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.