Acquiring 30-meter DEMs and using SURFER to view them

The United States Geological Survey (USGS) produces and distributes digital elevation models (DEMs) for most of the United States. The USGS DEM files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. These DEMs and lots of associated public data are available via a link at the USGS GeoData site. For detailed information on DEMs, including precision, availability, processing, and the like, see the USGS pages devoted to explanations of digital elevation models.

The DEM format is standardized; maps are generally available at two scales. Large scale DEMs are gridded samples of topography at 30 meter intervals, correspond to 7.5-minute USGS quadrangles, and use Universal Transverse Mercator (UTM) coordinates. These large-scale DEMs are commonly called 1:24,000 because their names and geographic coverage are the same as the traditional 1:24,000 paper quadrangles. However the DEMs represent digital samples of those areas and, since you can stretch them, or zoom on them, to any size you care to, you can present them at a variety of scales. The smaller scale DEMs, with samples at 3 arc seconds (about 92.5 meters on a great circle), correspond to 1:250,000 scale maps referenced to the USGS two-degree sheets. It is important to keep in mind that maps of both scales have plenty of smaller scale errors in them – lakes are likely to have various elevation surfaces, topography might be corrugated, etc. If you are doing detailed calculations with these data you will have to consider the error as in any scientific problem.

There are two sources for Montana’s 30-meter DEMs. You can get 30-meter DEMs for most of Montana from the Montana Natural Resource Information System (NRIS). These are bundled into huge groups (3-5 megabytes) of quadrangles so it takes a while to download them. On the plus side, if you need a whole area, you can get most of it with one click by selecting on a map. The USGS recently started providing 30-meter DEMs for the United States on their web site. Montana DEMs from the USGS are listed by quadrangle name and are supplied in the new SDTS data format.

Acquiring and manipulating 30-meter DEMs from Montana’s Natural Resource Information System:

• Go to the NRIS GIS web page for 30-meter DEMs and click on the map area of your choice and, as prompted, save the file to your hard disk. It should be 3-5 megabytes and have a .zip file extension.
  
• Find the file in Windows Explorer and uncompress it. Usually this just means double clicking on it but it depends on your particular machine. The result should be an expanded file of about 15-16 megabytes with a .dem file extension. SURFER will read that DEM. However, SURFER may not be able to contour it depending on the complexity of the DEM and the RAM in your computer. If SURFER flashes up a screen that says "Too many points", refer to the previous exercise for some possible solutions. In short you will need to use SURFER’s grid/utility choices to increase the sample interval or to reduce the area you intend to work with. You can almost always view it with the Map/image menu-option.

Acquiring and manipulating 30-meter DEMs from the USGS:

• Go to the web site that supplies the USGS 30-meter DEMs. The USGS used to provide the maps themselves but they recently lost interest in this (10/15/01) and entered into an agreement with a third party to provide the DEMs.
  
• Once you have decided on a map, make a subdirectory on the computer under your name. The name of the subdirectory should be the name of the quadrangle (e.g., C:/Dir/yr name/Abbot Lake). The reason for this is the USGS now supplies 30-meter DEMs in their SDTS format. Thus when you expand the file, you will end up with about 20 files with arcane names rather than one simple DEM.
  
• Now save the selected DEM (in compressed form) to the subdirectory you just made on your hard disk. These files are 100-200kb.
  
  • To uncompress the file on a PC with WinZip:
  • Find the file in Windows Explorer; it will end with a .gz file extension. Double-click the filename and WinZip will come up with a screen reminiscent of the figure to the right.
  • You should now have a long list of .ddf files in your subdirectory.
    
• SURFER will read the SDTS files without any further ado. However, if for some reason, you want to go from the USGS SDTS file format to a standard DEM file you can do so with SDTSEDEM.EXE, a DOS program available from the Bureau of Land Management's ftp site to go from the USGS SDTS format to a DEM format:
  
  • Put a copy of SDTSEDEM.exe in the same directory as your .ddf files (use copy with Windows Explorer).
  • Open a DOS window and go to that directory (type cd:\directory_name at the command line).
  • Type SDTSEDEM at the command line and answer the three questions that come up. For the second question, supply the name of the quad but do not include the .dem file extension, e.g. supply SE_MSO. When SDTSEDEM finishes, you should have SE_MSO.DEM in the directory. SURFER will also read that DEM.
    

Exercises:

1. Download a 30-meter DEM of your choice from both (NRIS and USGS) servers and show me good looking contour, surface, and image plots of both.
2. Download a 1:250,000, 3-arc second DEM for the same area as one of the 30-meter DEMs for exercise one and show me side by side contour maps demonstrating the difference in sampling intervals.