U.S. Survey Feet vs. International Feet in ENVI

ENVI (prior to 4.8) uses International Feet (and the 0.3048 m/ft conversion factor) , as opposed to the U.S. Survey Feet. While there isn't any built-in support for U.S. Survey Feet, users can add it as "custom user units" to any installation of ENVI.

To do this, users need only add a line to the ASCII file "menu\useradd.txt" and re-start ENVI. This process is documented in the online help under the Index Search "User Defined Units". For example, the following line could be added to the "useradd.txt" file:

{units} {Survey Feet} {0.3048006096012192} {0}
 
In most cases, the difference between U.S. Survey Feet and International Feet is quite small. For example, the Map Coordinate Converter (UTM projection zone 13N, NAD27 datum) converts the distance 12345.6789 meters as follows:

- 40504.1962 international feet
- 40504.1151 survey feet

The difference between the two is less than an inch (0.9732" to be exact), but there is indeed a small difference.

Additional background information:

An International Foot is one-third of the Imperial standard yard, which is defined to be 0.9144 meters (0.3048 m/ft conversion factor). This means that an International Foot is exactly 30.48 cm.

In contrast, a U.S. Foot (a.k.a. "Survey Foot") is defined by the National Geodetic Survey as the unit of length that is 1/3 of the U.S. yard (defined in 1866), which makes its length equal to 30.48006096012192 cm (0.3048006096012192 m/ft conversion factor).

For more information, the following webpage provides some additional background:

http://www.ngs.noaa.gov/INFO/Policy/st_plane.html

Note: ENVI 5.0 fully supports US Survey feet through the Esri® Projection Engine.

Review on 12/31/2013 MM