movement on published heights in various regions of the United States.

    In my last column (in the April 2021 Survey Scene), I mentioned that the National Geodetic Survey (NGS) announced that it is suppressing height information in Southeast Texas.

    The April column also highlighted one of NGS’ 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 column highlighted the potential effects of subsidence on published heights in the Houston region, which implied that most of the published heights that are based on older surveys in the region are not current or accurate.

    This column will provide more details of the suppression of heights in the Southeast Texas region, and potential effects of crustal movement on published heights in other regions of the United States.

    NGS announcement to suppress height information for Southeast Texas. (Image: NGS)
    NGS announcement that it suppressed height information for Southeast Texas. (Image: NGS)

    According to NGS’ announcement, only 28 marks will have publicly available orthometric heights on NGS datasheets in Southeast Texas.

    The “Link to Map: SE TX Valid Ortho. Heights” button provides the benchmarks available to users (see the box titled “Link to Map SE TX Valid Ortho Heights”). The website provides links to the published stations.

    Link to Map SE TX Valid Ortho Heights. (Image: NGS website)
    Link to Map SE TX Valid Ortho Heights. (Image: NGS website)

    Clicking on an icon provides the PID and name of the station with a link to a datasheet. Click  “Get Datasheet” for a datasheet of the station. Below is an excerpt from the datasheet of Station P 1200.

    Excerpt from Datasheet of Station P 1200.(Image: NGS Website)
    Excerpt from Datasheet of Station P 1200.(Image: NGS Website)

    Let’s address why NGS is suppressing the stations in Southeast Texas. My last column provided plots depicting the amount of movement in the Harris-Galveston, Texas, region. See the box titled “Estimate of Amount of Subsidence in 5 Years in Harris-Galveston, Texas, Region – Units Feet.”

    As indicated in the plot, some of the marks are estimated to have moved almost ½ foot (approximately 0.15 meters) in 5 years. In addition, some of the relative height differences approach 1/3 of a foot (approximately 0.1 meter) between neighboring stations. See the highlighted stations in the box titled “Estimate of Amount of Subsidence in 5 Years in Harris-Galveston, Texas, Region – Units Feet.”

    Estimate of Amount of Subsidence in 5 Years in the Harris-Galveston, Texas, Region – Units Feet. (Image: David Zilkoski)
    Estimate of Amount of Subsidence in 5 Years in the Harris-Galveston, Texas, Region – Units Feet. (Image: David Zilkoski)

    The last major releveling incorporated into NGS’ Database in the Harris-Galveston, Texas, region was performed more than 30 years ago in the 1986/1987 timeframe. Therefore, some of the published stations in the region could have subsided more than three feet (or about a meter).

    As stated in NGS’ Blueprint 3, “Most leveling data in NGS archives comes from the mid-20th century, in support of the NAVD 88 project.” Of course, most regions of the United States are not subsiding at the same rates as in the Houston-Galveston, Texas, region.

    In a previous newsletter, I discussed NGS’ second Multi-Year CORS Solution of the National CORS (MYCS2). I downloaded the coordinates and velocities from NGS’ website and created a plot of the vertical velocities. For those who prefer to use feet as opposed to meters, I provided velocities with units in feet/year and mm/year.

    See the boxes titled “Estimate of Velocity Rates Based on MYCS2 – CONUS (feet/year),” “Estimate of Velocity Rates Based on MYCS2 – Alaska (feet/year),” “Estimate of Velocity Rates Based on MYCS2 – CONUS (mm/year)” and “Estimate of Velocity Rates Based on MYCS2 – Alaska (mm/year).”

    It should be noted that the intent of these four plots is to provide a wide-ranging view of the values and some of the variation in rates across the United States.

    Estimate of Velocity Rates Based on MYCS2 – CONUS (feet/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – CONUS (feet/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – CONUS (feet/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – CONUS (feet/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – CONUS (mm/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – CONUS (mm/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – Alaska (mm/year). (Image: David Zilkoski)
    Estimate of Velocity Rates Based on MYCS2 – Alaska (mm/year). (Image: David Zilkoski)

    The rates appear to be small in most regions of the United States. As an example, the rates are all less than -0.0062 feet/year (-0.0019 meters/year) in the Lake Norman region in North Carolina (see the box titled “Potential Subsidence Rates in the Lake Norman Region in North Carolina). It would take many years for the crustal movement to make a difference to some projects in this region.

