Difference between GPS ellipsoid and sea level heights (N)
Counties (red), railroads (green) and NGS (geoid control points for GEOID03 (black)
Gravity stations used in NGS Geoid models
Gravity data and geoid control used in Geoid03
Free-air gravity anomalies in west Texas
Free-air gravity anomalies of West Texas (used for geoidal computations) (CI=6 mgals)
Elevation of gravity stations (approximate topography) (Contour interval 5 meters)
Computing geoidal corrections Gravity data ( a version of the Free-air gravity anomaly) is used to compute the surface of the equipotential of gravity representing sea level, the geoid. GPS heights H are corrected by the geoid undulation correction N by h (geoid height)=H+N The surface N is then MODIFIED with a CORRECTOR SURFACE consisting of locations where both sea level (geoid) heights and GPS heights are known, which is N (considered control points).
The reliability of the geoid computation is dependent on the distribution of the gravity data—data gaps are interpolated across— - If sparse gravity data-- less reliable.
Another reliability factor is corrector surface. - If widely spaced control, incorrect corrector surface.
Recommendations Gravity and control data in Big Bend area are most sparse and Free-air gravity anomalies and topography the most variable. Therefore original gravity values and corrector surface are less reliable. For 1 minute (lat/lon) grid prefer approximate 1 minute data distribution.
Recommendations At least--add regionally distributed gravity data in the west - along railroad/road.
- Fill in data gaps at 1-5 km spacing along and on both sides of route.
- 10-20 stations/day—20 days.
- Idea--Use university during the summer.
Then provide data NGS to compute geoid or university could do it. Integrate with Geoid03.
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