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Tag: North American-Pacific Geopotential Datum of 2022

  • Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 4

    Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 4

    My last column focused on the National Geodetic Survey’s (NGS) current plans for estimating North American-Pacific Geopotential Datum of 2022 (NAPGD2022) GNSS-derived orthometric heights and incorporating geodetic leveling data into 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.

    Recently, NGS published its second blueprint for the 2022 document titled “Blueprint for 2022, Part 2: Geopotential Coordinates.” The report addresses NAPGD2022 in detail. The intent of the document is to provide to the public the current status of plans by NGS to modernize the geopotential component of the National Spatial Reference System (NSRS) in 2022. This particular document covers the definition and determination of orthometric heights, geoid undulations, gravity, deflections of the vertical, dynamic heights, and any other quantity directly related to the geopotential field of the Earth. As mentioned my previous columns, NAPGD2022 will be replacing the North American Vertical Datum of 1988 (NAVD 88). The executive summary of report NGS 64 is provided in the box titled “Executive Summary, NOAA Technical Report NOS NGS 64, Blueprint for 2022, Part 2: Geopotential Coordinates.” Surveyors and mappers should obtain a basic understanding of the four interrelated products of NAPGD2022. They are GM2022, GEOID2022, DEFLEC2022, and GRAV2022. I’ve highlighted them in executive summary box below.

    Executive Summary

     


    NOAA Technical Report NOS NGS 64

    Blueprint for 2022, Part 2: Geopotential Coordinates
    In 2022, the entire National Spatial Reference System (NSRS) will be modernized. This document addresses the geopotential aspects of the NSRS, including every vertical datum, the geoid, gravity, deflections of the vertical, and other quantities related to Earth’s gravity field. Every one of these related, yet semi-independent sources of information will be replaced with an internally consistent geopotential datum called the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). Within NAPGD2022 four primary, interrelated time-dependent products will exist:

    • A global model of Earth’s geopotential field (GM2022)
    • Regional gridded geoid undulation models (GEOID2022)
    • Regional gridded deflection of the vertical models (DEFLEC2022)
    • Regional gridded surface gravity models (GRAV2022)

    The three regions for the gridded models will be North America (covering CONUS, Alaska, Hawaii, the Caribbean, Canada, Mexico, Central America and Greenland), American Samoa and Guam/Commonwealth of Northern Mariana Islands (CNMI).

    NAPGD2022 will be built upon the IGS frame, as only minor (entirely horizontal) differences will exist between the IGS frame and the four new terrestrial reference frames developed as part of the NSRS in 2022 (see NGS, 2017). Since these differences will be relatively small horizontal displacements (mainly due to Euler pole rotations), NAPGD2022 will operate equally well in any of four new frames.
    Orthometric heights in NAPGD2022 will be defined through ellipsoid heights and GEOID2022. This means NAPGD2022 orthometric heights will primarily be accessed through Global Navigation Satellite System (GNSS) technology. GEOID2022 will be defined in a manner that best fits global mean sea level at the epoch of NAPGD2022. When global sea level changes by a threshold level of 20 centimeters, a new geoid model, and thus geopotential datum, will be released. Until then, updates to any component of NAPGD2022 will result in updating all components of NAPGD2022 using sequential version numbering.

    Leveling in NAPGD2022 will retain its current role of providing high-accuracy local differential orthometric heights. The determination of absolute heights, however, which will provide the context of local differential heights, will reside in the GNSS domain (i.e., will be based on IGS ellipsoid heights).

    Find this entire report here.

    There is a lot of good information in the report and I would encourage everyone to download the report and read it. Some of the report is technical but most of it provides simple and easy to understand explanations of very technical terms. Pages 22 and 23 of NGS 64 provides a good summary of the different components of NAPGD2022 (see box tilted “Excerpt from Section 9 of NGS 64”).

    Excerpt from Section 9 of NGS 64

    9 The 2022 Geopotential Datum

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    1. In 2022, the NSRS will contain one geopotential datum, capable of providing (at a minimum) the geoid undulation, acceleration of gravity, geopotential number, and deflection of the vertical at any given latitude, longitude, ellipsoid height, and time in a global ideal reference frame, such as the International Terrestrial Reference Frame (ITRF) or International GNSS Service (IGS) frames. The name of this datum will be the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

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    2. The foundational component of NAPGD2022 will be a spherical13 harmonic model of Earth’s external gravitational potential, called (for now) the Geopotential Model of 2022 (GM2022).
       