    Potential Subsidence Rates in the Lake Norman Region in North Carolina. (Image: David Zilkoski)
    Potential Subsidence Rates in the Lake Norman Region in North Carolina. (Image: David Zilkoski)

    That said, let’s look at another region of the country. For example, in the vicinity of Maryville, Missouri, the rate of subsidence is around -0.0187 feet/year (-0.0057 meters/year). See the box titled “Potential Subsidence Rates in the Maryville, Missouri, Region.” These subsidence rates don’t appear to be large values but if you take into account the last time the height of a mark was established by leveling data it could result in a large difference from the true orthometric height.

    Potential Subsidence Rates in the Maryville, Missouri, Region. (Image: David Zilkoski)
    Potential Subsidence Rates in the Maryville, Missouri, Region. (Image: David Zilkoski)

    According to NGS’ database, it appears that many of the marks in the Maryville, Missouri, region were last leveled in 1935. I used NGS’ Passive Mark Lookup tool and Leveling Project Page tool to identify the marks and associated leveling lines in the area of the CORS stations in the Maryville, Missouri, region.

    I described the Passive Mark Lookup webtool in a previous column. As previously mentioned, these subsidence rates all seem very small, but if you take into account the last time the height of mark was established by leveling data, the subsidence value can be very large.

    See the box titled “Potential Subsidence in 86 Years in the Maryville, Missouri, Region.” The box indicates that, if you account for the last 86 years (2021 – 1935), the potential subsidence exceeds 1½ feet (-1.6082 feet, -0.4902 meters).

    Potential Subsidence in 86 Years in the Maryville, Missouri, Region. (Image: David Zilkoski)
    Potential Subsidence in 86 Years in the Maryville, Missouri, Region. (Image: David Zilkoski)

    Continuing across the country to Colorado, the box titled “Potential Subsidence Rates in the Grand Junction Region, Colorado,” provides the estimate of subsidence rates in Mesa County, Colorado. As the plot indicates, the rates vary between -0.0046 feet/year (-1.4 mm/year) and -0.0128 feet/year (-3.9 mm/year). Once again, these rates all seem relatively small but many of the marks near CORS MC06 were last leveled in 1985. This means the potential change in height could be as large as 0.2592 feet (0.0792 meters).

    Potential Subsidence Rates in the Grand Junction. Colorado, Region. (Image: David Zilkoski)
    Potential Subsidence Rates in the Grand Junction Region, Colorado. (Image: David Zilkoski)

    Obviously, this is only an estimate of the subsidence in the region and the actual amount of subsidence is unknown since the last time the mark was leveled. These estimates are based on the MYCS2, which uses current data to estimate the velocity. The processing included data spanning 1996 to 2016 (week 0834 to 1933), 1099 weeks or about 21 years in total.

    The point of this column is not to provide the exact change in height of a mark, but to highlight that the publicly available orthometric height on a NGS datasheet may not be up to date based on crustal movement. The new modernized National Spatial Reference System will enable users to determine an accurate, current height on a mark and be able to efficiently and effectively monitor changes in a mark’s height.

    As stated in NGS’ announcement to suppress the heights in Southeast Texas, the agency has developed tools to assist users in submitted data to NGS. See the box titled “Excerpt from NGS Announcement to Suppresses Height Information for Southeast Texas.”

    Excerpt from NGS Announcement to Suppresses Height Information for Southeast Texas. (Image: NGS website)

    This assistance is for every user, not just for individuals performing surveys in Southeast Texas. NGS has Regional Geodetic Advisors throughout the United States.

    NGS Regional Geodetic Advisors. (Image: NGS Website)
    NGS Regional Geodetic Advisors. (Image: NGS Website)

    The Regional Geodetic Advisors provide guidance and assistance to constituents within their region. They are subject-matter experts in geodesy and regional geodetic issues. These individuals can assist users that are planning GNSS campaigns to re-densify the network.

