NAME

       sphtriangulate  -  Perform  optimal  Delaunay  triangulation  or  Voronoi  construction of
       spherical data

SYNOPSIS

       sphtriangulate infiles [ -A ] [ -C ] [ -D ] [ -H[i][nrec] ] [ -Lunit ] [ -Nnfile ] [ -Qd|v
       ] [ -T ] [ -V ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -m[i|o][flag] ]

DESCRIPTION

       sphtriangulate  reads  one  or more ASCII [or binary] files (or standard input) containing
       lon, lat and performs a spherical Delaunay triangulation, i.e., it  find  how  the  points
       should  be  connected  to  give the most equilateral triangulation possible on the sphere.
       Optionally, you may choose -Qv which will do further  processing  to  obtain  the  Voronoi
       polygons.   Normally,  either  set  of  polygons will be written as fillable multi-segment
       output; use -T to write unique arcs instead.  As an  option,  compute  the  area  of  each
       triangle or polygon.  The algorithm used is STRIPACK.

       infiles
              Data  files  with  the point coordinates in ASCII (or binary; see -b).  If no files
              are given the standard input is read.

OPTIONS

       -A     Compute the area of the spherical triangles (-Qd) or polygons (-Qv) and  write  the
              areas  (in  chosen  units;  see  -L)  in  the multisegment output headers [no areas
              calculated].

       -C     For large data set you can save some memory (at the expense of more processing)  by
              only storing one form of location coordinates (geographic or Cartesian 3-D vectors)
              at any given time, translating from one form to the other when  necessary  [Default
              keeps both arrays in memory].

       -D     Used  with  -m  to  skip  the  last  (repeated) input vertex at the end of a closed
              segment if it equals the first point in the segment.  Requires -m [Default uses all
              points].

       -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.

       -L     Specify the unit used for distance and area calculations.  Choose among  e  (m),  k
              (km),  m  (mile),  n  (nautical  mile),  or  d  (spherical  degree).   A  spherical
              approximation is used unless ELLIPSOID is set to an actual ellipsoid.  When  degree
              is selected the areas are given in steradians.

       -N     Write  the  information  pertaining  to  each polygon (for Delaunay: the three node
              number and the triangle area; for Voronoi the unique  node  lon,  lat  and  polygon
              area)  to  a separate file [Default puts this information in the segment headers of
              the output file].  Required if binary output is needed.

       -Q     Select between BD(d)elaunay or BD(v)oronoi mode [Delaunay].

       -T     Write the unique arcs of the construction [Default  writes  fillable  triangles  or
              polygons].   When  used with -A we store arc length in the segment header in chosen
              unit (see -L).

       -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].

       -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 precision,
       consider switching to binary output (-bo if available) or specify more decimals using  the
       D_FORMAT setting.

GRID VALUES PRECISION

       Regardless  of  the  precision of the input data, GMT programs that create grid files will
       internally hold the grids in 4-byte floating point  arrays.   This  is  done  to  conserve
       memory and furthermore most if not all real data can be stored using 4-byte floating point
       values.  Data with higher  precision  (i.e.,  double  precision  values)  will  lose  that
       precision  once  GMT  operates  on  the  grid  or  writes out new grids.  To limit loss of
       precision when processing data you should always consider normalizing the  data  prior  to
       processing.

EXAMPLES

       To  triangulate  the points in the file testdata.txt, and make a Voronoi diagram via psxy,
       use

       sphtriangulate testdata.txt -Qv | psxy -Rg -JG30/30/6i -M -L -P -W1p -B0g30 | gv -

       To compute the optimal Delaunay triangulation network based on the multiple  segment  file
       globalnodes.d and save the area of each triangle in the header record, try

       sphtriangulate globalnodes.d -M -Qd -A > global_tri.d

SEE ALSO

       GMT(1), triangulate(1) sphinterpolate(1) sphdistance(1)

REFERENCES

       Renka, R, J., 1997, Algorithm 772: STRIPACK: Delaunay Triangulation and Voronoi Diagram on
       the Surface of a Sphere, AMC Trans. Math. Software, 23 (3), 416-434.