Like many things in Civil3D, they can be done but how it’s done isn’t so obvious.  In the process of adding a DEM surface to your drawing you may notice that you can’t project it to another coordinate system (that of the drawing).  Here is a step-by-step procedure including assigning a coordinate system to the current drawing.

First, the reason.  When a DEM (GridSurface) is accurately projected to another coordinate system it cannot remain a grid.  While you could (not recommended) approximate this by moving the grid to a known area, rotate and scale it, the grid intersections would still not hit exactly where they should.  The grid of points will have to become a TinSurface.

  1. In the ToolSpace panel, choose the Settings Tab, then right click the Drawing name and choose “Edit Drawing Settings”.
  2. In the Zone section of the dialog, choose your Category, then below that select the coordinate system, then click OK.
  3. In the ToolSpace > Prospector Tab, right click Surfaces, then choose Create Surface.  Change the Surface Name to something like”DEM Import” and set appropriate contour intervals in the Style area.
  4. Expand the Surface list (click [+]), expand “DEM Import [+]” (your surface name) and expand [+] the definition section.  Highlight “DEM Files”, right click and choose Add.
  5. Click the Select button in the upper right and choose choose your DEM file.  You may have to wait a minute while the application evaluates the file.
  6. Confirm the DEM and drawing systems are correct, you may have to choose your DEM file system using […] to the right of “CS Code”.
  7. Click OK to begin (this may take some time depending on the size of the file).

Evaluate your DEM surface file size before beginning.  Anything in the area of 100MB or more can lead to what appears to be a locked up Civil3D session.  As such it’s highly recommended you save and/or close any other drawings you may have open (you are likely going to need all available resources).  Even on a very capable machine you may find yourself considering a Task Manager > End Task!

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