SYNOPSIS

utm2ll [-L] | [utm_x utm_y zone [nad27 | nad83 | wgs84]]

DESCRIPTION

This program uses Redfearn's formulas to convert a given set of Universal Transverse Mercator (UTM) coordinates into the equivalent latitude and longitude geographical coordinates. (This operation is often referred to as inverse projection, since it projects a previously-projected flat surface back onto the curved surface from whence it originally came.) The inputs are the UTM "x" (also known as easting) value, the UTM "y" (also known as northing) value, and the utm zone. The "x" value includes the normal 500,000 false easting. The "y" value includes the normal 10,000,000 false northing, if the point is in the southern hemisphere. For points in the southern hemisphere, make the zone number negative.

Warning: Not all of the possible triples of utm_x, utm_y, and zone values represent correct UTM coordinates. The program will generally still produce latitude/longitude coordinates, even for incorrect inputs. You can check that your original inputs were correct by using ll2utm to convert the latitude/longitude coordinates back into UTM coordinates.

The output takes the form of a single line, containing the latitude and longitude, separated by white space. The values are in decimal degrees; with latitudes south of the equator being negative, and longitudes west of the prime meridian being negative.

If you provide just the "-L" option, the program will print some license information and exit.

Projections, and inverse projections, depend on defining an ellipsoid that approximates the shape of the earth (the reference ellipsoid) and defining reference coordinates (the datum) that allow measurements to be made. Different choices of the ellipsoid and datum can yield projections that differ by tens of meters. There are a wide variety of choices, due to both the historical progression of measurement technology, and the desire to maximize accuracy over a given region (such as North America, or one of the United States).

This program defaults to the North American Datum of 1927 (NAD-27) with the Clarke Ellipsoid of 1866, since these appear to be appropriate for much of the freely-available data. The data are apparently in the process of being converted to the Geodetic Reference System 1980 (GRS-80) ellipsoid and NAD-83. If you come across such data, you can specify "nad83" on the command line. The GTOPO30 data use the World Geodetic System 1984 (WGS-84) ellipsoid, which can be invoked by specifying "wgs84" on the command line.