11.3.3 GPS Heighting
GEOID HEIGHT DETERMINATION
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The techniques for geoid height determination have been discussed in
many reports and papers (references presenting an Australian perspective
include HOLLOWAY, 1988; KEARSLEY, 1988; MITCHELL,
1990). The basic geoid height determination techniques are:
- Astro-geodetic method: directly
related to the local geodetic network because the necessary data (the deflections
of the vertical) are the astronomic coordinates on the one hand, and the
geodetic coordinates of the same stations on the other. Profiles of
geoid heights are calculated between these astro-geodetic stations.
- Geopotential models
of the earth's gravity field: derived from a combination of satellite
and terrestrial data. The geoid height is one quantity that can be determined
from a mathematical formula whose input is the
, 
, h
of the point, and the coefficients of the geopotential model. Very
high order models (with up to 3602 coefficients)
are available.
- Use of surface gravity in techniques such as Stokes'
Integral: requires gravity data in the vicinity of point at
which the geoid height is to be evaluated. This is a severe restriction
in some parts of the world, but the technique is potentially the most accurate
of all the geoid determination procedures.
- Geometric or interpolation methods:
a local representation of the geoid is obtained only at points which have
both levelled (orthometric) heights and heights derived using GPS (ellipsoidal).
Geoid heights at other points are found by interpolation.
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© Chris Rizos, SNAP-UNSW, 1999