Existing Ground Surface Without a Survey Crew

Quite often, people ask me about the amount of information out there (on the web, since you can’t see my arms vaguely pointing as I type…) regarding free data to utilize for project design. Some people recently have asked how they would go about creating surfaces from contour data that was imported from GIS data, specifically an ESRI SHP file (I explained it here, by the way.) What people don’t realize is the fact that there is plenty of other data out there available for download that has never been considered before. Some of this data is very valuable to us in that it saves time in the preliminary design process. Today, I’d like to look at one form of free information, called the SDTS format. Click more to find out how this free data can help you create surfaces without the need of a survey crew.

Pretty much every surveying and engineering firm out there has a set of quad maps, whether they hang on the wall, are in a flat file, or on disk for easy lookup and retrieval. What is a quad map? It’s a map created by the US Geological Survey (USGS for short), and is more particularly described as follows:

The best known USGS maps are those of the 7.5-minute, 1:24,000-scale quadrangle series. A scale of 1:25,000 is used for maps based on metric units (1 centimeter = 0.25 kilometer). The area portrayed on each sheet ranges from 64 square miles at latitude 30 degrees north to 49 square miles at latitude 49 degrees north.

A scale of 1:24,000 allows considerable detail to be shown in the quadrangle areas. It takes about 57,000 maps to cover the conterminous 48 States, Hawaii, and territories. All of these maps are now for sale to the public. (For Alaska, 7.5-minute maps at 1:24,000 and 1:25,000 scale are available for areas around Anchorage, Fairbanks, and Prudhoe Bay.)

The sheet size is about 22 x 27 inches north of latitude 31 degrees and 23 x 27 inches south of that latitude.

For Puerto Rico, the scale of 1:20,000 (1 centimeter = 0.2 kilometer) was adopted in the 1950’s because it was a metric scale and the island had used metric measurements previously. The island was originally mapped at 1:30,000 scale, but this was changed to 1:20,000 to provide greater detail. The sheet size is about 29 x 32 ½ inches. The U.S. Virgin Islands of St Croix, St. John, and St. Thomas have been mapped at a scale of 1:24,000; central Pacific islands at scales ranging from 1:10,000 to 1:50,000.

Now, that was a good description. However, it may not sink in until you see the following:


If you look closely at the above map, you’ll see that it has contour lines on it. This area (the county seat where I live, by the way) is at an elevation in the 700′ range, as evidenced by the contour line on the northwest corner of the image. The great thing about these maps is the fact that they cover every square mile of the US, so you have to wonder how they got those contours on there. Is some frazzled mapper sitting there tracing these contours by hand, estimating where each one goes? Hardly. These contours are computer-generated, just like the surfaces that we generate on a daily basis. And you can download the files used to create these surfaces from the internet, for free. Yes, free.

There are many locations that offer free GIS data, but the one that I use most frequently is the Geo Community. The surface files used on USGS quad maps are called DEM files, or Digital Elevation Models. These are large files, and are often shared in a format called SDTS, or Spatial Data Transfer Standard. This standard allows information to be shared across multiple computing platforms. From the USGS website:

The purpose of the SDTS is to promote and facilitate the transfer of digital spatial data between dissimilar computer systems, while preserving information meaning and minimizing the need for information external to the transfer. Implementation of SDTS is of significant interest to users and producers of digital spatial data because of the potential for increased access to and sharing of spatial data, the reduction of information loss in data exchange, the elimination of the duplication of data acquisition, and the increase in the quality and integrity of spatial data. SDTS is neutral, modular, growth-oriented, extensible, and flexible–all characteristics of an “open systems” standard.

The SDTS provides a solution to the problem of spatial data transfer from the conceptual level to the details of physical file encoding. Transfer of spatial data involves modeling spatial data concepts, data structures, and logical and physical file structures. To be useful, the data to be transferred must also be meaningful in terms of data content and data quality. SDTS addresses all of these aspects for both vector and raster data structures.

