Leica GPS900 Series, Technical Reference Manual

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Managing, Creating, Editing Coord Systems GPS900 78
N value and geoid model
The geoid separation (N value) is the distance between the geoid and the reference ellip-
soid. It may refer to the WGS 1984 or to the local ellipsoid. It is not a constant except over
maybe small flat areas such as 5 km x 5 km. Therefore it is necessary to model the N value
in order to obtain accurate orthometric heights. The modelled N values form a geoid model
for an area. With a geoid model attached to a coordinate system, N values for the measured
points can be determined. Ellipsoidal heights can be converted to orthometric heights and
back.
Refer to the online help of LGO for more information on geoid models.
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Geoid models are an approximation of the N value. In terms of accuracy, they may vary
considerably and global models in particular should be used with caution. If the accuracy of
the geoid model is not known it might be safer to use local control points with orthometric
heights and apply a transformation to approximate the local geoid.
Geoid field file Geoid field files may be used in the field to calculate orthometric heights out of ellipsoidal
heights and vice versa.
CSCS model Description
Country Specific Coordinate System models
•
are tables of correction values to directly convert coordinates from WGS 1984 to local
grid without the need of transformation parameters.
• take the distortions of the mapping system into account.
• are an addition to an already defined coordinate system.
Types of CSCS models
The correction values of a CSCS model can be applied at different stages in the coordinate
conversion process. Depending on this stage, a CSCS model works differently. Three types
of CSCS models are supported by GPS900. Their conversion process is as explained in the