This is a beta version of DEM Reader. It will create a Macintosh pict image from a USGS DEM file. This version is limited to grayscale elevation or bas-relief images. It will only run on a Power Macintosh.
B. About Digital Elevation Models (DEMs)
1. A DEM is a file containing geographic elevation data. The format was developed by the U.S. Geological Survey (USGS). The most common are 7.5 minute and 1 degree. They correspond to 1:24,000 and to 1:250,000 scale USGS maps. Data points for 7.5 min. DEMs are 30 meters apart. 1 deg. DEM data point are 3 seconds of latitude and longitude apart.
2. For more information on Digital Elevation Models check the USGS WEB page :
Standard DEM files are in text format making for large files. A typical 7.5 min. DEM is ~1MB, while 1 deg. maps are over 9MB! The USGS stores DEMs in .gz format, so you'll need MacGzip to expand them. The file must be expanded as a text file, or DEM Reader will not see it. The image may be
reduced to fit on screen.
C. How to use DEM Reader.
1. The file menu:
Draw Map... This opens a file. If bas-relief is not checked in the options menu, a dialog box will open to allow you to change the minimum and maximum elevation. This is only useful if you are tiling multiple maps in an image editing program such as Adobe Photoshop. Set all maps to a common minimum and maximum elevation.
Save as... This converts the file to DEM Reader's own binary format. It is 3x smaller than the text file and loads much faster. You cannot overwrite the original text file.
Save Pict... This saves the image at full resolution to a pict file. This can be up to 1.4MB for a 1deg. DEM.
Close.
Quit.
2. The Options menu:
Bas-relief. If this is checked, DEM Reader will draw the map in relief form. Otherwise, it draws the map in standard gray levels, with black corresponding to low elevations and white to high.
Zero Elevation. If this is checked, DEM Reader will draw areas with zero elevation as light gray. You'll want to turn this off for areas below sea level.
3. This is a beta version. Future versions will include color maps and
conic projection for 1deg. DEMs. There may also be a 68K version. Comments,
suggestions and bug reports are welcome. Please send them to:
brianw@electriciti.com
4. DEM Reader is copyrighted to Brian Wagner, San Diego, California USA.
This version 1.1b is distributed without cost. Future versions will be
shareware. DEM Reader cannot be sold or distributed with any commercial