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6 gmtsimplify - Line reduction using the Douglas-Peucker algorithm
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9 gmtsimplify [ table ] -Ttolerance[unit] [ -V[level] ] [ -bbinary ] [
10 -dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags
11 ] [ -oflags ] [ -:[i|o] ]
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13 Note: No space is allowed between the option flag and the associated
14 arguments.
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17 gmtsimplify reads one or more data files and apply the Douglas-Peucker
18 line simplification algorithm. The method recursively subdivides a
19 polygon until a run of points can be replaced by a straight line seg‐
20 ment, with no point in that run deviating from the straight line by
21 more than the tolerance. Have a look at this site to get a visual
22 insight on how the algorithm works (‐
23 http://geometryalgorithms.com/Archive/algorithm_0205/algo‐
24 rithm_0205.htm)
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27 -Ttolerance[unit]
28 Specifies the maximum mismatch tolerance in the user units. If
29 the data is not Cartesian then append the distance unit (see
30 UNITS).
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33 table One or more ASCII (or binary, see -bi[ncols][type]) data table
34 file(s) holding a number of data columns. If no tables are given
35 then we read from standard input.
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37 -V[level] (more ...)
38 Select verbosity level [c].
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40 -bi[ncols][t] (more ...)
41 Select native binary input. [Default is 2 input columns].
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43 -bo[ncols][type] (more ...)
44 Select native binary output. [Default is same as input].
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46 -d[i|o]nodata (more ...)
47 Replace input columns that equal nodata with NaN and do the
48 reverse on output.
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50 -e[~]"pattern" | -e[~]/regexp/[i] (more ...)
51 Only accept data records that match the given pattern.
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53 -f[i|o]colinfo (more ...)
54 Specify data types of input and/or output columns.
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56 -g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (more ...)
57 Determine data gaps and line breaks.
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59 -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
60 Skip or produce header record(s).
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62 -icols[+l][+sscale][+ooffset][,...] (more ...)
63 Select input columns and transformations (0 is first column).
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65 -ocols[,...] (more ...)
66 Select output columns (0 is first column).
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68 -:[i|o] (more ...)
69 Swap 1st and 2nd column on input and/or output.
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71 -^ or just -
72 Print a short message about the syntax of the command, then
73 exits (NOTE: on Windows just use -).
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75 -+ or just +
76 Print an extensive usage (help) message, including the explana‐
77 tion of any module-specific option (but not the GMT common
78 options), then exits.
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80 -? or no arguments
81 Print a complete usage (help) message, including the explanation
82 of all options, then exits.
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85 For map distance unit, append unit d for arc degree, m for arc minute,
86 and s for arc second, or e for meter [Default], f for foot, k for km, M
87 for statute mile, n for nautical mile, and u for US survey foot. By
88 default we compute such distances using a spherical approximation with
89 great circles. Prepend - to a distance (or the unit is no distance is
90 given) to perform "Flat Earth" calculations (quicker but less accurate)
91 or prepend + to perform exact geodesic calculations (slower but more
92 accurate).
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95 The ASCII output formats of numerical data are controlled by parameters
96 in your gmt.conf file. Longitude and latitude are formatted according
97 to FORMAT_GEO_OUT, absolute time is under the control of FOR‐
98 MAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point val‐
99 ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for‐
100 mat in effect can lead to loss of precision in ASCII output, which can
101 lead to various problems downstream. If you find the output is not
102 written with enough precision, consider switching to binary output (-bo
103 if available) or specify more decimals using the FORMAT_FLOAT_OUT set‐
104 ting.
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107 To reduce the geographic line segment.d using a tolerance of 2 km, run
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109 gmt simplify segment.d -T2k > new_segment.d
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111 To reduce the Cartesian lines xylines.d using a tolerance of 0.45 and
112 write the reduced lines to file new_xylines.d, run
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114 gmt simplify xylines.d -T0.45 > new_xylines.d
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117 There is a slight difference in how gmtsimplify processes lines versus
118 closed polygons. Segments that are explicitly closed will be consid‐
119 ered polygons, otherwise we treat them as line segments. Hence, seg‐
120 ments recognized as polygons may reduce to a 3-point polygon with no
121 area; these are suppressed from the output.
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124 One known issue with the Douglas-Peucker has to do with crossovers.
125 Specifically, it cannot be guaranteed that the reduced line does not
126 cross itself. Depending on how many lines you are considering it is
127 also possible that reduced lines may intersect other reduced lines.
128 Finally, the current implementation only does Flat Earth calculations
129 even if you specify spherical; gmtsimplify will issue a warning and
130 reset the calculation mode to Flat Earth.
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133 Douglas, D. H., and T. K. Peucker, Algorithms for the reduction of the
134 number of points required to represent a digitized line of its carica‐
135 ture, Can. Cartogr., 10, 112-122, 1973.
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137 This implementation of the algorithm has been kindly provided by Dr.
138 Gary J. Robinson, Department of Meteorology, University of Reading,
139 Reading, UK; his subroutine forms the basis for this program.
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142 gmt, gmt.conf, gmtconnect, gmtconvert, gmtselect
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145 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
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1505.4.5 Feb 24, 2019 GMTSIMPLIFY(1)