Your point cloud is too dense in some parts and this defeats the interpolation. I would suggest that you clean it up by discarding a few points, at least those which are very close with different altitudes.
Anyway, here is the result for the simple Delaunay Triangulation.
[image: 6pCj_TerrainCheylardtriangulation.png]
A little bit shaky, but enough to extract isocontours and generate the terrain with a quadmesh style.
[image: Rgej_TerrainCheylardquadmesh.png]
Both are enclosed in the attached model (SU2017).
terrain Cheylard - triangulation.skp
Fredo