The vertical datum can be independent of the horizontal coordinate system. This means that a project may have:
- A datum for the horizontal coordinates (latitude, longitude or X,Y).
- A different datum for the heights (altitude Z).
Pix4Dmapper can only do conversions between coordinate systems described by mathematical models such as ellipsoids, spheroids, etc. A geoid can not be described by mathematical models. Therefore, Pix4Dmapper can not do conversions between two geoids or between an ellipsoid and a geoid. The user needs to provide additional information by giving the Geoid Height Above the Ellipsoid.
The following section illustrates some examples where this function can be used.
Image geolocation height | GCPs | Output horizontal coordinate system | Output height | |
Case 1 | ellipsoidal WGS84 height | no GCPs | UTM zone | geoid |
Case 2 | ellipsoidal WGS84 height | no GCPs | national coordinate system (based on an ellipsoid different than the WGS84 ellipsoid.) | geoid |
1. When selecting the image coordinate system, leave the Geoid Height Above the Ellipsoid to 0 (default value). For more information about the Select Image Coordinate System function: 202558099. By default, Pix4Dmapper will generate the outputs in UTM zone. UTM zones use as datum the WGS84 ellipsoid. So, no conversion will be done with respect to the heights.
2. On the Select Output Coordinate System window, on the Geoid Height Above the Ellipsoid, set the difference between the WGS84 ellipsoid and the desired geoid. For more information about the Select Output Coordinate System function: 202558099.
The outputs will have horizontal coordinates in the UTM zone and geoid height.
1. When selecting the image coordinate system, leave the Geoid Height Above the Ellipsoid to 0 (default value). For more information about the Select Image Coordinate System function: 202558099.
2. On the Select Output Coordinate System window, select the desired national coordinate system for the horizontal coordinates. For more information about the Select Output Coordinate System function: 202558099.
3. On the Select Output Coordinate System window, on the Geoid Height Above the Ellipsoid, set the difference between the ellipsoid that the national coordinate system (step 2) uses and the desired geoid.
Pix4Dmapper converts the heights from ellipsoid WGS84 to the ellipsoid used for the National coordinate system. Then, Pix4Dmapper converts the new ellipsoidal heights to geoid heights based on the Geoid Height Above the Ellipsoid value.
Figure 1. Conversions between Heights |