Pre-Processing:Obtaining DEM Data for Watershed Delineation
The United States Geological Survey (USGS) has converted their topographic maps into digital elevation model (DEM) files. These files represent the land surface as a matrix (grid) of elevation values at a given space (resolution) apart. The most commonly used maps are the 1:24,000 series that are commonly found in a 30-m resolution, with many areas now being converted to a 10-m resolution. In addition, the 1:250,000 map series has also been converted into 3 arc-second (approximately 90 m) resolution DEMs. Both DEM classes have been distributed by the USGS for a number of years. More recently free downloads are available from the World Wide Web (www). Many other local, state, and Federal agencies warehouse and deliver these DEM products. The EMRL at BYU has developed a single website that contains links to some of the most common and easy to use websites for free downloading of DEM data. This website is hosted at:
In addition to DEM data, this site also contains resources for locating, downloading, and preparing land use, soil textural classification, and image data. Each topic contains basic information, frequently asked questions, and detailed instructions for obtaining and preparing the data to be used in WMS.
A new exciting feature of WMS is the use of web services to automatically retrieve data. The USGS hosts the National Elevation Dataset on the internet. The information contained in the web site can be directly downloaded and accessed by WMS from within WMS.
DEM File Formats
The USGS has two different file formats. Historically, quadrangle map DEMs have been stored in a single file using what is often referred to as the “USGS Format.” In recent years, the Federal Geographic Data Committee (FGDC) has developed standards for sharing spatial data and most DEMs have been converted to this format, referred to as the “SDTS Format.” WMS can import either format, but the old-style USGS format is a little easier to work with. The SDTS is a format defined for vector as well as raster (grid) data. Multiple files are required to define the elevation grid with the SDTS format. In the case of DEMs, many of these required files are blank or small. The USGS DEMs often are named with a “. dem” extension while the SDTS DEM files will contain a “. ddf” extension. When reading the SDTS DEM, any of the “.ddf” files may be specified for the file. WMS will automatically extract the needed information from the various files.
Besides the two USGS file formats, the Arc/Info ASCII grid is another commonly used format by many GIS systems. If you are trying to obtain data stored in Arc/Info grid files, you must have the grid converted to ASCII, as the binary file format is proprietary to Environmental Systems Research Incorporated (ESRI) and unreadable by WMS. WMS also supports the DTED and GRASS grid file formats.
