The Geoid Slope
Validation Survey of 2011

The above quote refers to one of many ambitious goals in the NGS 10 year plan. While admirable, its achievability remains an open question. In order to begin addressing the actual achievable accuracy of a gravimetric geoid model, NGS will engage in a variety of testing methods prior to the 2022 date when the gravimetric geoid will serve as the reference surface for a new vertical datum. The first such testing method is an upcoming project known as the Geoid Slope Validation Survey of 2011, or GSVS11. As its name implies, this survey will test the accuracy of differential geoid undulations between points ("geoid slopes") from NGS's gravimetric geoid models. The baseline for comparison will be independently computed geoid slopes from two methods:

1. Differential orthometric heights and differential ellipsoid heights from leveling and GPS campaigns, respectively (both minimally constrained)
2. Astro-Geodetic deflections of the vertical from observations with the Swiss DIADEM camera

A variety of factors had to be considered when designing the final survey line. These were:

• It had to be under existing GRAV-D surveys, in order to test the impact of the aerial gravity on the gravimetric geoid model
• It should be at least 200 km long in order to compare directly to satellite-only (GRAce and/or GOCE) geoid models.
• It should be topographically "low" (near the geoid), safe, relatively straight, and in an area of few trees (for GPS work) and during a time of year of clear skies (for camera work).

The final line chosen was a 300 km stretch of mostly U.S. highways running north/south between Austin, Texas and Corpus Christi, Texas. A 2 1/2 month survey campaign was designed, running from mid-July until late September, 2011. Reconnaissance began in February 2011, and mark setting began in May 2011. While many existing marks (most NAVD 88 bench marks) were found, many were unusable by GPS or other survey instruments. New marks were set as necessary, with the final count of official marks in the survey coming to around 220 (about 1/mile or 1 / 1.6 km).

The following surveys will be performed along the survey line during the course of GSVS11:

1. Differential Leveling
2. Campaign GPS
3. RTN-based GPS
4. Absolute Gravity
5. Gravity Gradients
6. Deflections of the Vertical
7. Airborne LIDAR
8. Airborne Imagery

For more information, contact Dru Smith (dru.smith@noaa.gov)