STAR*NET V6 - Circe

Chapter 10 Adjustments in Grid Coordinate Systems
10.9 Entering a Geoid Height and Vertical Deflections
In general, you work with elevations in STAR*NET, which is the height above the geoid
usually determined by differential leveling. However, in the grid calculations performed
during an adjustment, distances are reduced to the ellipsoid, not the geoid. Therefore
STAR*NET must know the heights of points above the ellipsoid.
The geoid height at a station is defined as the height above the ellipsoid minus the height
above the geoid. Therefore, by entering the average geoid height for your project, the
program can determine reasonably accurate ellipsoid height for all points to perform the
necessary geodetic calculations.
Enter the average geoid height value for your project in the “Grid Jobs” options group in
the Project Options/Adjustment dialog as shown below.
The geoid height value is always entered in meters, and don’t forget to use the proper
sign! In the conterminous US, for example, the geoid is below the ellipsoid, so the geoid
height is negative. In Alaska, the opposite is true, so the value is positive.
Note that providing an approximate “average” project geoid height is adequate for
working with conventional observations since their elevations are based on the geoid
anyway. When working with GPS vectors, however, accurate geoid heights must be
known for every point since vectors are based on the ellipsoid. The geoid height at each
point is the single value that properly connects a GPS point’s ellipsoid height to its actual
orthometric elevation! The STAR*NET “Professional” edition, which handles GPS
vectors, includes the ability to enter geoid heights at every station as well as perform full
geoid modeling.
The average vertical deflections define known deflections from the vertical. These values
represent the north and east deflections in seconds from a vertical line normal to the
ellipsoid. Positive deflections are in the north and east directions. A positive deflection
in the north direction means that the top of a plumb line leans north of its bottom when
compared to a vertical line normal to the ellipsoid. In the example above, the plumb line
would be leaning north and west.
In grid projects, vertical deflections affect zenith angle, turned angle, direction and
azimuth computations. These values should normally be left as zero if you don’t know
the deflections for your project area. However, if they are known and are significant (i.e.
several seconds), entering average values should give better results.
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