DEM Surface vs. TIN Surface

I've been working with Dan and Dave on the demo set we are going to use during the Civil 3D portion of the ISD breakout session at AU this year, and have a couple of DEM files that we are going to use. While I was building one of the drawings that will have the DEMs in them, I was reminded of a tip that I've been meaning to share for quite some time on the differences between a DEM surface and a TIN Surface with a DEM added to it.

You see, we have two ways of adding DEM information as a surface in Civil 3D; from a DEM file directly, or by adding a DEM file to a TIN Surface definition.

Now I don't know too much about the technical specification of each surface type, but as far as i can tell at a high level, when you create a surface by simply importing a DEM file, it uses a Grid Surface as its definition, then pulls the DEM "points" to define the surface values. With a TIN surface, you add the DEM as a data type, and it uses the "points" contained in the DEM file to build a Triangulated Irregular Network (TIN).

Now which one do I use and when? Well that depends on what I need to do with the surface. In the case of the dataset I mentioned above, I need to create the surface from the data, and I need to translate that surface to a state plane coordinate system. To do this properly, I need to add the DEM file to a TIN Surface definition, instead of importing the DEM directly.


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I've imported the DEM in both ways here, and as you can see, they are nowhere near each other when I zoom extents. This is because that a DEM import doesn't do any transformation when it builds the grid based surface. But when I add the DEM to a TIN surface as a data type, I can set the coordinate zones, and the transformation is done for me.


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This is the window you get when you add a DEM to a TIN surface. Note the ability to see statistics on the DEM file, and the ability to set a coordinate zone for transformation purposes.

So basically, if you aren't working with a local coordinate system, or just don't care about transformation, then importing the DEM file directly to a grid surface will work just fine. But if you are working in a Coordinate system, or are taking advantage of other Geospatial data to analyze you site conditions, then adding the DEM as a data type in a TIN Surface is the way to go. Also, you can't export a grid based surface to a LandXML file, so if you plan on sharing this surface via LandXML at some point, TIN surface is the way to go as well.

With the DEM data added through the TIN Surface, I can now continue with my project in a State Plane coordinate system, and do other cool things like underlay a MrSID image using Raster Design.


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So anyway, that's today's wicked cool tip; know that there two different ways to work with DEM files In Civil 3D, and what they are good for.