Elevation Data Accuracy

Elevation Data Accuracy

Q: Why are the elevation values from TAP different from the elevations on my map?

A: There are several factors that can affect the accuracy of the topographic elevations read with TAP.

Be sure you are using the correct coordinate datum (NAD27 or NAD83). The elevation values in different data files are based on the datum used to generate those files. When you specify NAD27 or NAD83, the software makes any necessary adjustments in the datum shift to get the correct elevation for the location from the files. If the datum of your coordinates is not known, you can select the "No Adjustment" option. The coordinates will be used with the data files with no datum adjustment.

Note that if your coordinates are based on another datum, such as WGS72, WGS84, etc., you can still select the NAD83 option. The horizontal location difference between NAD83 and the other datum systems is typically only a few meters.

The topographic data you are using can also have a dramatic effect on the accuracy of the elevation values reported by TAP.

If you are using only 3-second resolution data for areas in the U.S., you can probably improve the accuracy considerably using more recent and higher resolution databases. TAP 4.5 and later can read the recently released USGS National Elevation Dataset (NED) and Shuttle Radar Topography Mission (SRTM) data. These topographic elevation databases are available from SoftWright.