The 7.5 minute files cover a smaller portion of the land surface. The files are smaller, about a megabyte a piece uncompressed. The data points are only 90 feet appart. This means that these files will show much finer detail than the one degree maps. 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 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.
At this point, there is a USGS web site that contains a large repository of 7.5 minute DEM data for the United States. These files are free of charge and are availble on the internet. They are in a special new format called SDTS. Inorder to use these new files, you have to convert them to the original DEM format.
If you don't want to go to the trouble of converting the file or if the files you want aren't on the net, you can also buy the 7.5 minute files on CD-ROM The cost is a $90 setup fee plus $7.50 per quad. This means that if you put together of a group order of 30 quads, the price drops around $10 per quad.
Converting SDTS Files
The USGS is now offering a large selection of 7.5 minute DEM quadrangles on the World Wide Web free of charge. The only problem is that these files are in a new format called Standard Data Tranfer Standard (SDTS). The data is available at:
The COMPASS DEM Reader will not directly read files in this format, but you can convert the SDTS files to the regular DEM format. In the original DEM format, each quadrangle consisted of one file. In the SDTS format, each quadrangle consists of many files, each containing part of the DEM data. For this reason, the files are archived in a "TAR" format file. They have also been compressed using GZIP.
Inorder to use the DEM files, you will need to decompress it, extract the files from the "TAR" format file, and then convert it to DEM. Here is are the steps necessary to do this:
1. DECOMPRESSING AND EXTRACTING. The easiest way to decompress and extract the "TAR" archived files is to use WinZip. This utility program can both decompress and extract the "TAR" files. It is on the COMPASS CD-ROM and it is also available at:
Before you decompress the files, make sure the files have the extension "GZ". This will enable WinZip to understand that the file is a compressed file. If it does not have the extension "GZ" rename the file with this extension.
After you have selected the file for WinZip to decompress, it will then ask you for an extension for the file it will produce after decompression. Besure to add the extension "TAR" to the filename. This will enable WinZip to understand that it is a TAR format archive file and it will automatically extract all the files inside.
2. CONVERTING TO DEM FORMAT. When you have finished decompressing and extracting the files, the directory should contain about a dozen files that look something like this:
1109CATD.DDF 1109CATS.DDF 1109CEL0.DDF 1109DDDF.DDF
Inorder to convert these files into a old format DEM file, you will a conversion program. The conversion program is available on the COMPASS CD-ROM. It also available at this web site:
ftp://ftp.blm.gov/pub/gis/sdts/dem/win95/
There are two versions of the program, one for DOS and one for Windows. The Windows version is easier to use.
To run the program, locate it in Windows Explorer and then double click on it. You then press the buttons and simple enter the proper parameters. First, you select the SDTS filename. 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.
Next, you will select output filename. This will be the name that will be used for your DEM files. For example, you might want to use the name CACA for the Carlsbad topographic quadrangle. You do not need to add the extension, the program will automatically added the extension "DEM" to the file.
Once you have entered this information, press the "Convert" button and the data will be converted and written to disk.
