GPS World

Tag: geoid

  • Plate tectonics and NGS’s new NSRS terrestrial reference frames

    Plate tectonics and NGS’s new NSRS terrestrial reference frames

    The adoption of the new, modernized National Spatial Reference System (NSRS) is rapidly approaching, with official implementation now expected in the first quarter of 2027.

    One of the most common questions I receive during presentations is: How will the National Geodetic Survey (NGS) account for plate tectonics in the modernized NSRS, and what does that mean for my geospatial products and services?

    First, I have some very sad news to share.

    Dr. Chris Pearson
    Dr. Chris Pearson

    Our friend and colleague, Dr. Chris Pearson, unexpectedly passed away while in Cape Town attending the May 2026 International Federation of Surveyors (FIG) conference. At the time, he was serving as a Geodetic Advisor for Trimble and as co-chair of FIG Commission 5.2.

    Chris previously worked for the National Geodetic Survey (NGS) as a Geodetic Advisor, where he played a key role in developing the comprehensive block model of crustal deformation — widely known as HTDP — across the western United States, including Alaska.

    He was an active and respected member of several professional organizations and will be greatly missed by the entire geodetic and surveying community.

    Now, let’s discuss how the National Geodetic Survey (NGS) will handle plate tectonics in the modernized National Spatial Reference System (NSRS) and what this will mean for users’ geospatial products and services.

    Map of tectonic plates (Image: Dave Zilkoski)

    Plate tectonics is the scientific theory that describes how Earth’s outer shell, known as the lithosphere, is divided into large, rigid pieces called tectonic plates. These plates float atop the hotter, more ductile rock in the mantle below and move very slowly — roughly at the same rate as your fingernails grow, about 1 to 10 centimeters per year.

    So why does plate tectonics matter for geodetic coordinates? Because the most significant geological activity — including earthquakes, volcanic eruptions, and crustal deformation — occurs primarily at the boundaries where these plates interact.

    My last newsletter highlighted several activities by the North Carolina 2022 Reference Frame Working Group (NC RFWG) that are addressing this issue and other challenges related to the implementation of the new NSRS.

    During my presentations on the modernized NSRS, I always show the National Geodetic Survey (NGS) maps that illustrate the approximate horizontal and vertical changes expected when the new Terrestrial Reference Frames (TRFs) are adopted, with coordinates referenced to epoch 2020.00. These maps provide a high-level (“30,000-foot”) overview of the anticipated changes. However, they do not include the level of detail that many users are looking for.

    Participants at these seminars and meetings consistently want to know the expected coordinate differences for their specific state or local region, and how the time-dependent components will impact their work.

    Most geospatial users now understand that International Terrestrial Reference Frame (ITRF) coordinates include a velocity component caused by tectonic plate movement. To manage these changing coordinates, the National Geodetic Survey (NGS) plans to incorporate time-dependent modeling. NGS has developed two key models — EPP2022 and IFDM2022 — to make time-dependent geodetic control practical and usable.

    • EPP2022 (Euler Pole Parameters) describes the rigid rotation of tectonic plates.
    • IFDM2022 (Intra-Frame Deformation Model) computes the internal deformation and drift within a tectonic plate.

    As shown in the figure below, the NOAA CORS Network station COLA in Columbia, South Carolina — located on the North American Plate — is moving at approximately 0.05 feet (14 mm) per year.

    This velocity is provided on the published ITRF2020 position and velocity data for the station  (NGS CORS Position and Velocity Sheet for COLA).  As a result, a surveyor working in June 2026 would observe a shift of about 0.3 feet in the ITRF2020 horizontal coordinates compared to the 2020.00 reference epoch, solely due to tectonic plate motion.

    Motion due to plate movement (rates per year) – based on ITRF2020 velocity rates

    Image: Dave Zilkoski
    (Image: Dave Zilkoski)

    The National Geodetic Survey (NGS) provides detailed information for all NOAA CORS Network (NCN) stations on the NGS NCN Station Pages. 

