So, you’ve heard that there is a lot of free data that you can use in your GIS software. Well, some of that free data, you can also use in Civil 3D. Civil 3D is built on top of MAP so all that free GIS data, you can use as GIS data in Civil 3D. But, I don’t want to use it as GIS data, I want it to be Civil 3D data! Well, if you have a DEM (Digital Elevation Model), you can add that to a surface and away you go! Follow the link to find out more.
Finding Free GIS Data
If you already have the DEM file, you can skip on down to “Using the DEM in a Surface”. So, where can you find all this free data? Well, that’s what Google is for. Do a search for the county name your project is in coupled with Digital Elevation Model. In my example, I live in Larimer County, Colorado so I Googled “Larimer County Digital Elevation Model”. Don’t search for DEM or you’ll end up finding a lot of information about the local Democratic Party. You can see the results of this search HERE. One of the links about half way down is to the Larimer County page at geocomm.com (follow link HERE). Follow the link to the Digital Elevation Model and choose which one you want. Well, I chose the Big Narrows (an oxymoron you may ask? Not if you’ve been there!) because it’s a great spot for fly fishing (if you read this and want to join me for some fishing, let me know) and was also the site of the devestating flood that killed 143 people (wikipedia). Sign up for the site, it’s free, and download the file. I’m sure there are other sites to get this type of data but, this is the one I used.
The file that you download will look like it has a lot of extensions to it – 1704113.DEM.SDTS.TAR.GZ is the file I downloaded. As you can see, it does not end in the DEM format. Typically, the file you download will need to be converted to a DEM file. Depending on the source of the file, you’ll have to use different methods. Here is what I did to convert the file I downloaded. First thing, I had to decompress it. the .GZ file is a compressed file similar to a .zip file. I used 7zip to decompress it. The file that was decompressed is 1704113.DEM.SDTS.TAR. This is also a compressed file I decompressed using 7zip. This time I decompressed it to a folder for for the DEM. There are a lot of files in here that I will then use to create the DEM file. To do this, I downloaded a piece of software called SDTStoDEM from the University of Arizon (HERE). I placed the executable in the same folder, ran it, and it created a DEM file of whatever name I choose.
Using the DEM in a surface
Now that I have this great DEM file, what can I do with it? Well, in Civil 3D, I can use it in a surface. There is an option in the program to create a surface from a DEM file. This works great if your drawing is in the same coordinates as your DEM file.
If your drawing is in a different coordinate system, this routine will not work. If you need to translate your DEM file to a different coordinate system, simply add the DEM file to a regular ol’ surface. The DEM file can be added to a surface as data just as a point file can be added. Right click on DEM file and choose add.
In the dialog box, browse to the DEM file. Now, here’s the part that most people seem to miss, in the Add DEM File dialog box, you must set the coordinate system of the DEM file. Simply make it match the meta data provided with the DEM file and you’ll be good to go.
Here is the result. I’ve created a drawing, assigned NAD 83 Colorado North Zone Foot as the coordinate system, brought in the DEM file that was in NAD27 Zone 13 Meter, and it comes into the right location. I then drew a rectangle around the surface, exported it to Google Earth, and then imported the Google Earth image into my drawing and it comes in just as expected.
If you want to only use a portion of the DEM file for your surface, you can apply a Data Clip Boundary to the surface before adding the DEM.