Spectra Precision FAST Survey Reference Manual - Manual (Page 294)

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DTM Import - DTM from Land XML (TXML):
Allows you to import 3DFACE entities from an XML File and save them as a triangulation (TIN) file and also draw
them as 3D faces.
The DTM file is stored in FAST Survey as a TIN file and can be used for commands such as Elevation Difference
(obtaining cut/fill by comparing field measurements to the DTM).
The 3DFaces are placed on a layer (TRI_FACE by default), and that layer can be turned off and removed from view. If
you choose E for Erase, you can pick the 3D Face entities and erase them on command. There is no particular value to
seeing the 3D Faces, so it is not recommended that they be drawn. The main value is to capture the TIN (triangulation)