This error message indicates that invalid or crossing triangles have been detected in the LandXML file. Often this is caused by very thin triangles (slivers). Some 3rd party TIN modeling software, allows these very thin triangles to be created. Terrain does not, since they can lead to inaccurate results.
Also, check to make sure that you have exported triangles with sufficient numeric precision (decimals). LandXML files without enough decimal accuracy perturb the vertices of triangles making them invalid.
All TIN problems must be corrected before the TIN model can be used by Terrain. Luckily many TIN problems can fixed using the Repair option. Choosing the Repair option will fix the TIN model as follows:
1) Sliver triangles are removed.
2) Break-lines are created for the remaining triangle sides in the LandXML file.
3) The TIN is recalculated from the breaklines.
In most cases this will produce an exact match of the imported TIN model.