      The GM2022 will be created for the entire Earth and will contain two components:
      1. The first component will be time independent, fixed at some epoch (TBD14) to a at least degree and order of 2160,15 called (for now) the Static Geopotential Model 2022 (SGM2022).
      2. Complementing SGM2022 will be a time-dependent model of Earth’s external gravitational potential, capable of capturing both secular and episodic changes of significance. This time-dependent model will be called (for now) the Dynamic Geopotential Model 2022 (DGM2022).

       

    3. Three derivative products, based upon GM2022, but requiring additional information and providing higher-resolution regional information than is contained in GM2022 will be created:
      1. A gridded geoid model GEOID2022,16 which will contain two components:
        1. The first will be time independent, fixed at some epoch (TBD) called (for now) the Static Geoid model of 2022 (SGEOID2022).
        2. Complementing this will be a time-dependent geoid undulation model, encompassing permanent geoid changes >= 1 millimeter per year, called the Dynamic Geoid model of 2022 (DGEOID2022).
      2. A gridded deflection of the vertical, DoV, model (at the surface of the Earth) DEFLEC2022, which will contain two components:
        1. The first will be time independent, fixed at some epoch (TBD) called (for now) the Static Deflection of the Vertical model of 2022 (SDEFLEC2022).
        2. Complementing this will be a time-dependent DoV model, called the Dynamic Deflection of the Vertical model of 2022 (DDEFLEC2022).
      3. A model for interpolating surface gravity GRAV2022, which will contain at least one, possibly two components:
        1. The first will be time independent, fixed at some epoch (TBD) called (for now) the Static Gravity model of 2022 (SGRAV2022).
        2. As a second, possible component, NGS will investigate the feasibility of a time-dependent surface gravity model.

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    The three derivative-gridded products (GEOID2022, DEFLEC2022, and GRAV2022) will encompass three non-global areas. These three areas will be (latitude and longitude convention being positive north, positive east):

    The boxes titled “Figure 9-1 From NOS NGS 64,” “9-2 from NOS NGS 64,” and “9-3 from NOS NGS 64” depict the regions that GEOID2022, DEFLEC2022 and GRAV2022 will cover.

    Figure 9-1 From NOS NGS 64

    The North American region for GEOID2022, DEFLEC2022 and GRAV2022
    Figure 9-2 From NOS NGS 64

    The American Samoa region for GEOID2022, DEFLEC2022 and GRAV2022
    Figure 9-3 From NOS NGS 64

    The Guam and CNMI region for GEOID2022, DEFLEC2022, and GRAV2022

    So, what does this mean to the surveying and mapping community? First, as mentioned in my previous columns, there will be significant differences between NAPGD2022 and NAVD 88. Figure 1 depicts the approximate differences between NAPGD2022 and NAVD 88 in the conterminous United States.

    Figure 1 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). [Figure 1 is from June 2017 Survey Scene column.]

    For those still referring their products to NGVD 29, figure 2 depicts the approximate differences between NAPGD2022 and NGVD 29 in the conterminous United States.

    Figure 2 – Approximate Change Between NAPGD2022 and NGVD 29 Using GPS on BMs Data (units = cm). [Figure 2 is from the June 2017 Survey Scene column].

    My April 2017 Survey Scene column provided an estimate of the change between NAPGD2022 and NAVD 88 at bench marks with GNSS-derived ellipsoid heights in Alaska. Figure 3 is a plot of the GPS on BMs residuals computed using xGeoid16b geoid values, IGS08 ellipsoid heights, and NAVD 88 orthometric heights.

    Figure 3 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). GPS on Bench Mark Residuals Using xGeoid16b in the State of Alaska – Referenced to IGS08 (units = cm) – Green Line Represents the Leveling Lines [Figure 3 is from the April 2017 Survey Scene column.

    As outlined in NOS NGS 64 report and previously mentioned in this column, there are four interrelated products of NAPGD2022 – GM2022, GEOID2022, DEFLEC2022, and GRAV2022. What most surveyors will be using is GEOID2022 (SGEOID2022 and DGEOID2022). As explained in my last column, and part of NGS’ frequently asked questions about the new datums, users will access the NSRS using GNSS-derived ellipsoid heights and GEOID2022.

    How will accessing the National Spatial Reference System (NSRS) change with the release of the new datums?
    The NSRS will be accessed using Global Positioning System (GPS) technology that references Continuously Operating Reference Stations (CORS) and relies on a time-dependent gravimetric geoid model. This method of accessing the NSRS is a paradigm shift from accessing NAD 83 and NAVD 88 through the use of geodetic survey marks.

    It will not be necessary to connect to a geodetic monument, i.e., a bench mark, because the NATRF2022 ellipsoid height (hNATRF2022) is determined using the NGS CORS and the geoid model (NGEOID2022) is consistent with NATRF2022. In other words, GNSS ellipsoid heights (e.g., NATRF2022) combined with the geoid model (e.g., GEOID2022) will become the primary means for deriving orthometric heights on marks.

