# Digital terrain models (DTMs)

You can view electronic representations of a topograpical surface or digital terrain model (DTM) in the map.

The Trimble Access software supports digital terrain models (DTMs) in the following file formats:

- gridded 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)

You must define a projection and datum transformation before you use a DTM in a GNSS or conventional survey.

DTMs are topographical surfaces. For information on non-topographical surfaces in 3D models, see BIM models.

Viewing DTMs in the map

To display a DTM in the map, tap in the map toolbar to open the Layer manager and select the Map files tab. Tap the DTM file once to make it visible (), and tap it again to make points in the file selectable (). If the file is a DXF or LandXML file, tap the arrow next to the file name and then tap the appropriate layer once to make it visible and again to make it selectable.

When a surface is enabled in the map, a color gradient shows the elevation changes.

You can change the how the surface appears in the map and specify an offset to raise or lower the surface in the Map settings screen or the Stakeout options screen:

- From the map, tap and select Settings to open the Map settings screen and scroll to the DTM group box.

- From the Stakeout screen, tap Options to open the Stakeout options screen and scroll to the DTM group box.

To change the appearance of the surface, select an option from the Display field. To show only an outline of the surface, select the Outline option from the Display field. See Surface options in Map settings.

To offset the surface, enter the offset value in the Offset to DTM field and then tap and select whether the offset is to be applied Vertical or Perpendicular to the DTM, and whether the offset is Up or Down. When your current position is on the surface, the map shows the V.Offset or Perp.Off) value and the Fill value.

To configure the software to show the vertical or perpendicular distance deltas (or both) in the stakeout navigation screen, during stakeout tap Options and then tap Edit in the Deltas group. See Stakeout navigation deltas.

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).

To rotate the surface in the map, tap and then tap the map and drag to rotate the view. The icon in the center of the map indicates the point of orbit.

Working with DTMs

From the map, you can select points and lines in the surface and then use them in other software functions, for example to perform a Cogo calculation, create a surface, or stakeout.

If you have three or more 3D points in the job, you can create a surface and store it as a triangulated terrain model (TTM) file in the current project folder. You can then use the surface to calculate a volume. See Creating a surface from existing points.

During a survey, you can use the Measure to surface measurement method to calculate and store the closest distance from the measured point to the selected surface model.

Staking out using DTMs

You can stake out points on a DTM or you can stake out points, lines, arcs, or alignments relative to the DTM. See To stake out a DTM and To show the cut/fill to a DTM during stakeout.