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DTM interpolation

When creating a DTM, Pix4D mapper gives an option to choose the Resolution of the DTM in multiples of GSD of the orthophoto. I want to know what is the benefit of choosing the resolution of DTM at least five times GSD of orthophoto?
My second question is which interpolation technique (like nearest neighbourhood or average of the surrounding pixels) is used by Pix4D mapper if Resolution is chosen 10 times of GSD instead of 5 times of GSD?
Kindly reply.

Dear Alam,

The DTM is generated from the DSM, so it follows the same spatial interpolation algorithm. In Pix4Dmapper, you can choose between IDW and Triangulation. The difference between DSM and DTM is that only the Ground and Road Surface classes are preserved in the DTM, while the other point cloud classes are smoothed.

Due to the smoothing nature of the DTM generation algorithm, results are better when the DTM resolution is moderately lower than the resolution of the project. By default, the resolution of the DTM is set to 5 x GSD of the project. It is possible to edit it to a higher multiple of the GSD or any value higher than the default one. Please read more here: How to automatically generate a Digital Terrain Model (DTM).

Cheers,
Teodora

Thanks Teodora,
What do you mean by smoothening nature of DTM generation?

Hi Alam,

When I say smoothing nature, I mean the fact that the DTM will be generated by smoothing the point cloud classes which are not Ground and Road Surface. Everything is explained in the article I just mentioned.

Best,
Teodora

Hi Teodora,
As per the article, smoothening means the point cloud class except ground class and road class will not be REMOVED but SMOOTHED to generate DTM.
Now it has become more confusing. If vegetaion class of point cloud will not be taken into account for DTM generation, then what hight will be assigned to the ground points under these vegetation points (for example dense forest area)?

Hi,
The dense forest area will also be smoothed (i.e. flattened). The DTM, as the name suggests, is approximating the terrain of the area, so it is “cutting” all the elements above it.
Cheers,
Teodora