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DTM height on vegetation

In fields with relatively constant vegetation, what is the output of the DTM?

I suppose the Z coordinate of the point cloud will have some points distribution between ground and top of vegetation.

When the DTM is generated, does Pix4D try to output ground height, or will DTM show height of vegetation?

Example of field we are trying to capture. We are only interested in the ground.,-7.7260363,3a,75y,253.68h,84.81t/data=!3m6!1e1!3m4!1sJ8A62M9y5nx4JPCq4TiyRQ!2e0!7i13312!8i6656

A Digital Terrain Model describes the reconstructed 3D terrain in a suitable geographic coordinate system. Unlike the Surface Model (see DSM), it should contain only points that belong to the terrain layer. This means that it’s implicitly or explicitly performing a crude segmentation of the scene between terrain vs non-terrain points.

In the case of Pix4Dmapper, the algorithm uses the classes terrain (ground), vegetation, building and unknown to locally adapt the DTM extraction process. 

Looking at the area, you presented in the link, that medium height grass should be assigned to the vegetation class. Afterwards, the software will use the mild smoothing over that class and get to the terrain. 

As the algorithm to differentiate the classes uses geometry and colour characteristic of the objects, you would need to ensure that your point cloud is properly classified (as the area you are planning to reconstruct is very uniform).

Useful articles:
How to generate the point cloud classification
How to edit the point cloud in the rayCloud