A Digital Elevation Model (DEM), consists of a sampled array of elevations for ground positions that are normally at regularly spaced intervals. Fifteen second DEMs are simlar to the 3" DEMs, except they are one 500-meter-resolution. These DEMs are available via CD-ROM or ftp on the website. To get to the necessary information, click on "How to Order" after connecting to the 15" DEM website.
Go to 15" DEM Web Site
Go to Obtaining Hydrologic Data through the Internet