1SPHDISTANCE(1) Generic Mapping Tools SPHDISTANCE(1)
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6 sphdistance - Calculate nearest distances from Voronoi construction of
7 spherical data
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10 sphdistance infiles -Ggrdfile [ -C ] [ -D ] [ -E ] [ -F ] [ -H[i][nrec]
11 ] [ -Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Lunit ] [ -Qvoronoi.d ] [
12 -Rwest/east/south/north[r] ] [ -V ] [ -:[i|o] ] [
13 -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -m[i|o][flag] ]
14
16 sphdistance reads one or more ASCII [or binary] files (or standard
17 input) containing lon, lat and performs the construction of Voronoi
18 polygons. These polygons are then processed to calculate the nearest
19 distance to each node of the lattice and written to the specified grid.
20 The Voronoi algorithm used is STRIPACK. As an option, you may provide
21 pre-calculated Voronoi polygon file in the format written by sphtrian‐
22 gulate, thus bypassing the memory- and time-consuming triangulariza‐
23 tion.
24 infiles
26 Data files with the point coordinates in ASCII (or binary; see
27 -b). If no files are given the standard input is read.
28 -G Name of the output grid to hold the computed distances.
30
32 -C For large data set you can save some memory (at the expense of
33 more processing) by only storing one form of location coordi‐
34 nates (geographic or Cartesian 3-D vectors) at any given time,
35 translating from one form to the other when necessary [Default
36 keeps both arrays in memory]. Not applicable with -Q.
37 -D Used with -m to skip the last (repeated) input vertex at the end
39 of a closed segment if it equals the first point in the segment.
40 Requires -m [Default uses all points].
41 -E Instead of computing distances, return the ID numbers of the
43 Voronoi polygons that each grid node is inside [Default computes
44 distances].
45 -F Force pixel node registration [Default is gridline registra‐
47 tion]. (Node registrations are defined in GMT Cookbook Appendix
48 B on grid file formats.)
49 -H Input file(s) has header record(s). If used, the default number
51 of header records is N_HEADER_RECS. Use -Hi if only input data
52 should have header records [Default will write out header
53 records if the input data have them]. Blank lines and lines
54 starting with # are always skipped.
55 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
57 append a suffix modifier. Geographical (degrees) coordinates:
58 Append m to indicate arc minutes or c to indicate arc seconds.
59 If one of the units e, k, i, or n is appended instead, the
60 increment is assumed to be given in meter, km, miles, or nauti‐
61 cal miles, respectively, and will be converted to the equivalent
62 degrees longitude at the middle latitude of the region (the con‐
63 version depends on ELLIPSOID). If /y_inc is given but set to 0
64 it will be reset equal to x_inc; otherwise it will be converted
65 to degrees latitude. All coordinates: If = is appended then the
66 corresponding max x (east) or y (north) may be slightly adjusted
67 to fit exactly the given increment [by default the increment may
68 be adjusted slightly to fit the given domain]. Finally, instead
69 of giving an increment you may specify the number of nodes
70 desired by appending + to the supplied integer argument; the
71 increment is then recalculated from the number of nodes and the
72 domain. The resulting increment value depends on whether you
73 have selected a gridline-registered or pixel-registered grid;
74 see Appendix B for details. Note: if -Rgrdfile is used then
75 grid spacing has already been initialized; use -I to override
76 the values.
77 -L Specify the unit used for distance calculations. Choose among e
79 (m), k (km), m (mile), n (nautical mile), or d (spherical
80 degree). A spherical approximation is used unless ELLIPSOID is
81 set to an actual ellipsoid. -N Read the information pertaining
82 to each Voronoi polygon (the unique node lon, lat and polygon
83 area) from a separate file [Default acquires this information
84 from the ASCII segment headers of the output file]. Required if
85 binary input via -Q is used.
86 -Q Append the name of a file with pre-calculated Voronoi polygons
88 [Default performs the Voronoi construction on input data]. For
89 binary data -bi you must specify the node information separately
90 (via -N).
91 -R west, east, south, and north specify the Region of interest, and
93 you may specify them in decimal degrees or in
94 [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and
95 upper right map coordinates are given instead of w/e/s/n. The
96 two shorthands -Rg and -Rd stand for global domain (0/360 and
97 -180/+180 in longitude respectively, with -90/+90 in latitude).
98 Alternatively, specify the name of an existing grid file and the
99 -R settings (and grid spacing, if applicable) are copied from
100 the grid.
101 -V Selects verbose mode, which will send progress reports to stderr
103 [Default runs "silently"].
104 -: Toggles between (longitude,latitude) and (latitude,longitude)
106 input and/or output. [Default is (longitude,latitude)]. Append
107 i to select input only or o to select output only. [Default
108 affects both].
109 -bi Selects binary input. Append s for single precision [Default is
111 d (double)]. Uppercase S or D will force byte-swapping.
112 Optionally, append ncol, the number of columns in your binary
113 input file if it exceeds the columns needed by the program. Or
114 append c if the input file is netCDF. Optionally, append
115 var1/var2/... to specify the variables to be read. [Default is
116 2 input columns].
117 -bo Selects binary output. Append s for single precision [Default
119 is d (double)]. Uppercase S or D will force byte-swapping.
120 Optionally, append ncol, the number of desired columns in your
121 binary output file. [Default is same as input].
122 -m Multiple segment file(s). Segments are separated by a special
124 record. For ASCII files the first character must be flag
125 [Default is '>']. For binary files all fields must be NaN and
126 -b must set the number of output columns explicitly. By default
127 the -m setting applies to both input and output. Use -mi and
128 -mo to give separate settings to input and output.
129
131 The ASCII output formats of numerical data are controlled by parameters
132 in your .gmtdefaults4 file. Longitude and latitude are formatted
133 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
134 according to D_FORMAT. Be aware that the format in effect can lead to
135 loss of precision in the output, which can lead to various problems
136 downstream. If you find the output is not written with enough preci‐
137 sion, consider switching to binary output (-bo if available) or specify
138 more decimals using the D_FORMAT setting.
139
141 Regardless of the precision of the input data, GMT programs that create
142 grid files will internally hold the grids in 4-byte floating point
143 arrays. This is done to conserve memory and furthermore most if not
144 all real data can be stored using 4-byte floating point values. Data
145 with higher precision (i.e., double precision values) will lose that
146 precision once GMT operates on the grid or writes out new grids. To
147 limit loss of precision when processing data you should always consider
148 normalizing the data prior to processing.
149
151 To construct Voronoi polygons from the points in the file testdata.txt
152 and then calculate distances from the data to a global 1x1 degree grid,
153 use
154 sphdistance testdata.txt -Rg -I1 -Gglobedist.grd
156 To generate the same grid in two steps using sphtriangulate separately,
158 try
159 sphtriangulate testdata.txt -Qv > voronoi.d
161 sphdistance -Qvoronoi.d -Rg -I1 -Gglobedist.grd
162
164 GMT(1), sphinterpolate(1) sphtriangulate(1) triangulate(1)
165
167 Renka, R, J., 1997, Algorithm 772: STRIPACK: Delaunay Triangulation and
168 Voronoi Diagram on the Surface of a Sphere, AMC Trans. Math. Software,
169 23 (3), 416-434.
170GMT 4.5.6 10 Mar 2011 SPHDISTANCE(1)