DTM

1. Set the max slope of the terrain to choose the type of terrain that has to be extracted (Flat/Steep). It represents the general slope that can be seen in the terrain.

2. Choose the Direction to extract the terrain.

3. Set the extraction grid size between points to process the point cloud as a grid. This will give the level of details in the final mesh.

4. Choose the extraction strategy according to the type of terrain you want to extract.

5. Choose what you want as outputs: clouds corresponding to the ground, the noise or with unclassified points.

6. Choose if you want a mesh of the ground by checking ground mesh, and if so, choose the meshing strategy.

7. Validate the command with OK, display the result with Preview or close the dialog box with Cancel.

Slope of the terrain

If the project concerns a rather flat urban area, the cursor can be put to the left, in order to choose a small angle for the general slope.
If the project concerns an open pit mine or a cliff, then the cursor needs to be put on the right side in order to choose a wide angle for the general slope.

Direction

For a typical DTM (Digital Terrain Model), the direction to use would always be the Z axis, as the ground is mainly horizontal. However, another direction can be used if this function is used; for example, to clean the point cloud of a vertical wall and remove all vegetation points. In more complex projects where a direction cannot be found easily, it is useful to use the option Best Plane, as it will take the normal direction of the best plane of the point cloud.

Level of details

The average distance between points is computed by default to suit the point cloud selected. It can, of course, be modified to adjust the value to the needs of the project. A low value will produce a result with more points, so more details. In this case the computation will also be longer.
The option Refine the final mesh will help to preserve the small details on the ground and reorganize the triangles along the sharp edges. In an urban projet, for example, this option is necessary to see the edge of the sidewalks in the final mesh.

Extraction strategy

Choose the Fast extraction strategy if your data is fairly smooth. The option Check noisy points should be used otherwise. The last option, Local steep slopes, should be used in case of a terrain with some steep slopes (ditches for example).

Point clouds

This option allows you to categorize the points of the initial point cloud and export them into your project. The points will be sorted into 3 categories according to LAS format specifications: Ground, Unclassified, Noisy points.

Created mesh

You can choose to compute a mesh of the ground. This mesh can be either a grid (vertices located at regular positions), a mesh which vertices are located on points from the initial point cloud (Measured points option) or a Refined mesh. The refined mesh option is mostly interesting in case of sidewalks and ditches and is not compatible with the Fast extraction strategy.