Three Geoid Slope Validation Survey (GSVS) were planned by the National Geodetic Survey (NGS) for validating geoid improvement gained by incorporating airborne gravity data collected by the "Gravity for the Redefinition of the American Vertical Datum" (GRAV-D) project in a flat, a medium and a rough topography area. The flat area one was conducted in Texas and it confirmed that the 1-cm differential accuracy over baseline lengths between 0.4-320 km is achievable by combing the GRAV-D data (Smith et al. 2013). The second survey, GSVS14, was carried out in Iowa in 2014 where the topography is moderate but the gravity has strong variation. Two sets of geoidal heights were computed from GPS/leveling data and observed astrogeodetic deflections of the vertical (DoV) at 204 GSVS14 official marks. They agree with each other in ±1.2 cm which attests high quality of the GSVS14 data. Three geoid models were computed by combing the satellite gravity model GOCO03/5S with terrestrial and GRAV-D gravity. One additional model, called xGEOID15A had no airborne gravity data, and served as the benchmark to quantify the contribution of GRAV-D to the geoid improvement.The comparisons showed that each model agrees with the GPS/leveling geoid height by 1.5 cm in mark-by-mark comparisons. In differential comparisons, all geoid models have a predicated accuracy of 1-2 cm at baseline length between 1.6 km to 247 km. The contribution of GRAV-D is not apparent due to a 9-cm slope in the western 50 km section of the traverse for all gravimetric geoid models, and it was determined that the slopes have been caused by a 5 mGal bias in the terrestrial gravity data. If that western 50 km section of the testing line is excluded in the comparisons, then improvement by GRAV-D becomes evident: In that case, 1-cm differential accuracy on baselines of any length is achieved by the GRAV-D 1 enhanced geoid models, a clear improvement over the geoid models without GRAV-D data. GSVS14 confirmed that the geoid differential accuracies are in 1-2 cm range at various baseline length. The accuracy is improved to 1 cm with GRAV-D gravity when the west 50 km line is not included.With the high accuracy and the traverses over 300 hundred kilometers, the GSVS data are not only useful for validating the local and regional geoid accuracy, but also useful for other high accuracy applications, e.g., GPS height accuracy estimation, development and validation of ultra-precise clocks used in the chronometric leveling.
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