Leica GPS900 Series, Technical Reference Manual

Page: 186 / 287

Working with Determine Coord System GPS900 186
Characteristic Description
Principle Transforms coordinates directly from WGS 1984 to local grid and vice
versa without knowledge about the local ellipsoid or the map projec-
tion. Procedure:
1. The WGS 1984 coordinates are projected onto a temporary
Transverse Mercator projection. The central meridian of this
projection passes through the centre of gravity of the common
control points.
2. The results of 1. are preliminary grid coordinates for the
WGS 1984 points.
3. These preliminary grid coordinates are matched with the local
grid control points in order to compute the Easting and Northing
shifts, the rotation and the scale factor between these two sets
of points. This is known as a classic 2D transformation.
4. The height transformation is a single dimension height approxi-
mation.
Positions
and heights The position and height transformations are separated.
Use When measurements are to be forced to tie in with local existing
control. For example:
A site where the coordinates of the control points are based on a
purely local grid. The coordinate values within this grid are totally
arbitrary and are in no way connected with any ellipsoid or map
projection. Obviously a Classic 3D transformation cannot be used
here, as cartesian coordinates cannot be calculated from such a grid.