    There will be a static geoid model of 2022, denoted as SGEOID2022, which will be fixed at a specific epoch. Since the geoid model changes due to various factors, such as changes in sea level, glacial rebound, and seismic activities, there will be a dynamic aspect of the 2022 geoid model, denoted as DGEOID2022. The permanent changes to the geoid model are small and will take several years to become significant to affect the typical survey and mapping product. Saying that, it is important to understand that there is a static and a dynamic aspect of the National geoid model. NGS will provide a single GEOID2022 value which will apply the appropriate static and dynamic components of the geoid model.

    Even though, the primary access to NAPGD2022 will be using GNSS and a geoid model, users will still want to perform precise leveling observations and incorporate the results into NAPGD2022. My last column discussed incorporating leveling data into NAPGD2022. Differential leveling of high precision is used to observe elevation differences which are then used to establish precise heights of vertical control points (bench marks) above or below a reference surface, e.g., the North American Vertical Datum of 88 (NAVD 88) or North American-Pacific Geopotential Datum of 2022 (NAPGD2022). Differential leveling, conceptually a simple procedure, in practice lends itself to many types of small errors. To detect, reduce, and control these errors, specific procedures need to be adhered to and corrections must be applied. FGCS has documented the necessary procedures to be used in first-, second- and third-order geodetic leveling projects. Procedures do not always reduce error to tolerable values; therefore, additional corrections are applied by the office processing the data to remove known systematic errors.

    The box titled “Excerpt from Special Report Results of the General Adjustment of the North American Vertical Datum of 1988” provides a summary of the corrections applied to the leveling data used in NAVD 88. As you can see, gravity (highlighted in the box) plays an important role in estimating accurate orthometric heights. This is where GRAV2022 is important, it is used during the process of converting observed leveling height differences into orthometric height differences.

    Excerpt from Special Report – Results of the General Adjustment of the North American Vertical Datum of 1988

    (https://www.ngs.noaa.gov/PUBS_LIB/NAVD88/navd88report.htm)
    David B. Zilkoski, John H. Richards, and Gary M. Young
    American Congress on Surveying and Mapping Surveying and Land Information Systems, Vol. 52, No. 3, 1992, pp.133-149
    Corrections Applied to Leveling Data

    The leveling observations used in NAVD 88 were corrected for rod scale and temperature, level collimation, and astronomic, refraction, and magnetic effects (Balazs and Young 1982; Holdahl et al. 1986). All geopotential differences were generated and validated, using interpolated gravity values based on actual gravity data. Geopotential differences were used as observations in the least-squares adjustment, geopotential numbers were solved for as unknowns, and orthometric heights were computed using the well-known Helmert height reduction (Helmert 1890): H = C/(g + 0.0424H), where C is the estimated geopotential number in gpu, g is the gravity value at the benchmark in gals, and H is the orthometric height in kilometers. The weight of an observation was calculated as the inverse of the variance of the observation, where the variance of the observation is the square of the a priori standard error multiplied by the kilometers of leveling divided by the number of runnings.

    This column highlighted two components of NAPGD2022 – the geoid undulation model of GEOID2022 and gravity model of GRAV2022. It expressed that these two models will be very important to future surveyors and mappers that are incorporating geodetic data into the North American-Pacific Vertical Datum of 2022 (NAPGD2022). As previously mentioned, I would encourage everyone to download and read NGS recently published second blueprint for 2022 document, titled “Blueprint for 2022, Part 2: Geopotential Coordinates.” This column also emphasized the significant differences between NAPGD2022 and the U.S. National Vertical Datums of NAVD 88 and NGVD 29. My next column will provide the latest details of NGS’ 2018 GPS on BMs campaign which will be used to develop transformation tools for converting products and services from NAVD 88 to NAPGD2022.

  • Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 2

    Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 2

    My last column highlighted some of the feedback provided by guest presenters at the NGS’ 2017 Geospatial Summit held on April 24-25 in Silver Spring, Maryland. That column also provided a discussion on the approximate differences between NAPGD2022 and NAVD 88 (and NGVD 29) at a national and local level. It was mentioned that to prepare for the new datums and develop implementation plans, users should obtain an understanding of the differences between NAPGD2022 and NAVD 88. The last column provided figures that depicted the approximate absolute and relative differences between the new vertical reference frame, North American-Pacific Geopotential Datum of 2022 (NAPGD2022) and NAVD 88. This column is the second in a new series of columns addressing topics associated with transitioning to the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

    The name of the National Geodetic Survey’s new vertical reference frame is the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). So, what is a geopotential model? The following is the definition of a geopotential model from Wikipedia: “In geophysics, a geopotential model is the theoretical analysis of measuring and calculating the effects of Earth’s gravitational field.” [See the box titled “Definition of geopotential and geopotential model from Wikipedia.”]

