The SoftWright 30-meter elevation data files are produced from the USGS 7-1/2 minute DEM files. The original format and content of these files are described in National Mapping Program Technical Instructions, Data Users Guide 5, "Digital Elevation Models". Like the USGS files, the SoftWright files are supplied in units of one 7-1/2 minute topo map, or "quad", per file.

The SoftWright file naming convention for 30-meter elevation data files is as follows:

CwwwnnNW.DTA

where:

This naming convention is illustrated by the following excerpt from the USGS Index to Topographic and Other Map Coverage for Colorado:

The SoftWright 30-meter elevation data files are binary files using a fixed record length in each file, but the record length varies between different files. Each record in the file consist of a column of points (from south to north) along a line of constant easting. The records are arranged in the file in order of increasing easting. In other words, after the header record described below, the first data record in the file is the south to north column eastern-most points in the file, and the last record is the south to north column of western-most points in the file.

The USGS 7-1/2' DEMs are based on the Universal Transverse Mercator (UTM) coordinate system for the locations of grid points instead of a latitude/longitude grid, and the SoftWright file structure maintains this convention. Since the UTM grid system is not orthogonal to the latitude-longitude system that defines the boundaries of the quad (or map), certain allowances must be made for the number of 30-meter data points along each north-south profile in the file. For example, as shown in Figure 1 from the USGS Data Users Guide 5, columns of data near the edges of the quad may be partially outside of the vertical boundaries of the quad (defined by lines of longitude), and only the points within the quad boundaries are included in the file.

For this reason, the first record in each SoftWright 30-meter elevation data file is a header record containing information about the structure of that particular file. This header information consists of the following information:

Typically, the file header record should be read first to determine the record length of the file. All records in the file (including the header record) are of uniform length as indicated by bytes 3-4 of the header record. The header record also contains other information necessary for proper operation with the TAP system. No changes should be made to the header record of the file.

The record length given in the header record defines the constant length of the records in the file. In succeeding each record, the first four bytes contain a 4-byte integer that defines the easting of all points on that profile (or south to north column of data defined by that record). Bytes 5-8 contain a 4-byte integer that defines the northing of the first point on the profile. (The northing of each successive point on the profile is obtained by adding 30 meters to the northing of the previous point on the profile.) The 2-byte integer elevation values (in meters) begin with bytes 9-10 of each record.

Bytes 25-64 (40 characters) contain the USGS quad name and/or description from the file. Since this is an ASCII field, the name of the quad is usually recognizable even though most of the rest of the content of the binary data in the file is not. Bytes 65-75 (11 characters) contain the geodetic datum used for the coordinate locations in the file. Byte 76 is a single ASCII character indicating the DEM level (1-3) assigned to the file by the USGS. The value indicates the USGS "level of quality" of the data in the file. A complete description of the DEM level classifications is found in the "Digital Elevation Model Data User’s Guide 5", available from the USGS.

Beginning in February of 1995, bytes 77-78 contain a two-byte integer value of the minimum elevation in the file, and bytes 79-80 contain the maximum elevation in the file, both values in meters. Bytes 81-120 (40 characters) contain an ASCII string indicating the source of the data conversion from the USGS DEM files to this format.

Beginning February of 1995, bytes 121-122 contain the UTM zone value.

Beginning June of 1995, bytes 123-128 contain the x,y and z spatial resolution. The default values (if the file contains zeroes in these bytes) are 30,30,1.

Bytes 201-220 (20 bytes) are reserved for internal use by the software.

Note the process of fitting elevation points on a UTM grid into a rectangle defined by geographic coordinates causes the skewing effect illustrated on page 3 of this document. As a result, profiles in the file do not all have the same number of elevation points, nor do they all necessarily have the same northing for the starting point of the radial. For example, profiles near the east and west edges of the quad may "clip the corner" of the quad, resulting in only a few points on that profile in the quad. In the original USGS ASCII DEM format this is accomplished by using variable length records. However, since the SoftWright format requires all profile records to be of uniform length, records with fewer than the maximum number of points are "padded" with the value -32000. It should be noted that the northing value for each profile record in the SoftWright file (defined by bytes 5-8 of each record) defines the northing of the first actual data point on the profile, not necessarily the first point in the record if that first point is a "padding" value (-32000).

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