# Surfaces

A surface can be a topographical or non-topographical:

- A topographical surface is a digital representation of the shape of a land surface, formed by a mesh of contiguous triangles. The surface may be existing terrain, proposed grade surfaces, or a combination of both.

- A non-topographical surface is a representation of an object or the face of objects in a 3D model or BIM file.

The Trimble Access software supports topographical surfaces in the following file formats:

- gridded digital terrain models (.dtm)
- triangulated terrain models (.ttm)
- triangular 3D faces in a DXF file (.dxf)

- triangulated DTMs in a LandXML file (.xml)

- triangulated DTMs in a 12da file (.12da)

When the offset is applied perpendicular to the DTM, the cut/fill value is computed using the following steps:

1. Determine the triangle that the current position lies on (1).

2. Offset that triangle at a right angle by the specified offset value (2) to define a new triangle.

3. Compute the elevation of the same position on the new triangle (3).

4. Compute the cut/fill value from the computed elevation to the staked position (4).