1. Difference between GPS ellipsoid and sea level heights (N)
GPS heights need to be corrected by the geoidal undulation N in order to integrate conventional and GPS heights.

2. Geoid03 heights from the National Geodetic Survey

3. Counties (red), railroads (green) and NGS (geoid control points for GEOID03 (black)
Area of interest

4. Gravity stations used in NGS Geoid models

5. Geoid03 in West Texas

6. Gravity data and geoid control used in Geoid03

7. Free-air gravity anomalies in west Texas

8. Free-air gravity anomalies of West Texas (used for geoidal computations) (CI=6 mgals)
Proposed road
NGS geoid control are black triangles. Approximately 6 mgal gravity change results in about 2cm geoid undulation. Note sparse data along the western part of road. Geoid control are used to force N surface through those known N values so if control and/or gravity data are sparse then N is less reliable. .

9. Elevation of gravity stations

10. Elevation of gravity stations (approximate topography) (Contour interval 5 meters)
Note similarity to Free-air gravity anomalies.

11. 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).

12. Reliability of geoid corrections
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.

13. 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.

14. Recommendations Coordinate with NGS
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.