    Definition of geopotential and geopotential model from Wikipedia

    In order for a height to a have physical meaning, the height system must have some relation to the Earth’s gravity field. Basically, for geodesists, a geopotential model is a way of measuring the effects of Earth’s gravitational field and the means to deriving a geoid model. So, what does the Earth’s gravity field look like? The box titled “Static Gravity Field – Anomalies” is a good image of the Earth’s gravity field created by the GRACE program.

    Static Gravity Field – Anomalies
    (Figure obtained from https://grace.jpl.nasa.gov/resources/28/)

    It was mentioned in the last column that stakeholders across the federal, public and private sectors provided feedback and impacts of NGS New 2022 Datums on their products and services. All of these presentations are now available on NGS’ website. [See box titled “Website that contains the NGS 2017 Geospatial Summit Presentations.“] NGS did an excellent job of recording these presentations. The website allows the user to download the video and/or slides, as well as watch the presentations on their computer.

    Website that contains the NGS 2017 Geospatial Summit Presentations
    (https://www.ngs.noaa.gov/geospatial-summit/presentations.shtml)

    Many surveyors and mappers will be providing services to Federal, state, and local agencies to assist them in their transitioning activities. I would encourage all users to watch the presentations by the partners to obtain an understanding of how these agencies’ products and services are going to be effected by a datum change. For example, the presentation by the Federal Emergency Management Agency (FEMA) can be found here.

    This column will focus on two of the presentations by NGS employees – “Modernizing the Geopotential or Vertical Datum” and Monitoring Changes in the Geoid.” These two presentations are very important to obtaining an understanding of NAPGD2022. [See box title “NGS Presentation at the 2017 Geospatial Summit – “Modernizing the Geopotential or Vertical Datum.”]

    NGS Presentation at the 2017 Geospatial Summit – “Modernizing the Geopotential or Vertical Datum”
    (https://www.ngs.noaa.gov/geospatial-summit/presentations/modernizing-geopotential-vertical-datum.shtml)

    Why is the Earth’s gravity field important to estimating GNSS-derived orthometric heights? Guidelines and procedures for estimating GNSS-derived heights were discussed in great detail in previous columns, such as Establishing Orthometric Heights Using GNSS — Part 1, Establishing Orthometric Heights Using GNSS — Part 2, Establishing Orthometric Heights Using GNSS — Part 3 and Establishing orthometric heights using GNSS — Part 4.

    Slide 33 from the presentation titled “Modernizing the Geopotential or Vertical Datum” depicts the relationship between the ellipsoid, geoid, and orthometric heights. (See box titled “Slide 33 From “Modernizing the Geopotential or Vertical Datum.”)

    Slide 33 From “Modernizing the Geopotential or Vertical Datum”
    (https://www.ngs.noaa.gov/geospatial-summit/presentations/modernizing-geopotential-vertical-datum.shtml)

    A previous column discussed how NGS developed their scientific and hybrid geoid models. The NAPGD2022 will begin with the best 3-dimension geopotential model available and derive the most accurate geoid model, e.g., GEOID2022, for establishing NAPGD2022 GNSS-derived orthometric heights. Just like NAVD 88 leveling derived heights need accurate gravity values to compute accurate orthometric heights and height differences, the geopotential model needs accurate, current gravity data to estimate local variations in the global model. The bottom line is that an accurate geopotential model is necessary for deriving an accurate geoid model that is necessary for establishing accurate GNSS-derived orthometric heights and height differences.

    In the presentation “Modernizing the Geopotential or Vertical Datum,” Monica Youngman discussed the NGS project called “Gravity for the Redefinition of the American Vertical Datum (GRAV-D).” The goal of GRAV-D is to create a gravimetric geoid accurate to 1 cm where possible using airborne gravity data. The overall target is to enable users to obtain 2-cm accuracy orthometric heights from GNSS and a geoid model. View this website for more information on GRAV-D.

