gmtsimplify(1) GMT gmtsimplify(1)
NAME
gmtsimplify - Line reduction using the Douglas-Peucker algorithm
SYNOPSIS
gmtsimplify [ table ] -Ttolerance[unit] [ -V[level] ] [ -bbinary ] [
-dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags
] [ -oflags ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
gmtsimplify reads one or more data files 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 seg-
ment, 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/algo-
rithm_0205.htm)
REQUIRED ARGUMENTS
-Ttolerance[unit]
Specifies the maximum mismatch tolerance in the user units. If
the data is not Cartesian then append the distance unit (see
UNITS).
OPTIONAL ARGUMENTS
table One or more ASCII (or binary, see -bi[ncols][type]) data table
file(s) holding a number of data columns. If no tables are given
then we read from standard input.
-V[level] (more a|)
Select verbosity level [c].
-bi[ncols][t] (more a|)
Select native binary input. [Default is 2 input columns].
-bo[ncols][type] (more a|)
Select native binary output. [Default is same as input].
-d[i|o]nodata (more a|)
Replace input columns that equal nodata with NaN and do the
reverse on output.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (more a|)
Determine data gaps and line breaks.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-ocols[,a|] (more a|)
Select output columns (0 is first column).
-:[i|o] (more a|)
Swap 1st and 2nd column on input and/or output.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
UNITS
For map distance unit, append unit d for arc degree, m for arc minute,
and s for arc second, or e for meter [Default], f for foot, k for km, M
for statute mile, n for nautical mile, and u for US survey foot. By
default we compute such distances using a spherical approximation with
great circles. Prepend - to a distance (or the unit is no distance is
given) to perform aFlat Eartha calculations (quicker but less accurate)
or prepend + to perform exact geodesic calculations (slower but more
accurate).
ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters
in your gmt.conf file. Longitude and latitude are formatted according
to FORMAT_GEO_OUT, absolute time is under the control of FOR-
MAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point val-
ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for-
mat in effect can lead to loss of precision in ASCII 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 FORMAT_FLOAT_OUT set-
ting.
EXAMPLES
To reduce the geographic line segment.d using a tolerance of 2 km, run
gmt simplify segment.d -T2k > new_segment.d
To reduce the Cartesian lines xylines.d using a tolerance of 0.45 and
write the reduced lines to file new_xylines.d, run
gmt simplify xylines.d -T0.45 > new_xylines.d
NOTES
There is a slight difference in how gmtsimplify processes lines versus
closed polygons. Segments that are explicitly closed will be consid-
ered polygons, otherwise we treat them as line segments. Hence, seg-
ments recognized as polygons may reduce to a 3-point polygon with no
area; these are suppressed from the output.
BUGS
One known issue with the Douglas-Peucker has to do with crossovers.
Specifically, it cannot be guaranteed that the reduced line does not
cross itself. Depending on how many lines you are considering it is
also possible that reduced lines may intersect other reduced lines.
Finally, the current implementation only does Flat Earth calculations
even if you specify spherical; gmtsimplify will issue a warning and
reset the calculation mode to Flat Earth.
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, Department of Meteorology, University of Reading,
Reading, UK; his subroutine forms the basis for this program.
SEE ALSO
gmt(1), gmt.conf(5), gmtconnect(1), gmtconvert(1), gmtselect(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 gmtsimplify(1)
gmt5 5.4.2 - Generated Wed Jun 28 18:02:21 CDT 2017