DEM File Format Errors
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Many DEM files have errors in them. These errors come from a variety of sources. Some errors are introduced when the original data was created. Other errors are introduced when the data is translated between different formats.
The DEM Reader can compensate for many DEM errors so that a defective file can still be used. However, the errors can distort the surface terrain or produce inaccurate geographic coordinates. To find detailed information about the DEM file errors, select the “View|DEM Error Info” option from the menu bar. The window will display a list of DEM errors. Here is a detailed description of each error type:
Carriage/Line Feed Return Embedded. The DEM file format specifies that there should be no Carriage Return or Line Feed (CR/LF) characters embedded in the DEM data. In some cases, DEM files will have CR/LF embedded in them, presumably to make them more easily edited with a text editor. (DEM files are divided into 1024 byte blocks, which is a throwback to when the data was stored on magnetic tapes with 1024-byte record size. For some reason, some programs insert CR or LF characters at the end each block, however, this is not a part of the USGS standard.)
These CR/LF characters will shift the position of certain fields in the file, making them difficult for most programs to read and display the data. The DEM Reader removes these characters so that the file can be read, displayed and manipulated. This process generally leaves no residual problems, but it does indicate that the file is non-standard and other inconsistencies may exist, making the data unreliable.
Invalid Corner Values (0). Each DEM files specifies the geographic location of the quadrangles boundaries. If this information is missing, the DEM Reader cannot accurately locate the DEM’s position on the earth. It also cannot display or manipulate the DEM. For this reason, the program will substitute dummy corner coordinates. This at least allows the DEM to be displayed and manipulated. However, all location information will be inaccurate.
Profile outside boundaries. The elevation points in a DEM file are organized into north-south columns called “profiles.” Every point in a profile should fall inside of the boundaries specified by the file. This error occurs when the location of an individual profile or data point falls outside the boundaries specified in the file. This can cause part of the data hidden off the edge of the screen.
To solve the problem, the DEM Reader will adjust the data so that all points will fall within the boundaries and will be displayed, however this may cause certain columns to be shifted northward, distorting the image. The error also indicates an internal consistency problem and it may indicate other aspect of the data is inaccurate.