    Once a geoid model is computed, e.g., GEOID2022, it will need to be validated to estimate the accuracy of the derived product. What does this mean to surveyors and mappers? In my opinion, the NAPGD2022 will help the surveying community maintain a vertical reference frame that’s reliable and traceable. Saying that, it is extremely important to know the relative accuracy of the geoid model used to establish GNSS-derived orthometric heights in NAPGD2022. As mentioned in my April column, NGS is performing geoid slope validation surveys (GSVS) to evaluate the current experimental geoid models being developed using GRAV-D data. In the presentation “Modernizing the Geopotential or Vertical Datum,” Derek Van Westrum discussed the GSVS projects. Evaluation of the experimental gravimetric geoid model is critical to the implementation of NAPGD2022 and should be part of a transition plan to the NAPGD2022. Performing a geoid slope validation project similar to NGS may be too expensive to be performed by most agencies. However, some agencies may be able to perform low budget geoid slope evaluation surveys. These surveys could include performing combined GNSS and leveling surveys to evaluate the relative accuracy of the gravimetric geoid model in areas that require accurate orthometric heights. Performing several of the gravimetric geoid evaluation surveys in major cities and/or areas that require accurate heights would help to facilitate the implementation of NAPGD2022.

    These types of geoid evaluation surveys should be performed in areas of the country that are influenced by crustal movement. For example, in southern Louisiana and other parts of the Gulf Coast of the United States that are being influenced by subsidence (https://www.ngs.noaa.gov/heightmod/NOAANOSNGSTR50.pdf, https://www.ngs.noaa.gov/PUBS_LIB/Subsidence_at_Houston_Texas_TR_NOS131_NGS44.pdf). There is no doubt that NAPGD2022 will provide a more efficient and cost-effective way to maintain consistent and accurate orthometric heights; however, evaluating the relative accuracy of the geoid model is critical to a successful implementation of NAPGD2022.

    The first phase of the GRAV-D project is the airborne gravity survey of entire country and its holdings; the second phase is the long-term monitoring of the change in the geoid. Not only is the NAVD 88 being replaced with a new datum but the geoid model, the underlying foundation of establishing GNSS-derived orthometric heights in NAPGD2022, will be constantly changing. The geoid will change but it will change very slowly. Saying that, it is still important for NGS to monitor changes in the geoid if users are going to establish and maintain GNSS-derived orthometric heights at the centimeter level. As part of the modernization of the vertical reference frame, NGS has outlined four components of a long-term monitoring plan. [See box titled “Components of a Long-Term Monitoring Plan.”]

    Components of a Long-Term Monitoring Plan
    (From presentation titled “Monitoring Changes in the Geoid” given by Dr. Theresa Damiani at the NGS 2017 Geospatial Summit)

    1. What and Where to Monitor
    2. How to Monitor in the Near-Term (next 1 to 3 decades)
    3. Which Products Need to be Available
    4. Long-Term Program Adaptation

    The two most important components of the plan, in my opinion, are “What and Where to Monitor” and “How to Monitor in the Near-Term.” There are small changes in the geoid that occur over long periods of time. [See box titled “Slide 5 from presentation titled “Monitoring Changes in the Geoid.”]

    Slide 5 from presentation titled “Monitoring Changes in the Geoid”
    (From presentation titled “Monitoring Changes in the Geoid” given by Dr. Theresa Damiani at the NGS 2017 Geospatial Summit)

    Dr. Damiani presented a slide that outlined NGS’ vision for vertical datum products as they are related to the geoid model. [See the box titled “NGS’ Vision for Vertical Datum Products, 2022 +.”] NGS will be publishing both static geoid models (S) and dynamic geoid models (D). The “S” static model will be a typical geoid model, aimed to capture the 1 cm-accurate model at a specific epoch, and the “D” dynamic model will capture the rate of change of the geoid at all places. Dr. Damiani mentioned in her presentation that NGS has initiated a program called “The Geoid Monitoring Service.” This service is a new project, initiated in January 2017, that is planned to be operational and produce NGS’ first “D” dynamic geoid by 2022.

    NGS’ Vision for Vertical Datum Products, 2022 +
    (From presentation titled “Monitoring Changes in the Geoid” given by Dr. Theresa Damiani at the NGS 2017 Geospatial Summit)

    ➢ In 2022, NGS will release “S” and “D” geoid models: static (S) and dynamic (D).

    ➢ The “S” static will be a typical geoid model, aimed to capture the 1 cm-accurate model at a TBD epoch.

    ➢ The “D” dynamic will capture the rate of change of the geoid at all places. In 2022, it will capture at least the continuous, permanent change signals such as Glacial Isostatic Adjustment (GIA).

    ➢ Both models will be integrated into OPUS, mostly invisible to users. Orthometric heights provided by OPUS will be time-sensitive, so that they are the combination of the static geoid model plus the geoid rate of change indicated by the dynamic model.

    ➢ NGS will provide separate tools to directly access both the “S” and “D” models.

