Getting DEMs

Before you can create a surface model of the terrain above a cave, you must acquire DEM files for the area you are working in. This data is available from United States Geological Survey (USGS) and several other commercial sites.

DEM Formats. The USGS has two format for DEMs. The first format is the original DEM format, which is usually referred to as DEM. The second format is a new format called "Spatial Data Transfer System" (SDTS.)

Converting SDTS Files. The DEM Reader will directly read the original format. A special converter is included with the DEM Reader that will convert SDTS files to the original format. For detailed information about working with SDTS files, click here.

File Sizes. The DEM reader will handle two kinds of DEM files: One degree and 7.5 minutes files. Each of these sizes has advantages and disadvantages.

One-Degree Files. The One-Degree files cover one square degree of the earth's surface. The data points are spaced about 300 feet or 90 meters apart. This means that sinkholes, hills, valleys and other features smaller than 90 meters, may not be visible on these maps. The one-degree maps do have three advantages. First, they are available for the whole United States. Second, they more readily available on the internet. And, finally, they are free. The USGS maintains a repository of DEM files covering the whole United States.

USGS One-Degree File Repository. Currently, one-degree files are available here:

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

The files are organized by state and have names that correspond to the nearest large town or landmark. Each file covers one degree of longitude and latitude. If you have a map with longitude and latitude lines on it, it is fairly easy to pick the proper file. Please check the Compass web site for the most current links. https://www.fountainware.com/compass/

Downloading Files. Each file is very large; about two megabytes compressed and 9 megabytes uncompressed. At 56K Baud, a compressed files takes about 7 minutes to download and an uncompressed file can take 30 minutes.

Unpacking Files. Unfortunately, these files are not compressed in the standard "zip" format. However, the Winzip program will decompress these files. Winzip is available at the following web site: http://www.winzip.com/

7.5-Minute Files. The 7.5-minute files cover a smaller portion of the land surface. The files are smaller - about a megabyte apiece uncompressed. The 7.5-minute files are also higher resolution with both 10 and 30 meter (30 and 90 feet) spacing available. This means that these files will show much smaller features than the one-degree maps.

On the other hand, there are several disadvantages to these maps. First of all, not all of the US is available in 7.5-minute maps. Second, the quality of the 7.5-minute data is poorer than the one-degree maps. For example, some of the files have artifacts and errors in them that distort the image and even make the files unreadable.

Getting 7.5-Minutes Files. At this point, the USGS has turned responsibility for 7.5-minute files over to several commercial enterprises that distribute some of the files for free. To try to encourage you to buy their products, they have some limitations on the free downloads. For example, one site limits the download speed and you may have to register. Of course, the links to the free materials may be hard to find. Here are some current sources for 7.5-minute files.

http://data.geocomm.com/dem/demdownload.html

http://www.mapmart.com/DEM/DEM.htm

Please check the Compass web site for the most current links. https://www.fountainware.com/compass

More than 100 DEMs of cave related locations are available on Compass CD-ROM.