    NGS also provides a website detailing how users can help densify the network to prepare for the new, modernized North American-Pacific Geopotential Datum of 2022 (NAPGD2022). See the box titled “NGS GPS on Bench Marks Webpage.”

    As mentioned in previous newsletters, a benefit of the new modernized National Spatial Reference System (NSRS) will facilitate the establishment of consistent, accurate NAPGD2022 GNSS-derived orthometric heights.

    NGS GPS on Bench Marks webpage. (Image: NGS Website)
    NGS GPS on Bench Marks webpage. (Image: NGS Website)

    This column provided details on the suppression of heights in the Southeast Texas region, and potential effects of crustal movement on published heights in other regions of the United States. NGS suppressed the heights in the Southeast Texas region because of the large amount of crustal movement since the last time the heights of the marks were established.

    As indicated by NGS’ MYCS2 velocities, every mark could be affected by crustal movement. In my opinion, the question a user should be asking is “How much has the height of the mark changed since it was last determined? Not, “Has the height of the mark changed?”

  • NGS revises NOAA report on working in the modernized NSRS

    NGS revises NOAA report on working in the modernized NSRS

    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.”

    NOAA Technical Report NOS NGS 67.(Image:NGS)
    NOAA Technical Report NOS NGS 67. (Image: NGS)

    On March 11, NGS held a webinar describing the revised document (see box titled “Working in the Modernized NSRS”). Download a video of the webinar and the presentation.

    Working in the Modernized NSRS. (Image: NGS}
    Working in the Modernized NSRS. (Image: NGS}

    The revised document added four use cases to describe how someone might access and use the NSRS in the future:

    • Use Case 1: Flood Mapping,
    • Use Case 2: Passive Control for a Multi-year Corridor Project,
    • Use Case 3: Transitioning Data to the Modernized NSRS, and
    • Use Case 4: Leveraging the Modernized NSRS for Airport and Other Infrastructure Monitoring.

    The box titled “Major Changes to NOS NGS 67” highlights the changes in the February 2021 revised version.

    Major Changes to NOS NGS 67. (Image: NGS)
    Major Changes to NOS NGS 67. (Image: NGS)

    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.

    Figure 10 from NOAA Technical Report NOS NGS 67 — Diagram of fictional case study location. The arrows correspond to hypothetical rates of ground subsidence. (Image: NGS)
    Diagram of fictional case study location for Use Case 1 (Figure 10), The arrows correspond to hypothetical rates of ground subsidence. (Image: NGS)

    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. 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)
    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.

    Download latest FEMA Flood Insurance Rate Map (FIRM). See box titled “Excerpt from FEMA FIRM Map Number 48201C0440N.”

    Excerpt from FEMA FIRM Map Number 48201C0440N. (Image: FEMA)
    Excerpt from FEMA FIRM Map Number 48201C0440N. (Image: FEMA)

    According to the latest Flood Insurance Study (FIS), the heights used in the study were based on a 2001 adjustment performed by the county. You can download the FIS from FEMA Flood Map Service Center | Search All Products, 48201CV001G (fema.gov) and map1.msc.fema.gov.

    I’d like to highlight a few statements in the FIS. First, the reports states that the FIS and DFIRM are referenced to the NAVD (2001 Adjustment). See the box titled “Page 111 from November 15, 2019 Flood Insurance Study 48201CV001G.” The report provides a link for users to obtain the latest vertical control data. Users can find information about the Harris County Floodplain Reference Marks here (See box titled “Harris County Floodplain Reference Marks.”) Users also can access the vertical control data at the county website.

    Page 111 from Nov. 15, 2019, Flood Insurance Study 48201CV001G. (Image: FEMA)
    Page 111 from Nov. 15, 2019, Flood Insurance Study 48201CV001G. (Image: FEMA)
    Harris County Floodplain Reference Marks. (Image: Harris County Flood Control District)
    Harris County Floodplain Reference Marks. (Image: Harris County Flood Control District)

    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.

    Snapshot of Vertical Control from Harris County Floodplain Reference Marks Website. (Image: (Image: Harris County Flood Control District))
    Snapshot of Vertical Control from Harris County Floodplain Reference Marks Website. (Image: Harris County Flood Control District))

    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”).