    This column discussed the basic foundation parameters of the North American-Pacific Geopotential Datum of 2022 (NAPGD2022); that is, a global geopotential model, the GRAV-D project, and the GEOID2022 geoid model. It emphasized that NAPGD2022 will provide a more efficient and cost-effective way to maintain consistent orthometric heights, but evaluating the relative accuracy of the geoid model is critical to a successful implementation of NAPGD2022. Performing GNSS/Leveling evaluation surveys will help in evaluating the relative accuracy of GEOID2022. NGS is developing geodetic routines and tools to assist users in transforming heights from NAVD 88 to NAPGD2022, and enabling the incorporation of geodetic leveling data into NAPGD2022 to establish NAPGD2022 orthometric heights. Future columns will address some of these tools and routines.

  • Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 1

    Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 1

    On April 24-25, 2017, the National Geodetic Survey (NGS) hosted the 2017 Geospatial Summit in Silver Spring, Maryland, to discuss its plans for replacing the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88) in 2022.

    The summit was a day and a half long and provided an opportunity for NGS to share updates and discuss the progress of projects related to National Spatial Reference System (NSRS) Modernization. Stakeholders across the federal, public and private sectors also provided feedback and impacts of New Datums on their products and services.

    The absolute differences between the new vertical reference frame, North American-Pacific Geopotential Datum of 2022 (NAPGD2022), and NAVD 88 are going to be large but, in most regions of the country, the relative differences over small areal extents will be small.

    NGS is developing geodetic routines and tools to transform heights from NAVD 88 to NAPGD2022, and to facilitate the incorporation of geodetic leveling data into NAPGD2022 to establish NAPGD2022 heights. To prepare for the new datums and develop implementation plans, stakeholders should obtain an understanding of the differences between NAPGD2022 and NAVD 88.

    My previous columns provided figures that demonstrated the approximate differences between NAPGD2022 and NAVD 88 heights at a national level. (See figure 1.) This column will provide feedback from stakeholders that participated in the Geospatial Summit and, using NGS’ GPS on BMs dataset, a discussion on the differences between NAPGD2022 and NAVD 88 (and NGVD 29) at a local level.

    Figure 1 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 1 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    Information about the summit and Summit Documents can be downloaded here.

    Read an excerpt from website here.

    If you check on the tab titled “Summit Documents” you can download the agenda and documents provided to participates. Read excerpts from the summit here.

    The first day consisted of presentations by NGS leadership and personnel providing updates and discussing the progress of projects related to the NSRS modernization. The presentations by NGS employees can be downloaded from NGS’ presentations library at this web link. View an excerpt from NGS’ presentations library here.

    The afternoon of day 2 were presentations by partners and stakeholders. (See box titled “Excerpt from NGS 2017 Geospatial Summit Agenda – Afternoon of Day 2.”)

    Excerpt from NGS 2017 Geospatial Summit Agenda – Afternoon of Day 2
    Day 2 Afternoon Agenda from NGS’ 2017 Geospatial Summit
    Day 2: Tuesday, April 25, 20171:30 – 3:05 Impacts of New Datums on Programs and Partners (Part 1)
    Coastal Mapping Program and VDatum: Mike Aslaksen and Stephen White, NOAA/NGS
    Federal Emergency Management Agency (FEMA): Kimberly Pettit, FEMA
    U.S. Geological Survey (USGS): Kari Craun, USGS
    U.S. Army Corps of Engineers (USACE): Jim Garster, USACE
    National Geospatial-Intelligence Agency (NGA): Stephen Malys, NGA
    3:05 – 3:25 Break
    3:25 – 4:55 Impacts of New Datums on Programs and Partners (Part 2)
    Geospatial and Remote Sensing Customers: Amar Nayegandhi, Dewberry
    Geographic Information System (GIS) Customers: Kevin Kelly, Esri
    Global Navigation Satellite System (GNSS) Equipment Customers: Hamid Mahmoudabadi, Trimble Kyle Snow, Topcon
    State Government Partners: Gary Thompson, N.C. Department of Public Safety
    Local Government Partners: Vickie Anglin, Fairfax County Government, Virginia; Patrick Simon, Baltimore County Land Survey, Maryland
    4:55 – 5:00 Wrap-up and closing

    In order for consistency, NGS provided guidance and a set of template slides for guest presenters to use. Guest presenters were allotted 10 minutes to present and limited to four slides. The presentation by the guest presenters are not on NGS’ Presentations Library but I’ve been told that they will be available on the Summit website later this year. Gary Thompson, Chief of the North Carolina Geodetic Survey (NCGS), provided me a copy of his slides and gave me permission to include them in this column. (See box titled “Power point Slides Presented by Gary Thompson, Chief of NCGS, at the NGS 2017 Geospatial Summit.”) North Carolina has been very proactive in addressing the impacts of the new datums on NC products and services. North Carolina Geodetic Survey has established a North Carolina Geodetic Survey Advisory Committee that reviews NCGS products and services, and they have established the North Carolina 2022 Reference Frame Working Group to prepare for the new datums.

