Transformation parameters
A transformation consists of shifts, rotations and scale factors, depending on
the type of transformation used. Not all these parameters are always required.
These parameters can already be known, or can be computed.
Description of transformations
•
Classic 3D, also called Helmert transformation
•
Onestep
•
Twostep
Transformation
Characteristic
Description
Classic 3D
Principle
Transforms coordinates from
WGS 1984 cartesian to local carte-
sian coordinates and vice versa. A
map projection can then be applied
to obtain grid coordinates. As a simi-
larity transformation, it is the most
rigorous transformation type and
keeps the full geometrical informa-
tion.
Positions and
heights
Positions and heights are linked. The
accuracy is fully maintained and does
not distort the measurements.
Use
When measurements are to be kept
homogenous.
Requirements
•
The positions and heights are
known in WGS 1984 and in the
local system for at least three
points. Four points or more are
recommended to obtain higher
redundancy.
•
Parameters of the local ellipsoid.
•
Parameters of the local map pro-
jection, to convert between grid
coordinates and geodetic coordi-
nates.
•
Parameters of the local geoid
model, to convert between
orthometric and ellipsoidal
heights. This information is not
compulsory.
Area
Especially wide networks with large
height differences. Local grid coordi-
nates must be accurate.
Advantage
•
Accuracy of the measurements is
maintained.
•
It can be used over any area as
long as the local coordinates,
including heights, are accurate.
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Glossary
