Impossible to cut terrain surface with knife project tool I am struggling to cut a terrain mesh with several triangles with the knife Project tool.



I defined the auxiliary geometry and selected the two entities, but when I apply the Knife Project button, I cannos see any face selected.





Thank you all in advance.

Best Regards

Jacob
I tried your STL file, imported (and scaled it down to the grid size, about .0001) then recalculated normals (face normals pointing up to +Z). 

Then added a simple plane (4 verts, face normal points up to +Z)

Placed the plane somewhat above the STL map



then selected the plane, and after also the map, entered edit mode and from ortho top view uset "knife project":



after hiding the plane:



imho, it should work, in this way...