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Working with DEM files

File Sizes.  DEM files come in several sizes, but the main types are 1-Degree and 7.5-Minute versions. The 1-Degree maps are also called 1:250,000 or 250K maps. The 7.5-Minute maps are also called 1:24,000 or 24K maps.

I. One-Degree Files. One-Degree files cover very large areas, typically about 55 by 70 miles in the middle of the US. Because the data covers such a large area, the spacing between data points is about 100 meters or 300 feet. This means that terrain features smaller than 300 feet will be invisible to a 1-degree file. The files are also quite large; typically about 9-megabytes in size. The 1-Degree files are available from the USGS here:

http://edcftp.cr.usgs.gov/pub/data/DEM/250/

The One-Degree File Format. One-degree files are available in the original "DEM Format". The Compass DEM Reader can read these files directly. They generally come as a compress file using the "GZ" format. To decompress these files, you will need to a program that understand the "GZ" format. I use the IZARC, which is free and works well: http://www.izarc.org/

The filename for a typical 1-degree looks something like this: "leadville-w[1]". Once you have uncompressed the file, you will probably want to add the "DEM" extension to the file: "leadville-w[1].dem". This will make the file easier for the DEM Reader to find the file.

II. 7.5-Minute Files. The 7.5 minute files cover a smaller portion of the land surface. The files are also smaller, about a megabyte a piece uncompressed. The data points are either 10 or 30-meters apart (30 to 90 feet). This means that these files will show much finer detail than the one degree maps. There are some disadvantages to these maps.  First, the quality of some of the 7.5 minute data is poorer than the one degree maps. Some of the files seem to have artifacts in them, like horizontal lines through the data or rectangular blocks equal elevation. Second, it is a bit more work to find these files because they are no longer found on the USGS website.

Getting 7.5-Minute Files. 7.5-Minute files are available from several commercial web sites. These companies have an agreement with the USGS to deliver the data free of charge to the public. However, since the websites are for-profit ventures, they do their best to hide the free data. As a result, you will dig and probe a bit to find the files you want. The following websites offer free 7.5-minute files to the public:

  • Map Mart - Commercial Site with free 7.5 minute downloads.
  • Geo Community - Commercial Site with some free 7.5 minute files.
  • ATDI - Commercial Site with free 1 degree and 7.5 minute files.

These sites provide free SDTS-DEM files for all of the US, but it is a little tricky to figure out exactly how to find the maps. You usually have to create an account with them and then browse through their data. However, registration won't cost any money unless you buy one of their other products. You are specifically looking for 7.5-minute or 1:24,000 (24k) DEM or SDTS elevation files. Ignore DRG, DLG, DOQ, Digital Raster Graphics files.

As an example here is a link on Geo Community for El Paso County, Colorado Maps: El Paso County. Again, you are looking for 24K DEM's so you would pick this link: Digital Elevation Models (DEM) - 24K. You will notice an entry for each 7.5-minute quadrangle, but it is unclear how to get the free data. In this case, the  small green square to the right of each entry: . is the only indication that the file is free. (The Gold Icon, on the other hand, links to a higher resolution DEM that costs money.) Clicking on "Green Link" link will allow you to download the free version of the file.

Compressed Files. Most SDTS files you find online are compressed. They are usually compressed using the "GZ" format. They may also be organized into "TAR" archive files inside the "GZ" files.  To decompress these files, you will need to a program that understand the "GZ" and "TAR" formats. There are several available, but I use the IZARC, which is free and works well: http://www.izarc.org/ (It is also available on the COMPASS CD-ROM.)

In most cases, your compressed SDTS file will have about dozen files inside. If it does, simply copy them to your hard drive. If the file has a single "TAR" file inside, double click on the Tar-file inside IZARC. IZARC will decompress the TAR file and show you all the files. Just copy them to your hard drive.

The 7.5-Minute File Format. Some 7.5-Minute files use the original "DEM" file format. These can be read by the Compass DEMReader program. You can usually tell that a file uses the original format by the fact that it is only a single file. Sometime it will have the "DEM" extension. If it does not, you should add the extension to make it easier for programs to find the file. For example, the file "Manitou" would be renamed to "Manitou.dem".

SDTS Files. Most the 7.5-minute files use a new format called SDTS. You can tell if the data uses the new format by the fact that the data is contained in about a dozen files. These files cannot be read directly by the Compass DEMReader. They must be converted to the original DEM format. 

Converting SDTS Files. There are several programs that will convert SDTS files to the original DEM format. First, Compass has special program that will do the conversion. The main advantage is that it will handle many corrupt SDTS files that would otherwise be useless. Click here, to get a copy of the Compass SDTS convert.

There is also another program available from the USGS that will convert the files. You can get a copy of it here: ftp://ftp.blm.gov/pub/gis/sdts/dem/win95/ There is no particular advantage to this program, but it is available as an alternative that, in some cases, it might read a corrupt file that would be unreadable otherwise. Both conversion programs are available on the COMPASS CD-ROM.

The conversion process is explained detail in the help files and documentation that accompany the programs. The process is similar for both programs, but the hardest part is understanding the file structure. You will notice that all the SDTS files begin with a four digit number. The master file for the SDTS data will have a name like: xxxxCEL0.DDF where xxxx is the four digit number. This is the file you must select to convert the data.

DEMReader. Once you have the data in the original DEM format, you will need to read it into the DEMReader. The DEM Reader is part of the Compass package and can be downloaded here. Complete instructions for using the Reader are available in its help files. Basically, you read the DEM data into the program. The program then displays a contour map of the terrain covered by the map. You then select a linking point between the cave and the Elevation data. This is usually the entrance and you specify it by specifying a station name from the cave survey. Once this is done, the program will generate a plot file with elevation data included. Finally, you can display the elevation data and cave passages by opening the new plot file in the Cave Viewer.

 
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