GMTDP(1) Generic Mapping Tools GMTDP(1)NAMEgmtdp - Line reduction using the Douglas-Peucker algorithm
SYNOPSISgmtdp 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] ]
DESCRIPTIONgmtdp 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 devi‐
ating 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 stan‐
dard 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 geo‐
graphical 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 (abso‐
lute 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.
ASCII FORMAT PRECISION
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 preci‐
sion, 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 carica‐
ture, 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 ALSOGMT(1)GMT 4.5.14 1 Nov 2015 GMTDP(1)
