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Glossary
Transformation
Characteristic
Disadvantage
Onestep
Principle
Positions and
heights
Use
Description
The local ellipsoid and map pro-
•
jection must be known for the
local grid coordinates.
•
In order to obtain accurate ellip-
soidal heights, the geoid separa-
tion at the measured points
must be known. This information
can be determined from a geoid
model.
Transforms coordinates directly from
WGS 1984 to local grid and vice versa
without knowledge about the local
ellipsoid or the map projection. Pro-
cedure:
1.
The WGS 1984 coordinates are
projected onto a temporary
Transverse Mercator Projection.
The central meridian of this pro-
jection passes through the cen-
tre 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 coordi-
nates are matched with the local
grid control points. The Easting
and Northing shifts, the rotation
and scale factor between these
two sets of points can then be
computed. This process is known
as a classic 2D transformation.
4.
The height transformation is a
single dimension height approxi-
mation.
The position and height transforma-
tions are separated.
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 arbitrary and are
in no way connected with any ellip-
soid or map projection. Obviously a
Classic 3D transformation cannot be
used here, as cartesian coordinates
cannot be calculated from such a
grid.
1055
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