    Harris County Floodplain Reference Mark Datasheet. (Image: Harris County Flood Control District)
    Harris County Floodplain Reference Mark Datasheet. (Image: Harris County Flood Control District)

    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.

    Page 113 from November 15, 2019 Flood Insurance Study 48201CV001G. (Image: FEMA)
    Page 113 from November 15, 2019, Flood Insurance Study 48201CV001G. (Image: FEMA)

    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. (Image: Harris-Galveston Subsidence District)
    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.)

    Estimate of Amount of Subsidence in 5 Years – Units: Feet. (Image: David Ziljoski)
    Estimate of Amount of Subsidence in 5 Years – Units: Feet. (Image: David Zilkoski)

    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.”)

    Estimate of Amount of Subsidence in 5 Years at Pam 1 – Units: Feet. (Image: David Ziljoski)
    Estimate of Amount of Subsidence in 5 Years at Pam 1 – Units: Feet. (Image: David Zilkoski)

    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.”)

    Estimate of Amount of Subsidence in 5 Years at Pam 46 – Units: Feet. (Image: David Ziljoski)
    Estimate of Amount of Subsidence in 5 Years at Pam 46 – Units: Feet.(Image: David Zilkoski)

    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.

    There’s one last item that I’d like to highlight in this newsletter. On March 12, NGS announced that they are suppressing height information in Southeast Texas. See the box titled “NGS Announcement to Suppresses Height Information for Southeast Texas” for more information.

    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.

    NGS announcement to suppress height information for Southeast Texas. (Image: NGS)
    NGS announcement to suppress height information for Southeast Texas. (Image: NGS)

    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.”

    NGS Southeast Texas Orthometric Heights. (Image: NGS)
    NGS Southeast Texas Orthometric Heights. (Image: NGS)

    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.

  • NGS releases modernized National Spatial Reference System updates

    NGS releases modernized National Spatial Reference System updates

    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.

    Updates notices from NGS Homepage

    Image: National Geodetic Survey
    Image: National Geodetic Survey

    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.

    Delayed Release of the Modernized NSRS

    (https://www.ngs.noaa.gov/datums/newdatums/delayed-release.shtml)

    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
    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”

    (https://www.govinfo.gov/content/pkg/FR-2020-07-24/pdf/2020-16068.pdf)

    Image: Federal Register
    Image: Federal Register

    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.

    Imposing Age Limits of Data in 2022

    (https://www.federalregister.gov/d/2020-16084)

    Image: Federal Register
    Image: Federal Register

    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

    (https://www.federalregister.gov/d/2020-16084)

    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.

    Webinar on Delayed Release of the Modernized NSRS

    (https://geodesy.noaa.gov/web/science_edu/webinar_series/delayed-release-nsrs.shtml?utm_medium=email&utm_source=GovDelivery)

    Image: National Geodetic Survey
    Image: National Geodetic Survey

    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.”

    Questions from NGS FAQ Website

    (https://www.ngs.noaa.gov/datums/newdatums/FAQNewDatums.shtml)

    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
    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.

  • Surveyors’ coordinate systems for 2022 and beyond

    Surveyors’ coordinate systems for 2022 and beyond

    Time.

    Ask anyone what time means to them, and they will give you a different answer. Benjamin Franklin famously stated that “time is money.” Time for the surveyor can mean being out in the field retracing a boundary, drafting a plat or working with a client to help them see their goals achieved. Just like any other profession, time can be a friend or foe for the surveyor. We seem to run out of it more than we have an excess of it. Either way, time marches on as we go about our business.

    Time, however, is changing the surveyor’s world and how we go about our methods of measurement. While it seems like a crazy concept, time is the major component requiring changes to geodetic procedural processes and how we will determine our locations in the future.

    We will continue to see advances in hardware and software along with new interfaces and ways to collect and display survey data almost daily, and we will continue to deal with adaptation. However, surveyors must be ready for the next big challenge: a national horizontal and vertical adjustment of the National Spatial Reference System (NSRS) into a new standard. The North American Terrestrial Reference Frame of 2022 (NATRF2022.) is currently being developed by NGS and will replace NAD83 and NAVD88. Most surveyors will ask why we are getting ready for a historic change in datums. Easy — it’s all about time.

