Subtract DSM by DTM or Ground Elevation

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hi,

I’d like to ask, what should I do to subtract DSM by DTM or Ground Elevation in Pix4dmapper so I can get the height model of captured object (DSM - DTM = Height Model)? or, do you have any suggestion what software should I use to make that happen?

Thanks.

2

Hi Beatrice,

You can use the Raster Calculator in QGIS by computing the difference between the raster layers:

image
image1510×1308 400 KB

Please make sure both models have the exact same spatial resolution and image size (i.e. number of pixels on the width and height).

Also note that the DTM generated in Pix4D is based on the point cloud classification, so make sure that the point groups are correctly assigned to each point cloud class: How to generate the point cloud classification.

Good luck!

Cheers,
Teodora

3

Thank you for your reply Teodora,

If Pix4Dmapper generate both the DSM and DTM which by default the DTM is 5 x GSD, will the DTM resolution be smaller that the DSM ?

Thanks

4

Hi Beatrice,

The default resolution for the DTM is indeed 5xGSD, whereas the resolution of the DSM is 1xGSD. During algorithm development, we empirically found that the results for the DTM are better at this (lower) resolution. Also, in general, the DTM does not need to be of the same resolution as the DSM to describe elevation changes in the terrain. One drawback of a higher resolution is that a denser DTM will be noisier. Thus, it is a pre-established feature that cannot be changed, the value needs to be equal or more than 5 GSD.

Cheers,
Teodora

1 Like