Photogrammetry on grass – Lower elevation of a point cloud

If you have made an aerial photogrammetric survey over a grassy field you may want to lower the points to ground level.
In order to do so you can use Cloud Compare and the “Apply Transformation” tool.
Set a negative translation along “z” axis of an amount equal to the height of the grass.

However, this procedure requires important precautions:

1. You must know precisely the height of the grass or turf;
2. To do this you need to measure points on the ground, close enough to each other and evenly distributed within the surface that you need to treat.
   Measurements must be taken with precision topographic instrumentation (GNSS or Total Station);
3. Check that there are no discontinuities, features or something particular in the ground covered by graas (a drainage ditch, a hole, …);
4. Limit this procedure only to grassy areas where you feel really confident on how the ground is, below the grass;
5. If the area you are dealing with is within a larger area, pay attention to the discontinuities and the possible steps of the point cloud at the edge;
6. This procedure is not topographically rigorous!

Ciao!

Paolo Corradeghini