    In the section titled “Coordinates and Velocities”, simply click the Position and Velocity button to view the station’s ITRF2020 coordinates and velocities (referenced to epoch 2020.00), as well as the NAD 83 (2011) coordinates and velocities (referenced to epoch 2010.00).

    NGS CORS position and velocity sheet for COLA

    NGS CORS position and velocity sheet for COLA

    So, what does this mean for users?

    As previously mentioned, the National Geodetic Survey (NGS) is expected to adopt the new modernized NSRS in the first quarter of 2027. The figure below shows the change in ITRF2020 coordinate values between epoch 2020.00 and 2027.00 for NOAA CORS Network (NCN) stations in South Carolina. This shift of approximately 0.33 feet (10 cm) is the result of seven years of tectonic plate motion.

    ITRF2020, Epoch 2020 to ITRF2020, Epoch 2027 (units ift)

    ITRF2020, Epoch 2020 to ITRF2020, Epoch 2027 (units ift) Image: Dave Zilkoski
    Image: Dave Zilkoski

    That said, what will the change in NATRF2022 coordinate values be between epoch 2020.00 and 2027.00?

    This is where NGS’s EPP2022 and IFDM2022 models become essential. My February 2022 and July 2024 GPS World newsletters discussed the Euler Pole Parameters (EPP) process in detail.

    The Beta NATRF2022 website provides the Euler Pole Parameters (EPP) needed to define the relationship between ITRF2020 and the new NATRF2022 frames for the North American, Caribbean, Pacific, and Mariana plates, as outlined in NGS’s Blueprint Part 1 document. The values in the table have proven especially useful to programmers developing and testing their software.

    Beta Values for EPP

    Beta Values for EPP (Image: NGS)
    (Image: NGS)

    As stated in Blueprint Part 1, the National Geodetic Survey (NGS) will define the official relationship between ITRF2020 and the four NSRS Terrestrial Reference Frames (TRFs) through Equation 59. This equation uses the rotation matrix provided in Equation 58, which results in Equation 60.

    See the box titled “Official Relationship Between ITRF2020 and the Four NSRS TRFs” for the equations.

    Official relationship between ITRF2020 and the four NSRS TRFs

    Official relationship between ITRF2020 and the four NSRS TRFs (Image: NGS Blueprint pt. 1)
    (Image: NGS Blueprint pt. 1)

    So, what does this mean for surveyors?

    The primary purpose of the EPP2022 model is to remove the rigid tectonic plate motion from the coordinates. After applying the EPP2022 model to the ITRF2020 coordinates at epoch 2027.00, the resulting NATRF2022 horizontal coordinates for station COLA (epoch 2027.00) will change by only 0.04 feet (12 mm).

    EPP applied

    NATRF2022, Epoch 2020 to NATRF2022, Epoch 2027 in SC (units ift)

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    As shown in the figure, the EPP2022 model removes most of the horizontal movement caused by seven years of tectonic plate motion — reducing it to just 0.04 feet (1.2 cm) at station COLA. In other words, the EPP model effectively removes the vast majority of plate tectonic effects.

    Additionally, the plot shows that the relative horizontal differences between nearby marks are very small — typically less than 0.01 feet (0.3 cm).

    As previously mentioned, the NGS maps provide a high-level (“30,000-foot”) view of the expected changes between the current NSRS and the new modernized NSRS. So, what are the anticipated differences between NAD 83 (2011) and NATRF2022 specifically in South Carolina?

    The figures below illustrate the differences in both horizontal position and ellipsoid heights between NAD 83 (2011) and NATRF2022 coordinates across South Carolina.

    NAD83 (2011), Epoch 2010 to NATRF2022, Epoch 2020 Horizontal Changes in SC (Units ift)

    NAD83 (2011), Epoch 2010 to NATRF2022, Epoch 2020 Ellipsoid Height Changes in SC (Units ift)

    The magnitude of these changes varies depending on your location. To illustrate this, I’ve provided two additional examples: one for Iowa and one for Washington State. As the plots clearly show, the differences in these states are noticeably different from those depicted for South Carolina.

