NAME¶
gmtdp - Line reduction using the Douglas-Peucker algorithm
SYNOPSIS¶
gmtdp infiles -Ttolerance [
-H[
i][
nrec] ] [
-V ] [
-:[
i|
o] ] [
-b[
i|
o][
s|
S|
d|
D[
ncol]|
c[
var1/...]] ] [
-bo[
s|
S|
d|
D[
ncol]|
c[
var1/...]]
] [
-m[
i|
o][
flag] ]
DESCRIPTION¶
gmtdp reads one or more data files (which may be multisegment files; see
-m) and apply the Douglas-Peucker line simplification algorithm. The
method recursively subdivides a polygon until a run of points can be replaced
by a straight line segment, with no point in that run deviating from the
straight line by more than the tolerance. Have a look at this site to get a
visual insight on how the algorithm works
http://geometryalgorithms.com/Archive/algorithm_0205/algorithm_0205.htm
WARNING: currently this program should be used only with geographical
coordinates.
- file(s)
- One of more data files. If none are supplied then we read standard
input.
OPTIONS¶
- -T
- Specifies the maximum mismatch tolerance in km.
- -H
- Input file(s) has header record(s). If used, the default number of header
records is N_HEADER_RECS. Use -Hi if only input data should
have header records [Default will write out header records if the input
data have them]. Blank lines and lines starting with # are always
skipped.
- -V
- Selects verbose mode, which will send progress reports to stderr [Default
runs "silently"].
- -:
- Toggles between (longitude,latitude) and (latitude,longitude) input and/or
output. [Default is (longitude,latitude)]. Append i to select input
only or o to select output only. [Default affects both].
- -bi
- Selects binary input. Append s for single precision [Default is
d (double)]. Uppercase S or D will force
byte-swapping. Optionally, append ncol, the number of columns in
your binary input file if it exceeds the columns needed by the program. Or
append c if the input file is netCDF. Optionally, append
var1 /var2/... to specify the variables
to be read. [Default is 2 input columns].
- -bo
- Selects binary output. Append s for single precision [Default is
d (double)]. Uppercase S or D will force
byte-swapping. Optionally, append ncol, the number of desired
columns in your binary output file. [Default is same as input].
- -f
- Special formatting of input and/or output columns (time or geographical
data). Specify i or o to make this apply only to input or
output [Default applies to both]. Give one or more columns (or column
ranges) separated by commas. Append T (absolute calendar time),
t (relative time in chosen TIME_UNIT since
TIME_EPOCH), x (longitude), y (latitude), or f
(floating point) to each column or column range item. Shorthand
-f[i|o]g means
-f[i|o]0x,1y (geographic
coordinates).
- -m
- Multiple segment file(s). Segments are separated by a special record. For
ASCII files the first character must be flag [Default is '>'].
For binary files all fields must be NaN and -b must set the number
of output columns explicitly. By default the -m setting applies to
both input and output. Use -mi and -mo to give separate
settings to input and output.
The ASCII output formats of numerical data are controlled by parameters in your
.gmtdefaults4 file. Longitude and latitude are formatted according to
OUTPUT_DEGREE_FORMAT, whereas other values are formatted according to
D_FORMAT. Be aware that the format in effect can lead to loss of
precision in the output, which can lead to various problems downstream. If you
find the output is not written with enough precision, consider switching to
binary output (
-bo if available) or specify more decimals using the
D_FORMAT setting.
EXAMPLE¶
To reduce the line segment.d using a tolerance of 2 km, run
gmtdp segment.d
-T 2 > new_segment.d
REFERENCES¶
Douglas, D. H., and T. K. Peucker, Algorithms for the reduction of the number of
points required to represent a digitized line of its caricature,
Can.
Cartogr., 10, 112-122, 1973.
This implementation of the algorithm has been kindly provided by Dr. Gary J.
Robinson, Environmental Systems Science Centre, University of Reading,
Reading, UK (gazza@mail.nerc-essc.ac.uk); his subroutine forms the basis for
this program.
SEE ALSO¶
GMT(1)