Data File Naming and Structure

The 3-second elevation data files are normally supplied as one degree data blocks in a single file (2.88 Megabytes), usually supplied on CD ROM.  The 30-second data files are provided as one data block per file (28 kilobytes). Files for 30-meter data are supplied in sections representing one 7½ minute topo map, or "quad".

This article describes the file format for SoftWright 30-second, 3-second, and 30-meter data files.  For details of the file format for USGS National Elevation Dataset (NED) 1-second files (usable with TAP 4.5 or later), see "TopoScript .BIL File Format."

Each data block includes the elevation values from the latitude and longitude indicated by the file name, and up to (but not including) the next degree latitude or degree longitude value. For example, the 3-second data base includes elevation data values every three seconds of latitude and longitude. The last latitude value in the file B10439_.DTA is 39:59:57. The next latitude value at 40:00:00 is in the file B10440_.DTA.  

Attention to the limits of the data files is especially important when retrieving area data with the "Area Grid" program. For example, if you have the data for block B10439 and you wish to plot that block, you should specify 104:59:57 for the west longitude boundary and 39:59:57 for the north latitude boundary. If you specify the next degree of longitude and latitude (i.e., 105:00:00 and 40:00:00) two additional blocks (B10539 and B10440) will be required to include the west and north borders of the area.

The file name suffix indicates a specific file in the block. Since the 30-second data base only requires a single file per block, all 30-second data files have the suffix "A" after the longitude and latitude (e.g., A10439A.DTA).

The data files for the 30-meter data base each represents one 7½ minute topographic map. The SoftWright file naming convention for 30-meter elevation data files is as follows:

  where:

C is the file name prefix indicating 30-meter data (for purposes of comparison, 30-second data file names have the prefix "A", and 3-second file names have the prefix "B").

www is the three digit western longitude degree value, including leading zeroes, for the southeast corner of the degree block containing the quad. Eastern longitudes are indicated by values greater than 180°. For example, longitude 170E would be indicated by the value 190 in the file name.

nn is the two digit northern latitude degree value, including leading zeroes, for the southeast corner of the degree block containing the quad. Quad files for southern latitudes are indicated in the file name suffix as described below.

N is the single letter designator of the quad latitude location in the degree block. The naming convention for northern latitude blocks follows the USGS format for map names, using the letters A-H to indicate the band of latitude above the bottom of the degree block:

Quads in the southern latitudes are indicated by the use of the letters I-P to indicate the band of latitude above the bottom of the degree block:

W is the single number designator of the quad longitude location in the degree block. The naming convention for all blocks follows the USGS format for map names, using the numbers 1-8 to indicate the band of longitude west of the right (eastern) edge of the degree block:

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

Terrain Elevation Interpolation

The topographic elevation data base includes elevation values on a grid of latitude and longitude (e.g., a 3-second grid for the 3-second data base). For a given path under study, it is likely that few if any of the data base points will be exactly on the path. Therefore, each desired point on the path is determined by interpolation of the surrounding data base points.

TAP provides three methods of data interpolation:

• The FCC specified method using four data points for the interpolation. This method should be used for pertinent terrain averaging calculations to be submitted to the FCC.

• A weighted interpolation method using twelve data points for the interpolation.

• A maximum elevation method using the elevation of the highest elevation of the four points around the radial point.

FCC interpolation

The FCC specified the interpolation procedure to be used with the point elevation data base in a public notice dated February 16, 1984. The prescribed method is a simple linear interpolation of the four data base "corner" points (A,B,C,D in the diagram) surrounding a desired point (G) as shown in Figure 3:

Following the FCC Public Notice, the interpolation is first performed between points AB to determine the elevation of point E, and between points CD to determine the elevation of point F. Finally, a linear interpolation is performed between points EF to predict the elevation of the desired point G.

This method is intended for data retrieval for the purpose of terrain averaging, and the averaged results for typical radials compare very favorably with the averages obtained using elevations taken manually directly from U.S.G.S. 7-1/2 minute topographic maps.

However, the FCC interpolation method includes an inherent limitation of the accuracy for individual data points. For example, if point "G" is a mountain peak, the four surrounding corner points will all be lower, and a simple linear interpolation of the four corners will result in a value lower than the actual elevation of the peak. Conversely, if point "G" is the bottom of a valley, the corner points will be higher, and the interpolated result will be higher than the actual elevation. Thus the FCC linear interpolation tends to "flatten" the terrain elevation values, lowering the peaks and raising the valleys.

While this effect is substantially overcome in the normal averaging process of fifty or more points on each radial, the use of the FCC linear interpolation for the retrieval of the elevation of a particular individual point may not provide sufficient accuracy. (The FCC accepts the use of the interpolated data for path elevation averaging, but requires the exact elevation of the end point, usually a transmitter site, to be taken from a topographic map.)

Weighted interpolation

The second interpolation method available attempts to reduce the impact of this "flattening" effect by using additional surrounding data points to determine the elevation of the point on the path under study. Twelve points, instead of four, are used, and the interpolation process uses data base points outside of the four immediate corner points to determine the slope of the terrain in different directions around the point under consideration.

 

The elevations are interpolated based on weighting factors computed from the slope of the terrain in the direction of each corner.

For example, using the point marked "H" in Figure 4, the weighed contribution is computed as follows:

The bearing from point H to the target point G is computed.

Using this bearing, the location of the crossing point I is determined.

A linear interpolation between points AC determines the elevation of point I.

The vertical slope of line HI is computed based on the horizontal distance between the points and the difference in their elevations.

Based on the slope of line HI, the extrapolated elevation of point G is computed.

This process is repeated for each of the outer points, for a total of eight predicted values of the elevation of point G. These values are averaged together to obtain a weighted value. Finally, the weighted value is averaged with the FCC value determined as described above.

Maximum Elevation

The maximum elevation method selects the highest elevation from the four closest points and uses that value. This method provides a "worst case" profile using the highest elevations within approximately 300 feet (in the 3-second data base) of the path.

   

Regardless of the interpolation method used, the software cannot represent topographic features that are not in the data base. Even in the case of topographic features that are in the data base, the elevation in the data base may differ from the actual elevation shown on topographic maps or determined by surveys or other means. (See the Technical Reference section Elevation Data Accuracy for more information on data base accuracy.) For this reason, all elevations that could critically affect the results of studies based on the topographic data should be determined from topographic maps or surveys or other means.