    Expanded Variables

    Just as early travelers thought the Earth was flat and learned it wasn’t through exploration and science, we are learning more everyday regarding how our world is changing. To get a better understanding of how our world is changing, NGS and the geodesy community have expanded the environmental variables of geographic location to areas including gravity, geoid undulations and geopotential data, plate tectonics and crustal evolution, and additional GNSS data analysis through satellites and continuously operating reference station (CORS) installations.

    By introducing new attributes affecting coordinate data, including horizontal motions induced directly or indirectly by adjoining tectonic plates, horizontal motions induced by Global Isostatic Adjustment, other horizontal motions and all vertical motions in their entirety (per NGS NOAA Technical Report NOS NGS 62), data captured will be used to create an Intra-Frame Velocity Model (IFVM). Data  following this format will be now be used to monitor the movement of survey positions from implementation forward. The key factor in which all the data is centralized is time.

    My GPS World colleague David Zilkoski presented a thorough explanation (“NGS to Replace NAVD88 in 2022: What GNSS Users Need To Know) of the nuts and bolts behind the changes. Here are the basic reasons behind the new adjustment as provided by NGS:

    NAD 83 and NAVD 88, although still the official horizontal and vertical datums of the National Spatial Reference System (NSRS), have been identified as having shortcomings that are best addressed through defining new horizontal and vertical datums.

    Specifically, NAD 83 is non-geocentric by about 2.2 meters. Secondly, NAVD 88 is both biased (by about one-half meter) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today. Both of these issues derive from the fact that both datums were defined primarily using terrestrial surveying techniques at passive geodetic survey marks. This network of survey marks deteriorate over time (both through unchecked physical movement and simple removal), and resources are not available to maintain them.

    The new reference frames (geometric and geopotential) will rely primarily Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) as well as an updated and time-tracked geoid model. This paradigm will be easier and more cost-effective to maintain.

    Plate tectonics
    Plate tectonics

     

    These proposed changes to the NSRS, however, are based upon how much we have learned about our changing Earth using GNSS equipment and data collection. Time, as it turns out, is a big factor in how we measure and document locations. A point that is determined exactly here on this day at a specific moment will have moved due to plate tectonics and other variables to there over a period of time.

    New Vertical Component

    Another aspect of the datum change will be the definition of a new vertical component. Surveyors are familiar with the establishment of NGVD29 based upon mean sea level, and also NAVD88 being based upon the benchmark at Father Point/Rimouski, Quebec, Canada with reference to the International Great Lakes Datum of 1985. What science has taught us in the years beyond NAVD88 is that there is a greater force at play when it comes to the vertical piece of geolocation: gravity.

    Yes, gravity keeps us on the ground and causes water to flow downhill, but the development of gravitational studies has led to incredible discoveries of how gravity affects elevation. It was always assumed that the gravitational pull on the earth was uniform worldwide, but with the development of instruments that can measure and map the variations in gravity, NGS will be redefining the vertical datum through a program called GRAV-D. NGS is currently flying in various portions of the U.S. and is scheduled to be completed by 2021 in order to roll out with the new horizontal program in 2022.

    So, it turns out that time has been affecting not just our productivity but also our positions on the earth. Another famous quote by Paulo Coelho does hold true: “Time neither moves nor is stationary. Time changes.” Time has passed since this article began; did you feel the earth move?

    What about our survey monuments and state plane coordinates?

    For many surveyors, the main question is simple: why now? What is so bad with our existing NAD83 and NAVD88 datums?

    Burch0517003
    Map courtesy of GISGeography, at http://gisgeography.com/state-plane-coordinate-system-spcs/

    The reason is very simple; staying current with our favorite tool in the toolbox: GNSS. Surveyors have always been about “monuments” and perpetuation of data from established points located on the face of the Earth with published and/or known values. This concept has become even more important to the surveying community once the proliferation of geographic-based and state plane coordinate data was published for all to utilize. I touched on the surveyor’s use and data collection/perpetuation of location values in a past column (GPS World November 2016). As long as NGS updated the national database with more information and a simple adjustment every so often, life was good and simple.

