Virtual Surveyor can help you align your photogrammetry outputs with your elevation datum

The proper elevation datum needs to be set up in your photogrammetry process, which needs to be done before you start working in Virtual Surveyor. Typically, your image processing software only exports the Ellipsoidal height unless you use Ground Control Points (GCPs) or adjust the 3D model to a global or local geoid model.

Virtual Surveyor's Offset Z tool provides a quick and easy way to adjust your data to local heights. 

Note: This situation only applies for small areas. Changing from an ellipsoidal elevation to a geoid elevation is not a constant, so for larger areas this process would not be ideal. But for small areas, which is often the case in surveying, this difference of offset can be seen as a constant.

In the example below, the Elevation Terrain is set to the drone's photogrammetric ellipsoidal height. By comparing the drone ellipsoidal height information with the vertical datum we're currently using, you can see that it is 13.37 feet below the vertical geoid Z height. 

Steps to Adjust the Elevation Datum

To adjust the drone's ellipsoidal height to the vertical geoid Z height:

  1. Select your Elevation Terrain item and click on the TOOLS tab.
  2. Set your Offset Z value to 13.37 ft.
    1. This moves the Elevation Terrain item up 13.37 ft, and the points now fit better to the surface of the terrain.


Tips and Tricks

  • Objects (points, polylines, surfaces...) initially placed on the terrain will not adjust with the offset terrain. 
    • The best practice is to first offset the terrain and then perform your survey.
  • You can remove the offset by entering a zero value as the Offset Z
    • This reverts the terrain to its originally imported state.
  • Refer to the site supervisor to determine the offset value between ellipsoidal height and local height. 
    • You can also use the geoid height value if you want to offset from ellipsoidal height to geoidal height. 
    • Refer to your national geographic institute to know more about the geoid height values.