3D assessment of rockfall hazard and risk mitigation

Managing unconventional DTMs
using RocPro3D



November 9th, 2023


Recently we received from a user an unconventional composite DTM consisting of two unconnected and overlapping meshes to be used for rockfall simulations. This DTM did not meet prerequisites* for RocPro3D simulations as it exhibited:

You might be thinking “It looks like a challenge to solve all these problems”! 

Not necessarily… let us show you how to:

In this case, the initial DTM was built from two separate meshes: a refined mesh in the zone of interest and a coarse mesh around this first mesh. Note that the two meshes are neither connected nor merged (Figure 1).

Figure 1: Initial composite mesh

Step 1: Preparation (correct orientation of mesh faces)

Diagnosis: The coarse mesh is defined with correctly oriented (upward) normal of faces, while the fine mesh is defined with downward oriented faces normal.

Solution: In RocPro3D, a specific tool can force an upward orientation of all the faces normal to obtain a correctly oriented mesh.

Step 2: Correction (creation of a single mesh without holes and overhangs)

Diagnosis: The two sub-meshes are not connected, leaving some holes between them. They also overlap in some areas and the fine mesh exhibits some overhanging faces.

Solution: By resampling this composite DTM in the XY plane, a single and connected mesh without overhangs is obtained. In this particular case, a random resampling is applied (without preserving domain boundaries) to obtain a final mesh that is consistent with the geometric structures (especially the benches) of the original model. Excepted at boundaries between the two initial sub-meshes, the geometric error (signed distance) between the resampled mesh and the points of the initial mesh is between -20 cm and +20 cm (Figure 2). 

Figure 2. Signed error between the initial mesh and the resampled mesh
Figure 3. Resampled mesh

Step 3: optimisation of the final mesh (optional)

Diagnosis: The resampled mesh (268 379 points and 217 400 faces) is more refined than the initial composite mesh (68 379 points and 137 059 faces), particularly in areas of the initial coarse sub-mesh, which would result in extra computation time for calculating block trajectories.

Solution: The number of faces of the resampled mesh can be reduced applying a “smart” decimation that preferentially reduces the number of faces in areas of low local curvature (i.e. almost flat). Here, the so-called “Pro” decimation with 50% reduction allows to preserve sufficient accuracy in the area of interest and to produce a coarser mesh in flat areas where mesh refinement is not required. The geometric error (signed distance) between the decimated mesh and the points of the resampled mesh is very small, is between -8 cm and +8 cm (Figure 5).

Figure 4. Signed error between the resampled mesh and the final mesh
Figure 5. Final mesh


RocPro3D version 6.2 includes new DTM edition tools to manage most meshes and make them compatible with the requirements, allowing them to be processed in just a few minutes.

Have a project and do not have a well designed DTM to start with? The solution may be easier than you think!

* Mesh prerequisites (from the RocPro3D User Manual):

  • Right orthonormal reference frame, with the Z-axis (altitude) upward oriented;
  • Mesh without any hole;
  • Mesh with a single domain, i.e. the mesh must not be composed of several unconnected domains;
  • Mesh of “manifold” type, i.e.:
    • Any edge is shared by 2 faces at most;
    • Any point belongs to only one surface;
  • Mesh without overhanging faces.