    NAD83 (2011), Epoch 2010 to NATRF2022, Epoch 2020 Horizontal Changes (Units ift)

    That said, the differences between NATRF2022 at epoch 2020.00 and epoch 2027.00 in Iowa and Washington State — after applying the EPP2022 model — are very similar to the values shown for South Carolina.

    However, readers should note that the differences in Washington State increase as you move toward the coast. This is because the area lies near the boundary between the North American Plate and the Pacific Plate. The Juan de Fuca Plate, a small microplate in the eastern North Pacific, is also actively involved in this region.

    (See the box titled “Juan de Fuca Plate.”)

    NATRF2022, Epoch 2020 to NATRF2022, Epoch 2027 (units ift)EPP Applied

    Juan de Fuca Plate

    The Juan de Fuca plate or Juan de Fuca microplate is a small oceanic tectonic plate (microplate) generated from the Juan de Fuca Ridge that is subducting beneath the northerly portion of the western side of the North American plate at the Cascadia subduction zone.

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    What about orthometric height changes in the new NSRS?

    As an example, the orthometric height differences between NAPGD 2022 and NAVD 88 in South Carolina are expected to range from approximately -0.8 feet to -1.3 feet.

    Difference between NAPGD2022 and NAVD 88 (Units ift) in S.C.

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    The differences between NAPGD 2022 and NAVD 88 vary significantly depending on your location. The figures below illustrate these orthometric height differences for Iowa and Washington State as examples.

    Difference between NAPGD2022 and NAVD 88 (Units ift)

    The new NSRS will use a gravimetric geoid (GEOID2022) rather than a hybrid geoid (GEOID18) to compute GNSS-derived orthometric heights.

    During my presentations, I always remind participants that a hybrid geoid is not a “true” geoid. It is simply a transformation model that converts ellipsoid heights in one reference frame to orthometric heights in a specific vertical datum. Specifically, GEOID18 is a transformation tool that allows users to derive NAVD 88 orthometric heights from NAD 83 (2011), epoch 2010 ellipsoid heights.

    The figure below shows the differences between the gravimetric geoid model GEOID2022 and the hybrid geoid model GEOID18.

    Important note: Users cannot use GEOID18 with NATRF2022 ellipsoid heights to obtain NAVD 88 orthometric heights. Instead, GEOID2022 must be used with NATRF2022 ellipsoid heights to compute orthometric heights in the new vertical datum, NAPGD 2022.

    Differences between GEOID2022 and GEOID18 in SC (Units ift)

    As noted at the outset of this newsletter, the transition to the modernized National Spatial Reference System (NSRS) is rapidly approaching, with official implementation scheduled for the first quarter of 2027.

    The National Geodetic Survey (NGS) released the following announcement on May 28, 2026:

    Public Testing Period Ends for Key NSRS Modernization Products

    NGS has declared the following products stable and ready for implementation planning and integration activities ahead of the official release:

    • North American-Pacific Geopotential Datum of 2022 (NAPGD2022)
    • New Terrestrial Reference Frames of 2022:
      • North America (NATRF2022)
      • Pacific (PATRF2022)
      • Caribbean (CATRF2022)
      • Mariana (MATRF2022)
    • State Plane Coordinate System of 2022 (SPCS2022)

    Additional modernization products, including NCAT, OPUS, and the Data Delivery System, are scheduled for release later in 2026.

    NGS news

    Public testing period ends on specific NSRS modernization products

    Image: NOAA

    Image: NOAA

    This newsletter highlighted the role of the EPP2022 model in accounting for plate tectonics and illustrated the anticipated local differences between the current National Spatial Reference System (NSRS) and the upcoming modernized version.

    Future editions will continue to explore additional NGS Beta products as they are released later in 2026.

  • 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.