# Confirming regional 1 cm differential geoid accuracy from airborne gravimetry: the Geoid Slope Validation Survey of 2011

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## Abstract

A terrestrial survey, called the Geoid Slope Validation Survey of 2011 (GSVS11), encompassing leveling, GPS, astrogeodetic deflections of the vertical (DOV) and surface gravity was performed in the United States. The general purpose of that survey was to evaluate the current accuracy of gravimetric geoid models, and also to determine the impact of introducing new airborne gravity data from the ‘Gravity for the Redefinition of the American Vertical Datum’ (GRAV-D) project. More specifically, the GSVS11 survey was performed to determine whether or not the GRAV-D airborne gravimetry, flown at 11 km altitude, can reduce differential geoid error to below 1 cm in a low, flat gravimetrically uncomplicated region. GSVS11 comprises a 325 km traverse from Austin to Rockport in Southern Texas, and includes 218 GPS stations (\(\sigma _{\Delta h }= 0.4\) cm over any distance from 0.4 to 325 km) co-located with first-order spirit leveled orthometric heights (\(\sigma _{\Delta H }= 1.3\) cm end-to-end), including new surface gravimetry, and 216 astronomically determined vertical deflections \((\sigma _{\mathrm{DOV}}= 0.1^{\prime \prime })\). The terrestrial survey data were compared in various ways to specific geoid models, including analysis of RMS residuals between all pairs of points on the line, direct comparison of DOVs to geoid slopes, and a harmonic analysis of the differences between the terrestrial data and various geoid models. These comparisons of the terrestrial survey data with specific geoid models showed conclusively that, in this type of region (low, flat) the geoid models computed using existing terrestrial gravity, combined with digital elevation models (DEMs) and GRACE and GOCE data, differential geoid accuracy of 1 to 3 cm (1 \(\sigma )\) over distances from 0.4 to 325 km were currently being achieved. However, the addition of a contemporaneous airborne gravity data set, flown at 11 km altitude, brought the estimated differential geoid accuracy down to 1 cm over nearly all distances from 0.4 to 325 km.

## Keywords

Geoid GRAV-D GSVS11 NAVD 88 Geoid accuracy Airborne gravity Geodetic surveying Gravimetry Deflections of the vertical Astro-geodesy## Notes

### Acknowledgments

Above all others, the authors wish to thank the 96 separate individuals from within the National Geodetic Survey who participated to some extent in this survey, including 46 who actually performed the field work. The authors also wish to thank the Texas governor’s office, the Texas State Troopers, the Texas Department of Transportation, the Texas Maritime Museum, the Texas General Lands Office, the Texas Natural Resources Information System, the University of Texas, Bowie High School and all other representatives of the Lone Star State for welcoming us, and allowing us to work safely and unhindered on the grounds of the capital building, along the highways of Texas as well inside the Stephen F. Austin Building, the Texas Maritime Museum, Pickle Research Campus and Bowie High School.

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