image image thumb6While importing surface data from Google Earth is probably the easiest way to get some conceptual topographical data into your drawings, DEM’s have to be a close second.  For the uninitiated, DEM’s or Digital Elevation Models could be described as a surface whose data is organized in a grid.  This is fundamentally different than DTM’s (standard Civil 3D surface) which still stores a collection of points, but in the form of triangles not a structured grid.  The great thing about DEM’s is they’re readily available for FREE from a wonderful website named the Geo Community.  Now don’t be fooled when you visit the site as you will see all sorts of premium content; never fear as there’s ample data that can be had for free!

Geo Community Download

The Geo Community DEM’s are organized first by state, then by county, and finally by quad name.  To download a DEM you will need to know each of these (state, county, and quad name)

  1. Go to the Geo Community DEM download page:
  2. Using the map, click on the state your site is located in.
  3. Find the county from within the state you selected.
  4. From the county page, select “Digital Elevation Model (DEM) – 24K
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  5. Click the green download button for the necessary Quad.
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    Note: You downloading from the Geo Community does require you to create an account (Free)

Extracting the DEM Download

Downloads from the Geo Community website are provided as a TAR.GZ file.  This is a compressed file format similar to a ZIP file. To get the DEM we must first unpack the file.  While there’s an endless list of programs that can do this, I prefer 7-Zip (Free Download).

To decompress the file using 7-Zip:

  1. Navigate to the file using Windows Explorer (My Computer)
  2. Right-Click on the TAR.GZ file and select “Extract Here
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    Note: I placed the file in an empty directory.  Depending on your files location you may choose to use one of the other extraction options.
  3. The .GZ file contains a .TAR file which also needs to be extracted. We’ll use the same process to extract the .TAR file
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You will now have a series of DDF files.  Civil 3D cannot read a DDF file, and therefore must be compiled into a DEM file.

Compiling into a Importable DEM File

Before we can use the surface in Civil 3D we must first compile it into a single DEM file.  This is done using yet another free download named sdts2dem.exe.  It’s just a small console application that will produce the DEM file necessary for Civil 3D.

Download sdts2dem.exe

  1. Place the sdts2dem.exe file in the same directory as the data you extracted above
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  2. Double-click the sdts2dem.exe file to run the executable. Accept any security prompts that may appear upon running the program.
  3. Upon running the program you will be prompted to enter the first 4 characters of the SDTS file name.  This is the first 4 characters of the DDF files, not the initial TAR.GZ file.
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  4. Next you’ll be prompted to enter the output file name.  While you can name this anything you want, it’s recommended you name it after the Quad name.
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After completing step 5, you will now have a .DEM file that can be used to build a surface in Civil 3D.

Creating a Civil 3D DEM Surface Definition

  1. Create/Open a Drawing in Civil 3D
  2. Make sure the correct coordinate system is assigned to your drawing. Settings tab, right-click / Edit Drawing Settings, Units and Zone tab.
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  3. Right-click on the Surfaces node within Prospector, and select Create Surface
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  4. Define your surface by giving it a name, assigning it’s style, etc.
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    Note: I prefer naming my surface something like EG-DEM to make it’s origin very obvious to other team members.
  5. Plus out your new surface in Prospector and navigate to the Definition>DEM Files node. Right-click on the DEM Files node and select Add…
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  6. Use the image image thumb18 button to browse to your DEM file from the Add DEM File dialog
  7. With our DEM file teed up, we now need to tell Civil 3D how to project the DEM surface to my current coordinate system.
    • Click on the […] button for CS Code under the DEM file tree node
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    • Look at the information listed under DEM file information to discern your source coordinate system.  Most DEM’s you download from the Geo Community will use a UTM with NAD27 datum.  Likewise the units for most DEM’s will be meters.  You can use this information to select the appropriate coordinate zone.
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      The illustration has been color coded to show how to match up the information inside the “DEM file information” with the information needed inside the “Select Coordinate Zone” dialog.
  8. [Ok] out of the dialogs, and Civil 3D will build & display your DEM surface.