The issue that we run into is the fact that Civil 3D doesn’t allow the use of SDTS files. Well, that’s actually a bit untrue. The Map functionality of Civil 3D does allow for the import of a SDTS file, but it gives us AutoCAD points in a grid format, which is OK….but not what I want (trust me, there’s a better way)

Notice that I said AUTOCAD points…not Civil 3D point objects. Yuck.

The good news is that there are programs out there that convert a SDTS file to a DEM file, which we can import into a surface. I use a command line program called SDTS2DEM, which can be found here. It’s fairly easy to use, provided that you young people who weren’t around for the DOS days aren’t afraid of the command prompt. First things first:

The SDTS file will come in a compressed format that will look something like 1634082.DEM.SDTS.TAR.GZ – sure, I know exactly what THAT is….seriously, it’s a compressed file. You will have to uncompress it using a program such as Winzip – standard Windows XP compressed folders won’t work with this format. And, like surfaces in Civil 3D, we have to explode it (unzip) it twice – the compressed file contains another compressed file. Once you get that extracted to a folder, you must copy the SDTS2DEM.EXE file into that folder in order to use it. This is because running the program makes it recognize ONLY files in the folder that it’s stored in (it’s basic, folks – nothing fancy here.) So, we go to the command prompt for this one, and navigate to the folder where the data was extracted to. For you young’uns, I’m not going to explain that part. If you can’t figure out the command prompt, ask that old guy in the corner who used to draw on paper to help you out. Once you’ve navigated to that folder, type in the command SDTS2DEM. That will bring up the following information:

Now, we have to go back to Windows Explorer to find out the characters used for this particular quad. I can see that it’s a series of 4 numbers, in my case those numbers are 9209, so I’ll enter that.

I want to name this the same as the quad name (for easy name recognition) so I enter ROCKFORD as the base output file name.


Now that the deed is done, we can see some really good information regarding this file, including the fact that this file is georeferenced. That’s right – this surface is going to come in where it should without any input from me (provided I have my drawing set up in the same zone…I’m not sure if it does coordinate transforms yet, but I plan on testing to see.)

Now that we have a DEM file, I can easily bring that into Civil 3D as a surface in one of two ways – I can create a surface normally and specify that I want a grid surface, or I can right-click on surfaces in Prospector and select “Import DEM.”


This will create a surface from that DEM file just as we wanted it to. Now, here’s the catch. Actually, there are two catches that I can think of to warn you about:

1) This is a surface the size of a quad map. For those of you that don’t know, that’s 8 miles by 6 miles, give or take. That’s 48 square miles. That’s over 37,000 acres. That’s a big surface. Here’s a way to make that drawing perform ok with that size surface – cut down on your contour interval. If you try to display this at 1′ intervals, you’re going to crash. Just FYI, my standard DEM Surface Style shows 20′ minors and 100′ majors, and it’s STILL a ton of contours in the area that I work. Don’t expect blazing speeds.

2) This one is pretty important. These files are interpolated from satellite imagery. Interpolated means that they may not be exact. I can prove this by showing you the triangles:

I don’t care how good your survey crews are, nobody shoots topo like that. These are all interpolated triangles. As such (read this next part very carefully) – Do not attempt to use this data for final design. In certain areas, I’ve heard of this data being off by tens of feet. That’s not good, especially when your pipe sticks out of the ground or is buried 100′ deep. Be very careful with what you do with this data.

With all that being said, have fun!

Ed. Note – be careful at work when you Google the term SDTS.   Google thinks they know everything and like to assume that you meant to search on diseases that are transmitted in a fashion not suitable for a family civil engineering website ;) 


  1. deanht says:

    Nice explanation. Good for doing quick estimates on an area that may not be “normal” for your office. We are lucky in our county, when we do jobs here, that we have local, flown GIS data we can use. It’s a little more acurate than the USGS. By the way, I resemble that guy in the corner and I’m only 37, yet feel sometimes like I must be 60+ when talking to the “young’uns” about command lines, etc.

  2. Paul Markas says:

    Great! I had most of the pieces, and now I can put them all together. To make it even easier, you can run (by double-clicking) SDTS2DEM.EXE from windows and it will open a DOS window for you and start the command. FWIW I work with someone who’s 85 and still uses his slide rule!