NAME¶
Planimeter -- compute the area of geodesic polygons
SYNOPSIS¶
Planimeter [
-r ] [
-s ] [
-l ] [
-e a
f ] [
--comment-delimiter commentdelim ] [
--version |
-h |
--help ] [
--input-file
infile |
--input-string instring ] [
--line-separator linesep ] [
--output-file outfile
]
DESCRIPTION¶
Measure the area of a geodesic polygon. Reads polygon vertices from standard
input, one per line. Vertices may be given as latitude and longitude, UTM/UPS,
or MGRS coordinates, interpreted in the same way as
GeoConvert(1).
(MGRS coordinates signify the center of the corresponding MGRS square.) The
end of input, a blank line, or a line which can't be interpreted as a vertex
signals the end of one polygon and the start of the next. For each polygon
print a summary line with the number of points, the perimeter (in meters), and
the area (in meters^2).
By default, polygons traversed in a counter-clockwise direction return a
positive area and those traversed in a clockwise direction return a negative
area. This sign convention is reversed if the
-r option is given.
Of course, encircling an area in the clockwise direction is equivalent to
encircling the rest of the ellipsoid in the counter-clockwise direction. The
default interpretation used by
Planimeter is the one that results in a
smaller magnitude of area; i.e., the magnitude of the area is less than or
equal to one half the total area of the ellipsoid. If the
-s option is
given, then the interpretation used is the one that results in a positive
area; i.e., the area is positive and less than the total area of the
ellipsoid.
Only simple polygons are supported for the area computation. Polygons may
include one or both poles. There is no need to close the polygon.
OPTIONS¶
- -r
- toggle whether counter-clockwise traversal of the polygon
returns a positive (the default) or negative result.
- -s
- toggle whether to return a signed result (the default) or
not.
- -l
- toggle whether the vertices represent a polygon (the
default) or a polyline. For a polyline, the number of points and the
length of the path joining them is returned; the path is not closed and
the area is not reported.
- -e
- specify the ellipsoid via a f; the equatorial
radius is a and the flattening is f. Setting f = 0
results in a sphere. Specify f < 0 for a prolate ellipsoid. A
simple fraction, e.g., 1/297, is allowed for f. (Also, if f
> 1, the flattening is set to 1/ f.) By default, the WGS84
ellipsoid is used, a = 6378137 m, f = 1/298.257223563. If
entering vertices as UTM/UPS or MGRS coordinates, use the default
ellipsoid, since the conversion of these coordinates to latitude and
longitude uses the WGS84 parameters.
- --comment-delimiter
- set the comment delimiter to commentdelim (e.g.,
"#" or "//"). If set, the input lines will be scanned
for this delimiter and, if found, the delimiter and the rest of the line
will be removed prior to processing. For a given polygon, the last such
string found will be appended to the output line (separated by a
space).
- --version
- print version and exit.
- -h
- print usage and exit.
- --help
- print full documentation and exit.
- --input-file
- read input from the file infile instead of from
standard input; a file name of "-" stands for standard
input.
- --input-string
- read input from the string instring instead of from
standard input. All occurrences of the line separator character (default
is a semicolon) in instring are converted to newlines before the
reading begins.
- --line-separator
- set the line separator character to linesep. By
default this is a semicolon.
- --output-file
- write output to the file outfile instead of to
standard output; a file name of "-" stands for standard
output.
EXAMPLES¶
Example (the area of the 100km MGRS square 18SWK)
Planimeter <<EOF
18N 500000 4400000
18N 600000 4400000
18N 600000 4500000
18N 500000 4500000
EOF
=> 4 400139.53295860 10007388597.1913
The following code takes the output from gdalinfo and reports the area covered
by the data (assuming the edges of the image are geodesics).
#! /bin/sh
egrep '^((Upper|Lower) (Left|Right)|Center) ' |
sed -e 's/d /d/g' -e "s/' /'/g" | tr -s '(),\r\t' ' ' | awk '{
if ($1 $2 == "UpperLeft")
ul = $6 " " $5;
else if ($1 $2 == "LowerLeft")
ll = $6 " " $5;
else if ($1 $2 == "UpperRight")
ur = $6 " " $5;
else if ($1 $2 == "LowerRight")
lr = $6 " " $5;
else if ($1 == "Center") {
printf "%s\n%s\n%s\n%s\n\n", ul, ll, lr, ur;
ul = ll = ur = lr = "";
}
}
' | Planimeter | cut -f3 -d' '
SEE ALSO¶
GeoConvert(1). The algorithm for the area of geodesic polygon is given in
Section 15 of C. F. F. Karney,
Geodesics on an ellipsoid of
revolution, Feb. 2011; preprint <
http://arxiv.org/abs/1102.1215>.
See also Section 6 of C. F. F. Karney,
Algorithms for geodesics, Sept.
2011; preprint <
http://arxiv.org/abs/1109.4448>.
AUTHOR¶
Planimeter was written by Charles Karney.
HISTORY¶
Planimeter was added to GeographicLib,
<
http://geographiclib.sf.net>, in version 1.4.