While subsidence, and its inverse, called uplift, do not affect NAVD 88 everywhere throughout the country, they do have an impact in coastal areas like the Gulf of Mexico, Chesapeake Bay, and some parts of California. Accepting the known errors in these benchmarks could be more damaging than not having any geodetic data at all.

The solution to NAVD 88 will be the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022). This new datum will consist of various interrelated, time-dependent, IGS-based models, including the foundational component of NAPGD2022, the global model of Earth’s geopotential field (GM2022).

The GM2022 will be created for the entire Earth and will contain two components. The first, the Static Geopotential Model 2022 (SGM2022), will be time independent and fixed at some epoch to at least some degree. The second, the Dynamic Geopotential Model 2022 (DGM2022), will be a time-dependent model of Earth’s external gravitational potential, which is capable of capturing both secular and episodic changes of significance.

There are three derivative products based on GM2022 that will be created, including a regional gridded geoid undulation model (GEOID2022), a regional gridded deflection of the vertical model (DEFLEC2022), and a regional gridded surface gravity model (GRAV2022).

The datum’s foundational element – GM2022 – will be a spherical harmonic model of Earth’s external gravitational potential field. The other three models will all be gridded models built from GM202 and will require higher resolution regional information. Accordingly, GEOID2022, DEFLEC2022, and GRAV2022 will cover the geographic regions of greater North America, Guam and the Commonwealth of Northern Mariana Islands, and American Samoa.

Image excerpted from the NOAA Technical Report NOS NGS 64 - Blueprint for 2022, Part 2: Geospatial Coordinates

The North American region for GEOID2022, DEFLEC2022, and GRAV2022

Orthometric height will be user determined by GNSS observations, combined with GEOID2022, via an equation: orthometric height = ellipsoid height (GNSS) – geoid height (GEOID2022). Therefore, it will define zero orthometric height by specifying the geopotential surface best fit to global mean sea level.

According to the Stone and Caccamise II poster, the development of NAPGD2022 will rely on gravity datasets including satellite, terrestrial, and airborne data – the latter being collected during the ongoing NGS airborne gravity data collection program called GRAV-D.

GRAV-D Model

In 2014, the NGS started publishing a series of experimental geoid models annually, called xGEOID. They contained gravity data from the latest satellite gravity models, the terrestrial gravity, and the airborne gravity from the Gravity for the Redefinition of the American Vertical Datum, or GRAV-D, project.

The GRAV-D project is an important aspect of the changing datums, made up of two components. The first, collecting airborne gravity above the United States to capture a snapshot of gravity across the country in order to build an accurate geoid model at a specific epoch. The second, it monitors the changes to the geoid over time using satellite and terrestrial measurements, as well as geophysical modeling.

The xGEOID models are categorized into two types: type A, without aerogravity, and type B, with aerogravity. These models provide a preliminary, but rather accurate view of the changes expected from the 2022 release of the new geopotential reference frames. They also provide a uniting view of what the final geoid model will look like before replacing NAVD 88. NGS has recently transitioned the GEOID18 model into production. The hybrid geoid model uses additional GPS on bench mark data, pinning the model to the surface of NAVD 88. It also boasts improvements to the underlying gravimetric geoid model, including better elevation data, new gravity data from satellite gravity missions, improved geoid modeling techniques, and more.

Hybrid geoid models such as this are used to convert NAD 83 ellipsoid heights from GPS observations to orthometric heights. Therefore, the GRAV-D project has an overall goal of a 2 centimeter orthometric height, which will include 1 centimeter of uncertainty from GNSS and 1 centimeter of uncertainty from GEOID18.

The NGS has sponsored annual crowd-sourced data collection campaigns called GPS on Bench Marks (GPSonBM) in order to help improve the accuracy and geographic coverage of GEOID18. Working with the NSPS, NGS has promoted participation and awareness around National Surveyors Week each March. In 2018, over 600 individuals and agencies submitted nearly 3,800 four-hour GNSS observations on about 2,500 bench marks. This additional help improved the model by closing gaps and aiding in solving conflicts with older data. The NGS intends for GEOID18 to be the last hybrid geoid model created before the current vertical datums are replaced by NAPGD2022.

Get Prepared

Moving forward, it will be important for professional land surveyors and geospatial professionals to be attuned to the changes of NAVD 88 and NAD 83, as the newly updated datums will affect their operations. There will be new terminologies, new coordinate names, and new ways and time periods to process data.

The most important aspect of the change is that the new geometric datum will change latitude, longitude, and ellipsoid height between 1 and 2 meters. And, the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022) will change heights, on average, around 50 centimeters, with a 1 meter tilt towards the Pacific Northwest. Locations will potentially move by approximately 4 to 6 feet, and elevation will change by up to 3 feet.

Another important aspect of the NSRS changes is time-dependency. According to the NOAA Technical Report NOS NGS 67 called, “Blueprint for 2022, Part 3: Working in the Modernized NSRS,” beginning in 2022, points in the NSRS with defined coordinates will have epochs associated with them that are based on the time actual data were collected at those points. This will be called the “survey epoch.” The coordinates will be known as “final discrete” coordinates, if associated with finite timespans of data collection, or “final running” coordinates, if they’re associated with continuous data collection. Passive control will have less reliability than active control, and the NGS will treat the NOAA CORS Network as having the definitive, most up-to-date coordinates within the NSRS. Therefore, both leveling and classic surveys will require GNSS components to ensure coordinates are correct and connected.

In order to bridge users into the new 2022 datums and a more time-dependent NSRS, the NGS will be estimating and providing to the public coordinates on points at five-year reference epochs. NOAA will be providing users with a number of tools, under the name OPUS, or Online Positioning User Service. Originally built as a GPS processing tool, OPUS will be used for uploading, processing, analyzing, and submitting survey data of all types. This includes GNSS, real-time kinematic (RTK), leveling, gravity and more. Users will be able to process their data in their own way. The tool will be browser-based and will fully integrate all data types. Professional land surveyors and geospatial professionals that process their data in OPUS will always receive preliminary coordinates from OPUS. Blueprint 3 mentions that, whereas it is possible that preliminary coordinates perfectly match the reference epoch or final discrete coordinates on a point, the NGS will only use those data turned into preliminary coordinates to make reference epoch or final discrete coordinates after they’ve have taken a few steps. Those steps include quality-controlling the data, and merging the data with other data from other submitted projects.

When these changes go into effect, it will be crucial for professionals to transform their data. The NGS will offer tools to transform coordinates from the older datums to the coordinates in the modernized NSRS on their website. It is also important to record metadata. Knowing the datums and epochs for geospatial files will simplify any transformations, so moving forward, geospatial professionals will have to require complete metadata in all surveying and mapping contracts. Next, users will have to perform GPS on any and all bench mark operations. Obtaining accurate NAD 83 ellipsoid heights on NAVD 88 benchmarks will improve the transformation tool for the new vertical datum, NAPGD2022.

Another very important consideration will be the changing of the State Plane Coordinate System. Conceived in 1930, the State Plane Coordinate System (SPCS) is a system of conformal map projections created by the NGS to support surveying, engineering, and mapping activities throughout the United States. The SPCS provided a way to perform geodetic surveys using plane trigonometry, making it the earliest means to access the NSRS. Due to the progression of technology since then, use of the SPCS has grown as a result of adoption of technologies like CADD, geographic information systems (GIS), and GNSS.