Research Geoids — USGG and xGEOID

NOAA’s National Geodetic Survey developed experimental geoids to demonstrate the geoid improvements as an intermediate product or as a prototype prior to publishing an official geoid product. Early research geoids (1990s and 2000s) were based only on gravity observations and shared with the public every 3-6 years. With the addition of GRAV-D data and by refining the geoid computation methods, from 2014 to 2020 NOAA’s National Geodetic Survey shared annual experimental geoid (xGEOID) models in order to demonstrate the contribution of airborne gravity from the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project. Note that experimental geoids are preliminary and are not considered an official product.

Early research geoid models (USGGxxx and GxxSSS)

The early geoid models developed at the National Geodetic Survey (NGS) were based on the National Geospatial-Intelligence Agency’s (NGA) Earth Gravitational Models (EGM). A key step for generating a domestic geoid model was the need to change the reference ellipsoid. The EGM geoid used a WGS-84 reference ellipsoid, whereas NOAA’s NGS geoids require reference to the Geodetic Reference System 1980 (GRS 80) normal ellipsoid in an International Terrestrial Reference System frame (ITRF).

Research Gravimetric Geoids

	USGG2012	USGG2009	USGG2003	G99SSS	G96SSS
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Experimental GEOID Modeling (xGEOID)

In preparation for the North American-Pacific Geopotential Datum of 2022 (NAPGD2022), NGS began publishing a series of experimental geoid (xGEOID) models annually, starting in 2014 and ending in 2020. The xGEOID models contain gravity data from the latest satellite gravity models, terrestrial gravity, and most importantly, airborne gravity from the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project. In contrast to experimental geoid models that were developed to support hybrid geoids that also included GPS observations on leveled bench marks, the xGEOID used only gravity observations. In fact the last hybrid geoid model that NGS created was GEOID18.

The purpose of the xGEOID models is to demonstrate the improvement to the models and the extent of GRAV-D coverage prior to the official release of GEOID2022. Another reason for the annual release of the xGEOIDs was to provide to the stakeholders a perpetually improving and converging view of what the final geoid model will look like as we approach its use in replacing NAVD 88.

The geoid models are categorized into two types: type A without airborne gravity and type B with airborne gravity. The differences between the two are the contribution of the airborne gravity, which are highlighted in many of the “difference” maps in the image galleries below. xGEOID models with type B geoid model coverage are only available for North America. The three key layers associated with xGEOIDs are the xGEOID geoid undulation and the deflection of the vertical components xDEFLEC xi and xDEFLEC eta.

The main xGEOID layer is the geoid undulation that provides a height relationship between the reference geopotential surface with respect to the International Terrestrial Reference Frame (ITRF). xGEOID19 and xGEOID20 contain a geoid velocity layer to capture time-dependent changes in Earth’s gravity field. xDEFLEC is a companion product derived from the xGEOID geoid model, and represents the deflections of the vertical (DoV) at the surface of the Earth. The xDEFLEC includes a north-south (meridian component, designated as "xi") and east-west components (prime vertical component, designated as "eta"). The values of the DoV are typically a few arcseconds, but in certain locations they can reach an arcminute (60 arcseconds) of departure. On the CONUS-centered grid, xi ranges from +41.6 arcseconds (Bahamas) to -39.1 arcseconds (Bermuda), while eta ranges from +46.3 arcseconds (Wyoming) to -44.3 arcseconds (Bermuda). Currently, xDEFLEC products are available for the Continental U.S., Hawaii, American Samoa and Guam and the Northern Mariana Islands.

After the release of the official GEOID22, additional layers were provided, including uncertainty and the source input layers (e.g., a global digital elevation model and a spherical harmonic coefficient global geopotential model .

Overall, seven xGEOID models were published by NGS from 2014 to 2020:

xGEOID

	xGEOID20	xGEOID19	xGEOID18	xGEOID17	xGEOID16	xGEOID15	xGEOID14
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Dynamic

xDEFLEC

	xDEFLEC20	xDEFLEC19	xDEFLEC18
Download	GGXF	GGXF	GGXF
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CONUSPAC