Onestep
Onestep
This transformation approach works by treating the height and position transformations separately. For the position transformation, the WGS84 coordinates are projected onto a temporary transverse mercator projection and then the shifts, rotation and scale from the temporary projection to the real projection are calculated.
The height transformation is a single dimension height approximation.
Because of how the position transformation approach works it is possible to define a transformation without any knowledge of the local map projection or local ellipsoid.
The height and position transformations are separate and therefore errors in height do not propagate into errors in position. Additionally, if knowledge of local heights is not good or non-existent you can still create a transformation for position only. Also, the height points and position points do not have to be the same points.
Because of how the transformation works it is possible to compute transformation parameters with just one point in the local and WGS84 system.
The combinations of the number of points in position and the position transformation parameters that can be calculated from them are as follows:
| No. of position points | Transformation parameters computed |
|---|---|
| 1 | Classic 2D with shift in X and Y only. |
| 2 | Classic 2D with shift in X and Y, rotation about Z and scale. |
| More than 2 | Classic 2D with shift in X and Y, rotation about Z, scale and residuals. |
The number of points with height included in the transformation directly affects the type of height transformation produced.
| No. of height points | Height transformation based on |
|---|---|
| 0 | No height transformation. |
| 1 | Constant height transformation. |
| 2 | Average constant between the two height points. |
| 3 | Plane through the three height points. |
| More than 3 | Average plane. |
The advantage:
The advantage:
- The advantages of this method are that transformation parameters may be computed using little information. No knowledge is needed of the local ellipsoid and map projection and parameters may be computed with the minimum of points. Care should be taken however when computing parameters using just one or two local points as the parameters calculated are only valid in the vicinity of the points used for the transformation.
The disadvantage:
The disadvantage:
- The area of the transformation is restricted to about 10 km2 (using four common points).
Other transformation approaches:
Other transformation approaches:
Classic 3D
Twostep
Quick Ground
See also:
See also:
Which Approach to Use
Minimum Requirements for Coordinates