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39
39.1
Description
☞
Requirements to
determine a transfor-
mation
Requirements for
control points
39.2
Description
Access
530
Determine Coordinate System
Overview
GNSS measured points are always stored based on the global geocentric datum
known as WGS 1984. Most surveys require coordinates in a local grid system.
For example, based on a country's official mapping datum or an arbitrary grid
system used in a particular area such as a construction site. To convert the
WGS 1984 coordinates into local coordinates a coordinate system must be cre-
ated. Part of the coordinate system is the transformation used to convert coor-
dinates from the WGS 1984 datum to the local datum.
The Determine Coordinate System app allows:
the parameters of a new transformation to be determined.
•
the parameters of an existing transformation to be recomputed.
•
With one common control point, it is still possible to calculate a Classic 3D
transformation, as long as the rotations and the scale parameter are fixed.
Such a transformation fits perfectly in the vicinity of the common control point,
but is degraded by the distance from that point. This degradation is because
the orientation of the local reference frame or any scale factor within the local
datum cannot be taken into account.
To determine a transformation it is necessary to have common control points
whose positions are known in both WGS1984 coordinates and local coordi-
nates. The more points that are common between datum, the more accurately
the transformation parameters can be calculated. Depending on the type of
transformation used, details about the map projection, the local ellipsoid and a
local geoid model can also be needed.
•
The control points used for the transformation should surround the area
for which the transformation is to be applied. It is not good practice to
survey or convert coordinates outside of the area covered by the control
points as extrapolation errors can be introduced.
When a geoid field file, and/or a CSCS field file is used in the determina-
•
tion of a coordinate system, the control points for the calculation must fall
within the areas of the field files.
Selecting the Transformation Method
Determine Coordinate System is the conventional method of determining a
coordinate system. Parameters such as the height mode must be set by the
user.
One or more control points for both the WGS 1984 and the local datum are
needed.
Depending on the number of control points and available information, a One-
step, Twostep or Classic 3D transformation can be used.
Select Create coord sys from the Leica Captivate - Home menu.
Determine Coordinate System
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