    Slide: National Geodetic Survey
    Slide: National Geodetic Survey

    Powerpoint slides presented by Gary Thompson, chief of NCGS, at the NGS 2017 Geospatial Summit

    All of the presentations by the invited guest speakers were interesting, and everyone followed NGS’ guidance which helped to focus the Summit on the main issues associated with a datum change. As expected, each stakeholder had their own set of issues and concerns about transitioning to a datum. The following are some common themes that I heard from the participants:

    (1) There are a lot of products and services that will be effected by a datum change,
    (2) An official transformation model between the old and new datum(s) published by NGS is critical for a successful transition to a new datum,
    (3) Guidance documents that are “easily” understood by “non-geodesists” is required for a smooth implementation of a new datum, and
    (4) More frequent geospatial summits and webinars are needed to provide updates on the status of the projects associated with NSRS modernization and to ensure user involvement in the process.

    I contacted a couple of the guest presenters to discuss their feedback on the New Datums. As NAVD 88 Program Manager, I collaborated with many of them during the development and implementation of the NAVD 88. As in the transition from NGVD 29 to NAVD 88, it’s not the conversion of coordinates that’s a problem; a good transformation tool should meet that requirement. Saying that, it was stated that many users rely on commercial and open source software to convert their data, so they would like NGS to collaborate with others to ensure that these software suppliers are using the appropriate algorithms/information in their products. The integration with legacy data referenced to older datums may be complicated for some products and services; therefore, the process of transforming each product and service will need to be addressed individually. If all data are in digital form with the appropriate metadata, then the transformation should be relatively easy to accomplish and maps with new contour lines or new base flood elevations referenced to the new datum could be generated. However, how these new maps are integrated with old maps is a different issue. I will address some of these potential issues in future columns.

    To prepare implementation plans, users must obtain a working knowledge of the differences between the old and new datums. As previous mentioned, the absolute differences between the new vertical reference frame, NAPGD2022, and NAVD 88 are going to be large but, in most regions of the country, the relative differences over small areal extents will be small. To evaluate the relative differences at the local level, the differences between NAPGD2022 and NAVD 88 (and NGVD 29) were computed for bench marks in the NGS’ GPS on BMs dataset. The NAD 83 (2011) latitude, longitude, and ellipsoid height of each station was transformed to the IGS08 reference frame using NGS’ HTDP web tool, and then the GNSS-derived orthometric height was computed using the following formula:

    Approximate NAPGD2022 GNSS-Derived Orthometric Height
    Equals
    IGS08 Ellipsoid Height minus xGeoid16b Geoid Height (referenced to IGS08).

    Figure 1 is a plot of the difference between the approximate NAPGD2022 height and the published NAVD 88 height for bench marks that are part of the GPS on BMs dataset and have the published attribute of “Adjusted.” It should be noted that these are only estimated changes because the final NAPGD2022 reference frame will not be exactly the same as the current IGS08 reference frame, but these estimates should serve the purpose of providing approximate changes for users to develop transition plans.

    Since some users are still converting NGVD 29 heights to NAVD 88 heights, the approximate change between NAPGD2022 and NGVD 29 is provided in figure 2. VERTCON values were used to convert the NAVD 88 published heights to NGVD 29 heights, and then the difference between the approximate NAPGD2022 orthometric height and the NGVD 29 orthometric height was computed.

    Figure 2 – Approximate Change Between NAPGD2022 and NGVD 29 Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 2 – Approximate Change Between NAPGD2022 and NGVD 29 Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    As shown in figure 2, the absolute differences between the new vertical reference frame, NAPGD2022, and NGVD 29 are also going to be large but, once again, in most regions of the country, the relative differences over small areal extents will be small.

    What does this look like in a local area? Figure 3 is a plot of the approximate change between NAPGD2022 and NAVD 88 in North Carolina and surrounding states, and figure 4 is plot of the approximate change between NAPGD2022 and NGVD 29 in North Carolina and surrounding states.

    Figure 3 – Approximate Change Between NAPGD2022 and NAVD 88 in North Carolina and Surrounding States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 3 – Approximate Change Between NAPGD2022 and NAVD 88 in North Carolina and Surrounding States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 4 – Approximate Change Between NAPGD2022 and NGVD 29 in North Carolina and Surrounding States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 4 – Approximate Change Between NAPGD2022 and NGVD 29 in North Carolina and Surrounding States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    Figure 5 provides a more detailed depiction of the change between NAPGD2022 and NAVD 88 along the North Carolina Atlantic Coast. The differences appear to vary by several centimeters but some of these differences are due to errors in published heights (both ellipsoid and orthometric). These differences can be used to develop a transformation model but the user will need to know the accuracy of the model, globally and locally.

