Compute height above ground

gverhoev

Hi All,

I have the following problem that I would like to solve. I need to compute the height above ground for all the points in a laser scan file.

I know that Global Mapper has the „Height Above Ground“ visualisation and even supports this when exporting a LAS file, but:

·         How exactly is this height computed?

·         Can one set the parameters on how this height is computed?

·         Is it possible to compute this yourself when an existing DTM (i.e. a raster or point cloud representation of the ground) is present?

Certainly the latter option is the most interesting to me, as we have already filtered our point clouds with high-end filtering software (OPALS, developed at the TU Vienna).

Global Mapper has a function to „Apply elevations from terrain layer to selected features“. However, this adds the height, instead of subtracting. Since I did not find a way to subtract this value (is it anyhow possible?) I imported the DTM point cloud as if it would be below sea level (i.e. multiplied by -1), gridded it and then added its value using the aforementioned function. This would in effect subtract the height as wanted, however, the result holds extremely high numbers instead of values on the [0m – 20 m] range, so obviously something is going wrong here.

 Any comment and tips are welcome!

 Cheers, Geert

946

Hi ,
You can create an elevation grid from your non-ground points and than substract the DTM from it.
You will have another grid with the height above ground.
Hope it helps!