The Geoid Slope Validation Survey 2014 and GRAV-D airborne gravity enhanced geoid comparison results in Iowa

Abstract

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.

A high-precision digital astrogeodetic traverse in an area of steep geoid gradients close to the coast of Perth, Western Australia

“…The RMS values of the differences never exceed ±1" for any of the three models (Table 1), which is commensurate with investigations using astrogeodetic VDs in Northern Hemisphere countries (e.g., Hirt et al 2010b;Smith et al 2013;Wang et al 2017). As such, our study corroborates the ability of global or regional models to predict VDs at the 1" precision level or better over topographically flat areas.…”

“…The observations were carried out in Perth with contemporary astrogeodetic QDaedalus instrumentation at the ~±0.2" accuracy level. The comparison between the newly measured and modelled VDs showed an RMS agreement between ~±0.5" and ~±0.9" for and better than ~±0.3" for , which is commensurate with results from Northern Hemisphere countries (e.g., Wang et al 2017;Hirt et al 2010a).…”

“…With the advent of the digital zenith camera, astrogeodetic VDs have been observed and used to compute quasi/geoid profiles (e.g., Voigt et al 2009;Voigt 2013;Smith et al 2013;Guillaume 2015;Wang et al 2017), but only for Northern Hemisphere countries. In the aforementioned works, sub-mm, mm and cm precision was achieved for quasi/geoid height differences depending on the traverse length, VD station spacing and instrument accuracy.…”

GNSS-Based Verticality Monitoring of Super-Tall Buildings

“…Therefore, the determination of the deflections of the vertical is very important work. Traditionally, the deflection of vertical components are determined from astro-geodesy [21][22][23], gravimetry [24,25], GNSS/leveling [26,27], etc. from which it is easier to calculate the deflections of the vertical with an Earth gravity field model and a geoid model.…”

Improvement of Downward Continuation Values of Airborne Gravity Data in Taiwan

“…There are many regions in the world where airborne gravity surveys have been carried out [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The Taiwan airborne gravimetry project was conducted over many hills and high mountains.…”