    But now we have worlds colliding; static monuments with published horizontal and vertical values in one corner, while in the other corner is the new paradigm of ever shifting crustal plates and changing positional values monitored by GNSS data through satellites and a network of CORS located worldwide.

    This situation makes me harken back to one of my favorite “Ghostbusters” lines from Bill Murray’s character, Dr. Peter Venkman: “Human sacrifice, dogs and cats living together – mass hysteria…”

    Okay, maybe it won’t be quite that bad but there will be many surveyors that will have trouble wrapping their minds around the new concept of “moving monuments.” Burch0517005Our reliance on state plane coordinate systems (SPCS) is at an all-time high with the sharing of data by various parties being more seamless than ever. The notion that a permanent monument’s positional values will be constantly changing is a head-spinner to most.

    NGS has also stated that their new system and procedures will not maintain data values for SPCS (see NGS State Plane Flyer). There are currently 125 SPCS zones and 3235 county systems throughout the US and territories in place that rely on NGS data as the main framework, so having tools for reference and conversion in place will be crucial. Thus, it will be a herculean task to create a procedure/process to easily pass coordinate values between our many static systems worldwide and the new dynamic but very robust system underway from NGS.

    Based upon information currently available about the NAD2022 system, it would be more efficient for all those who use geolocation data to modify their thinking to adapt to a dynamic coordinate system. However, this is a similar situation to early scientists and geographers throwing out all references to flat-earth maps and atlases. For surveyors in the twilight of their careers, these are radical items to consider and a far cry from the standardized chain and theodolite. (Maybe there will be mass hysteria…)

    The good news is that we have very intelligent people in the surveying and geodetic community who are working on solutions for the masses. The beauty of newer technology is how quickly hardware and software can be adapted to fit these new data conditions. Getting the word out on these changes and educating our profession will be a key factor to its success.

    Further Refinement of Coordinate Systems

    While the use of GNSS has enabled the discovery of time as a significant variable in geolocation, it has also expanded out coverage area of coordinate systems to much larger areas. Distances that could not be computed prior to GNSS are now easily attained and large projects can be managed within a common coordinate system. County, regional and state agencies can now create large-scale GIS databases that utilize a single coordinate system as well.

    However, there are two differing tracks being formed with the continued development of the new datum by NGS. While the new datum will become more precise and predictable, there are movements in opposing camps to make changes in user coordinate in the furthest possible ways: statewide single zone system versus county/regional low distortion projection (LDP) systems. They both have their strengths and weaknesses, and will depend on the application of the user to choose the appropriate system.

    • Most states currently have two or more zones so there potential to combine all zones into one, but a major drawback will be the loss of accuracy away from the defining points. For GIS users, this accuracy will more than adequate and will allow the merging of data from across the state into one unified system.
    • Surveyors, however, are an interesting bunch in that they accept only the most accurate AND precise measurements. The growing use of LDP is rapidly changing the implementation and management of coordinate system in smaller areas such as counties and regional DOT districts.

    Burch0517007
    However, both systems have a place in our surveying and mapping world. NGS has stated that while they will help with transformation software and apps, it will leave the decision of legislative standards to each state. It will be paramount that each state study what makes the most sense for its users and pass the appropriate legislation.

    Burch0517009

    “The days are long but the years are short”

    As I look back and realize how much has changed with modern technology and overall knowledge of our profession, it is with much anticipation how much more will change with advancements we don’t even know about yet. The electronic distance meter (EDM) was revolutionary for many surveyors and I’ve waxed poetic about my feelings regarding RTK GNSS in past columns (GPS World May 2016). Once again, however, technology and information based upon its use has revolutionized our data system.

    As a profession, surveyors have embraced GNSS use and data collection from the early implementation of the system. And while the advances of UAV use, laser scanners and LiDAR along with software improvements have revolutionized data collection, these proposed coordinate improvements by NGS will bring more potential quality information into the surveyor’s hands.

    And while time is money as Mr. Franklin famously stated, 2022 is just around the corner. A good friend of mine is famous for saying: “Good coordination begins with good coordinates.” The work performed by NGS is helping us do just that. The entire surveying, mapping and geodetic community has lots to accomplish to be ready for the changes from NGS. Let’s get to work.