NAME
utm2ll - Convert UTM coordinates to latitude/longitude geographical
coordinates
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.
SEE ALSO
The ll2utm(1) command provides the inverse conversion.
Jul 24, 2001