USGS Standard Format
For many years, the US Geological Survey (USGS) has distributed
several different DEM products in a special ASCII format that
organizes raster data into a series of records. Since the USGS
now utilizes newer formats such as the
SDTS Raster Profile
format and GeoTIFF, this original format is referred to as USGS
Standard, USGS Native or USGS ASCII. The products include:
(1) 7.5-minute DEMs (1:24K scale)The product name refers to the extent of coverage of a single file or tile. Of these, the first two are the most commonly used. Detailed information for these two products is given below.
The 7.5-minute DEM product differs from the others in that it uses Universal Transverse Mercator (UTM) Cartesian coordinates instead of Geographic coordinates. When embedded in a rectangular grid, jagged nodata regions along all four edges of the DEM result from the difference between true north and grid north in UTM coordinates. These DEMs were produced by a variety of automated methods and the quality differs widely. Most problems were corrected when these DEMs were recently converted to SDTS Raster Profile format and released on the Internet.
The 1-degree DEM product is a reformatted version of the Defense Mapping Agency's (DMA, now NGA) DTED Level 1 product.
A file in the USGS standard format consists of a single Type A record with descriptive header information, followed by any number of Type B records, followed by a single Type C record. The Type B records contain elevation values along south-to-north profiles while the Type C record contains very basic statistics on data accuracy. A logical record size of 1024 bytes is used for all three record types. Type A and Type C records never span more than one of these logical records, but a Type B record typically spans more than one. If necessary, the logical records are padded with blanks to maintain this format. This is one reason that files in this format require over three times the disk space as the same data in a flat binary format.
How to Import
Choose Import DEM → USGS Standard ASCII (.dem) from the File menu and then select the file, which will usually have the filename extension "*.dem" or no extension.
EROS Data Center (EDC) of the USGS (US Geological Survey)
Complete coverage of the United States for both 7.5-minute and 1-degree products. For the 7.5-minute DEMs, it is recommended to obtain the corrected data in SDTS Raster Profile format.
GIS Data Depot: http://www.gisdatadepot.com
Earth Explorer: http://earthexplorer.usgs.gov
WebGLIS: (Now obsolete)
USGS Home: http://www.usgs.gov
Regardless of what software you use, you may occasionally encounter difficulties when reading DEMs with the USGS Standard format. Problems can usually be traced to one of the following four causes:
1. The Type A header record is supposed to be 1024 bytes long but users are sometimes tempted to delete the unused "padding" at the end of this record. The number of bytes actually used can be either 864 or 896 depending on when the DEM was made. RiverTools can usually handle all such cases.
2. Since files in this format are ASCII, end of line characters may not be converted correctly if you transfer these files between two dissimilar platforms. End of line characters for PCs, Macs and Unix machines are all different. Programs such as FTP for transferring files between computers can usually be configured to perform these end-of-line conversions automatically.
3. Programs from some vendors allow you to write raster data to USGS Standard format in a manner that does not conform to the format specifications. This is usually due to using a nodata value that has six characters, such as -99999. Using a five character nodata value is preferable.
4. Files may simply be corrupt. DEMs in this format have been around for many years and they are often shared between users. Errors in the original DEM or errors introduced by a former user can make the file unreadable. When in doubt, try downloading the DEM again, directly from the USGS.
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