The USGS now maintains a Seamless Data Distribution System which is a convenient source of National Elevation Dataset (NED) and Shuttle Radar Topography Mission (SRTM) terrain data for the United States and other limited areas of the world. Area selection is seamless and is not constrained by latitude or longitude boundaries, and data can be downloaded immediately. Currently downloads are free of charge, with some size restrictions. Click on the image below and then select “View and Download United States Data” or "View and Download International Data" to bring up the map for area selection as shown below.
Use the mouse and the coordinate readouts at the lower left of the map window to select a location and zoom in to the desired area. Once you have found the desired area, choose the “Downloads - Define Download Area” function and outline the area of interest using the mouse. You will then see a “Request Summary Page” that indicates the data available for download for this area. The data that can be used by 3DEM will be the National Elevation Dataset (NED) and the Shuttle Radar Topography Mission (SRTM) data. Usually these are both initially offered as ArcGrid format for download. 3DEM requires the TIF (GeoTiff) data format instead of the ArcGrid format, so click the “Modify Data Request” button and change the requested format to TIF. If you are downloading SRTM data for 3DEM, you may also request the BIL data format. Then click “Save Changes and Return to Summary.” Now you can click “Download” to retrieve each data set as a TIF or BIL file.
The advantage of this site is that high-resolution (30 meter and 10 meter) terrain data can be easily located and downloaded for any area of the United States. These data are suitable for high resolution, low altitude 3D terrain scenes.
The disadvantage of this site is that all data are available only in geodetic (latitude-longitude) coordinates and generally must be converted by 3DEM to UTM coordinates before graphic overlays can be applied. However this is easily accomplished by 3DEM and usually will take only a few seconds of time.
USGS 7.5 Minute Digital Elevation Models
The USGS has designated GeoCommunity as a repository for 10 meter and 30 meter resolution, 7.5 minute, SDTS format DEMs for the entire United States. DEMs are cataloged here by State, County, and Map Name. Click on the image below to go directly to this site.
The DEMs found here are approximately 13.5 km (8.4 miles) square. These DEMs are not seamless, but are constrained to 1/8 degree latitude and longitude boundaries. Multiple DEMs can be merged by 3DEM to cover larger areas. See the 3DEM help file under "Freely Available Data - USGS SDTS DEM Files" for information on use of these data with 3DEM.
After selecting the appropriate state and county, you will see a list of available downloads including
- Digital Elevation Model (DEM) - 24k
- Digital Raster Graphics (DRG) - 24k
These are the 7.5 minute digital elevation models and topographic maps for the selected county. Select either DEM or DRG, and then click the green download button.
Often, the DEM selection will result in a choice of two downloads which are the 10 meter resolution and 30 meter resolution versions of the same terrain. The 10 meter DEM is usually about 3 MBytes in size while the 30 meter DEM is usually less than 300 KBytes in size. The 10 meter DEM is the best choice for high resolution terrain scenes.
The DRG is a matching topographic map for the selected area. This map can be quickly applied as an overlay to the 3D terrain using the procedure described in the 3DEM help file under "Overlay Application."
The advantage of this site is that the very highest resolution (10 meter) DEMs can be downloaded free of charge. These provide the very best high-resolution 3D terrain scenes and animation. It is also very convenient to be able to download a matching DRG for the same terrain area.
The disadvantage of this site is that DEMs must be located by State, County, and Map name. This requires that you have an external index for each state that will allow you to find County and Map names for your area of interest. An inexpensive map index named MapFinder covering all states is available from Digital Data Services.
Also, download speeds are limited to about 5 KB/sec for free downloads. This means that it will take at least 10 minutes to download the typical 10 meter DEM (3 MBytes) even with the very fastest Internet connection. DRGs will take longer. A fast download service is available for a fee.
