National Map Accuracy Standards: Coordinate and Elevation Accuracy

Map Accuracy


USGS topographic maps adhere to "National Map Accuracy Standards". As applied to the USGS 7.5-minute quadrangle topographic map, the horizontal accuracy standard requires that the positions of 90 percent of all points tested must be accurate within 1/50th of an inch on the map. At 1:24,000 scale, 1/50th of an inch is 40 feet. The vertical accuracy standard requires that the elevation of 90 percent of all points tested must be correct within half of the contour interval. On a map with a contour interval of 10 feet, the map must correctly show 90 percent of all points tested within 5 feet of the actual elevation.

For more information regarding the accuracy of USGS maps, please visit the USGS web page: http://pubs.er.usgs.gov/publication/fs17199

Position/Cursor Accuracy

The coordinates yielded by Terrain Navigator while viewing a 1:24,000 scale USGS map are calculated by the position of the cursor, relative to the nearest pixel in the map image. Because each pixel represents approximately 12.5 square feet, our display should be within 20 feet of the actual position - assuming that the position shown on the USGS map is 100% accurate. This can be verified by placing the mouse cursor over any corner on the map and checking the displayed coordinates against those printed on the map. The match will always be within five tenths of a second.

The size of a pixel is calculated in the following manner: 1:24,000 USGS maps are drawn where 1 inch represents 2000 feet. Terrain Navigator samples these maps at a rate of 160 pixels per inch. Therefore, the size of each pixel is 2000/160, or 12.5 feet x 12.5 feet.

Coordinate Accuracy

Internally, Terrain Navigator stores coordinate positions in Decimal Degrees, in the WGS84 Datum. This format was chosen because it is the easiest to maintain within the confines of the software. It is also a very popular choice employed by similar navigation products, web sites, software development kits, etc. Whenever coordinates are entered into Terrain Navigator, regardless of their method and format, they are automatically converted internally to Decimal Degrees. Then, depending on your coordinate preferences, they are converted back into the format of your choice.

For a discussion regarding basic conversions between latitude/longitude formats, please see this knowledge base article.

When matters are complicated through sophisticated conversions (such as UTM or MGRS), along with compensations and conversions for each of the various datums, you quickly see that the math during the conversion and storage process is quite extensive, and small anomalies are bound to appear.

If you are using a GPS unit, and comparing coordinates, it gets even worse. Like Terrain Navigator, most popular units store their coordinates in Decimal Degrees, using the globally accepted datum of WGS84. If you are using a different datum (such as NAD27, which is common on most USGS topographic maps), conversions have to take place. Because Terrain Navigator has a whole computer to use for processing, its conversions to and from NAD27 to WGS84 are more accurate than those done in a typical GPS unit. Thus, there exist even more possibilities for minor discrepancies to appear.

How much is my position going to change, due to these sorts of math anomalies?
As explained above, a 1:24,000 scale topographic map in our software has been "rasterized" into a series of "pixels", yielding a highly detailed representation of a USGS topographic map. Each pixel (the smallest element on the map, you have to zoom in to see them) is roughly 12.5 feet by 12.5 feet square in the real world. This translates to roughly one tenth of a second. Thus, the coordinates would have to differ by a tenth of a second for it to "move" in position on the map. While some of the conversions described here could cause discrepancies that exceed this threshold, it will only happen in the rarest of circumstances.

In conclusion, you may experience small discrepancies as you place coordinates on the maps, and compare these coordinates against other sources. However, these discrepancies should never be so great to cause a navigation error, nor will it exceed the reasonable accuracy of the maps contained in Terrain Navigator.

Elevation Accuracy

Terrain Navigator displays two distinctly different elevations. The primary elevation display is the contours as printed on the original USGS map. Like the rest of the map, the contour lines are subject to the same national map accuracy standards as outlined above. However, because this elevation is imprinted directly on the map, it is impossible to use it for computations such as conversion from feet to meters, generating a profile, computing rise/fall/grade, and displaying a 3D image. These require the use of a Digital Elevation Model (DEM.)

The DEM integrated into Terrain Navigator Pro is called a "30-Meter" model. This means that there is a sample elevation taken every 30 meters. Terrain Navigator Pro then interpolates this data to approximate the elevation at the cursor position, or along a profile, in the 3D display, etc. Note that in some rare cases, where a 30 meter DEM is not available, Terrain Navigator uses a 100-meter model, which is less detailed.

Terrain Navigator does not use any elevation data that was collected along a GPS track. Typically, such measurements are crude at best, and can be very inaccurate.

For more information regarding the accuracy of the USGS digital elevation model, please visit the USGS web page: http://pubs.er.usgs.gov/publication/fs04000

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