    Figure 5 – Approximate Change Between NAPGD2022 and NAVD 88 along North Carolina Atlantic Coast Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 5 – Approximate Change Between NAPGD2022 and NAVD 88 along North Carolina Atlantic Coast Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    Figure 6 is a detailed depiction of the change between NAPGD2022 and NGVD 29 in the same area as shown in figure 5. Comparing figures 5 and 6, the reader should notice that the differences between NAPGD2022 and NGVD 29 are about 30 cm larger (more negative) than the differences between NAPGD2022 and NAVD 88.

    Figure 6 – Approximate Change Between NAPGD2022 and NAVD 29 along North Carolina Atlantic Coast Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 6 – Approximate Change Between NAPGD2022 and NAVD 29 along North Carolina Atlantic Coast Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    Figure 7 is the difference between NAPGD2022 and NAVD 88 in western North Carolina. The local difference in the NC mountains is around -35 cm which is about 10 cm different from the NC Atlantic Coast. Questions that users need to address include: What is the accuracy of the transformation model? And What is the accuracy of the product or service being transformed? The transformation model will not replace the original survey results but may be useful for transforming some products and services.

    Figure 7 – Approximate Change Between NAPGD2022 and NAVD 88 in the Western North Carolina Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 7 – Approximate Change Between NAPGD2022 and NAVD 88 in the Western North Carolina Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    Table 1 provides the average difference between NAPGD2022 and NAVD 88 (and NGVD 29) by State using the GPS on BMs dataset. This table shows that there are large differences between NAPGD2022 and both NGVD 29 and NAVD 88. No matter which datum the product or service is referenced to, it will probably need to be transformed to NAPGD2022.

    Table 1 – Average Difference Between NAPGD2022 and NAVD 88 (and NGVD 29) by State Using GPS on BMs Dataset (units = cm). Click to enlarge. (Date: National Geodetic Survey)
    Table 1 – Average Difference Between NAPGD2022 and NAVD 88 (and NGVD 29) by State Using GPS on BMs Dataset (units = cm). Click to enlarge. (Date: National Geodetic Survey)
    Average Difference Between NAPGD2022 and NAVD 88 by State Using GPS on BMs Dataset (units = cm). Click to enlarge. (Date: National Geodetic Survey)
    Average Difference Between NAPGD2022 and NAVD 88 by State Using GPS on BMs Dataset (units = cm). Click to enlarge. (Date: National Geodetic Survey)

    Table 2 provides the standard deviation of the average difference between NAPGD2022 and NAVD 88 by State. For example, North Carolina has a sample size of 1600 stations and its average difference is -28 cm with a standard deviation of 4.8 cm. Looking at figures 5 and 7, there appears to be a difference of 10 cm across the State. The States in the northwestern region of the United States have a larger difference between NAPGD2022 and NAVD 88 as well as a larger standard deviation. Oregon has a sample size of 195 stations and its average difference is -100.7 cm with a standard deviation of 13.0 cm, and Washington has a sample size of 266 stations and its average difference is -108.8 cm with a standard deviation of 9.0 cm. Figure 8 is a plot of the approximate change between NAPGD2022 and NAVD 88 in the northwest region of the United States.

    As mentioned previously, these differences will vary from station to station because of a bias and trend between the two datums and due to remaining errors in published heights (both ellipsoid and orthometric). As I have noted in previous columns, many of the large relative differences between stations in a local area could be due to an invalid NAVD 88 published height because the bench mark moved since the last time the height of the bench mark was adjusted and published, and/or an undetected error in an ellipsoid height due to a weak GNSS project design. Either way, in my opinion, most of these stations with large relative differences don’t accurately represent the current NAVD 88. NGS’ modernization of the NSRS will provide a more accurate and consistent reference frame, and improve the user’s ability to obtain a current and accurate orthometric height.

    Figure 8 – Approximate Change Between NAPGD2022 and NAVD 88 in the Northwest Region of the United States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)
    Figure 8 – Approximate Change Between NAPGD2022 and NAVD 88 in the Northwest Region of the United States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

    This column highlighted some of the feedback provided by guest presenters at the NGS’ 2017 Geospatial Summit held on April 24-25, 2017, in Silver Spring, Maryland. The column also provided a discussion on the approximate differences between NAPGD2022 and NAVD 88 (and NGVD 29) at a national and local level. To prepare for the new datums and develop implementation plans, users should obtain an understanding of the differences between NAPGD2022 and NAVD 88. This column is the first in a new series of columns addressing topics associated with transitioning to the new North American -Pacific Geopotential Datum of 2022 (NAPGD2022).