More Information about DEMs
Complete USGS DEM coverage of over 10 billion, regularly-spaced elevation points, lying 30 meters or less in ground coverage from one another, and stored in 58,000 individual files are now available to the public. All of the data can be downloaded from the USGS' Geodata Page, and can also be ordered on 8mm tape or CD media at the EarthExplorer website.
- National Elevation Dataset
- 1:24,000-Scale DEMs
- 1:100,000-Scale DEMs
- 1:250,000-Scale DEMs
- Sample DEMs
- DEM Format Conversion
National Elevation Dataset
The National Elevation Dataset (NED) is a raster product assembled by the U.S. Geological Survey (USGS). The NED is designed to provide national elevation data in a seamless form with a consistent datum, elevation unit, and projection. Data corrections were made in the NED assembly process to minimize artifacts, permit edge matching, and fill sliver areas of missing data. The NED has been developed by merging the highest-resolution, best-quality elevation data available across the United States into a seamless raster format. NED is the result of the maturation of the USGS effort to provide 1:24,000-scale DEM data for the conterminous US and 1:63,360-scale DEM data for Alaska.
Note: This information comes from the USGS' NED Homepage. Visit the site for more information or to order available products and publications.
1:24,000-Scale DEMs
The United States Geological Survey (USGS) is in the process of developing 7.5-minute x 7.5-minute digital elevation models that correspond with the 1:24,000-scale topographic quadrangles. Much of the state of Wisconsin is already finished and available.
These DEMs are in the public domain, and are available for free download over the world wide web (www) or via file transfer protocol (ftp). The USGS dem are available in the spatial data transfer standard (SDTS) format. The SDTS is a format adopted by the USGS for transferring data. SDTS is the emerging format for transferring all types of USGS data.
Many private companies are willing to provide USGS DEMs in other formats and at the same or lower price than the USGS themselves. We acquired the DEMs for our demonstration from Micropath. Micropath and other private firms find their niche in providing data quickly and by providing more customer assistance.
7.5-minute DEMs have 30 meter x 30 meter spacing between elevation positions; with elevations rounded to the nearest meter. This was done in the computer, essentially by laying a grid over the corresponding topoquad contours and recording an interpolated elevation at each point. As a result, a 7.5-minute DEM in Wisconsin has about 160,000 elevation points.
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The SCO has prepared some perspective shots of a USGS DEM. Click on any of the white circles to see the view.
This example comes from a USGS 7.5-minute DEM of the "Baraboo" quad, which includes Devil's Lake (outlined in dark blue) located in Sauk County.
Click on image to enlarge
1:100,000-Scale DEMs
The USGS is in the process of creating DEMs for the entire United States at a scale of 1:100,000. This project is 35% completed and is expected to be finished in 2-3 years.
The USGS method for making these DEMs is different than for the 7.5-minute DEMs. These DEMs cover a 30-minute x 30-minute area which is half of the 30-minute x 60-minute 1:100,000-scale USGS topomaps. Instead of placing an arbitrary grid over the map and taking elevations, the measurements were taken every 2 arc-seconds by every 2 arc-seconds. This corresponds to 810,000 elevations per model. The goal is to produce a seamless coverage of these DEMs for the entire United States. The obvious draw back in this approach, however, is that the spacing varies in regards to latitude. That is, the east-west ground spacing between elevation points is tighter in Wisconsin than in Texas. Plus, in both states, the spacing is tighter east-west than north-south. For more information including the completion status of these DEMs, visit the USGS page on the subject of 2 arc-second DEMs.
1:250,000-Scale DEMs
These USGS DEMs were the first to be completed for the entire country. Each DEM corresponds to the east or west half of a sheet from the USGS 1-degree x 2-degree Series USGS topographic map series. These DEMs have about 1.4 million points per file and cover an area one degree in longitude by one degree latitude. The elevation points are spaced every 3 arc-seconds (about 300 feet in the north-south direction and 200 feet in the east-west direction). The completed 1:250,000-scale DEMs are available free from the USGS here.
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This is only the southwest corner of the Madison West 1:250,000-scale DEM, the image has been cropped from its original size. Keep in mind, that the blue color signifies the lowest elevation, not necessarily water features.
Click on image to enlarge
Here are two sample USGS DEM's shown at different scales. The one on the left is at a scale of 1:250,000, while the other appears at a scale of 1:24,000. Click on images to enlarge.
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Sample DEMs
Here are some sample USGS 1:24,000-Scale DEMs available for download. The DEMs are in MicroDEM format. We have selected DEMs of some interesting places we think you might want to view on your own. These DEMs were imported into MicroDEM from the USGS' SDTS format. These DEMs can only be viewed in MicroDEM. You are free to use them for what ever you wish, but we suggest you visit the USGS' Geodata page to get the data for your various projects.
You can download a copy of MicroDEM here.
Below are some perspectives that describe the DEM. You are invited to download the DEMs and explore them on your own. You can click on each image to enlarge its view. Click on a link to download that file.The files are zipped, so you will need a decompression tool. The zipped files are less than 150KB each.
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Rough terrain in the northeast corner of Lincoln County.
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View of the South Range from Lake Superior.
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View of the sandstone bluffs in the Camp Douglas area.
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View of the east shore at Lake Winnebago.
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The mouth of the Wisconsin River at Prairie Du Chien.
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Aerial view of Rock Island in Door County.
If you would like to make 3-D perspectives, like the ones we made above, check out the Quick guide to making 3-D views from DEMs.
DEM Format Conversion
The USGS offers their DEMs free over the internet using file transfer protocol (ftp). The format in which theses files are available for download is the Spatial Data Transfer Standard (SDTS) raster profile.
The USGS DEM viewer and MicroDEM are able to open the SDTS format. However, most other commonly used GIS packages can not. The standard .dem file format that is accepted by most of the GIS packages, is still available from the USGS on tape or CD. You can order them from EarthExplorer, which is the USGS' online store for their data.
Conversion tools are in the process of being made that will convert SDTS format to .dem format, Arc GRID format, XYZ format, and several other commonly used formats. We suggest you check out the U.S. Bureau of Land Management's ftp site. This site offers a multitude of data conversion tools written by Soloman Katz and others. These tools are free. There are even tools that will convert various formats to SDTS format.
Note: You may need to use a ftp program to download the conversion programs at this site.
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