A Geoid Slope Validation Survey (2017) in the rugged terrain of Colorado, USA

Abstract

In the summer of 2017, the National Geodetic Survey (NGS) conducted its third and final Geoid Slope Validation Survey in the rugged terrain of southern Colorado, USA. As in previous surveys, the intent is to acquire the most accurate and precise field observations to determine geoid slopes. In turn, these data can be used to quantify the accuracy of various geoid models as NGS looks ahead to creating a highly accurate gravimetric geoid model for use as a national vertical datum. Long period GPS sessions, spirit leveling, absolute gravity, and deflection of the vertical (DoV) observations were acquired along a 360 km line, ranging from 1900 to 3300 m in elevation, with a station spacing of approximately 1.6 km. Our absolute gravity and DoV datasets are unique in that they were collected at 222 field stations in highly mountainous terrain at an unprecedented observational accuracy of 10 µGal and 0.04″, respectively. Further, by employing tailored refraction corrections to the spirit leveling data, we improved the agreement between heights derived from the DoV and spirit leveling from ± 1.9 to ± 1.3 cm RMS, or by more than 30%, across the line. At all length scales, from 1.6 to 360 km, the agreement is better than 2 cm. Finally, as a description of the validation process, we compare the observations with recent NGS experimental geoid models. We find that typical agreement is at about 3–5 cm, with no single model being best at all length scales. The data from this project are freely available to the community and should serve as test beds for not only geoid modeling comparisons, but also the refinement of numerous field techniques.