Provided by: gmt_4.5.11-1build1_amd64 bug


       sphdistance - Calculate nearest distances from Voronoi construction of spherical data


       sphdistance  infiles  -Ggrdfile  [  -C  ]  [  -D  ]  [  -E  ]  [  -F  ]  [ -H[i][nrec] ] [
       -Ixinc[unit][=|+][/yinc[unit][=|+]]    ]    [    -Lunit    ]    [    -Qvoronoi.d    ]    [
       -Rwest/east/south/north[r]  ]  [ -V ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [
       -m[i|o][flag] ]


       sphdistance reads one or more ASCII [or binary] files (or standard input) containing  lon,
       lat  and performs the construction of Voronoi polygons.  These polygons are then processed
       to calculate the nearest distance to each node of the lattice and written to the specified
       grid.   The  Voronoi  algorithm  used  is  STRIPACK.   As  an option, you may provide pre-
       calculated Voronoi polygon file in the format written by  sphtriangulate,  thus  bypassing
       the memory- and time-consuming triangularization.

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

       -G     Name of the output grid to hold the computed distances.


       -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]. Not applicable with -Q.

       -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

       -E     Instead  of computing distances, return the ID numbers of the Voronoi polygons that
              each grid node is inside [Default computes distances].

       -F     Force  pixel  node  registration  [Default  is   gridline   registration].    (Node
              registrations are defined in GMT Cookbook Appendix B on grid file formats.)

       -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.

       -I     x_inc [and optionally y_inc] is the  grid  spacing.  Optionally,  append  a  suffix
              modifier.   Geographical (degrees) coordinates: Append m to indicate arc minutes or
              c to indicate arc seconds.  If one of the units e, k, i, or n is appended  instead,
              the  increment  is  assumed  to  be  given  in meter, km, miles, or nautical miles,
              respectively, and will be converted to the  equivalent  degrees  longitude  at  the
              middle  latitude of the region (the conversion depends on ELLIPSOID).  If /y_inc is
              given but set to 0 it will be reset equal to x_inc; otherwise it will be  converted
              to  degrees latitude.  All coordinates: If = is appended then the corresponding max
              x (east) or y (north) may be slightly adjusted to fit exactly the  given  increment
              [by  default  the  increment  may  be  adjusted  slightly to fit the given domain].
              Finally, instead of giving an increment you may specify the number of nodes desired
              by appending + to the supplied integer argument; the increment is then recalculated
              from the number of nodes and the domain.  The resulting increment value depends  on
              whether  you  have  selected  a  gridline-registered  or pixel-registered grid; see
              Appendix B for details.  Note: if -Rgrdfile is used then grid spacing  has  already
              been initialized; use -I to override the values.

       -L     Specify  the  unit  used  for distance 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.  -N Read the information
              pertaining to each Voronoi polygon (the unique node lon, lat and polygon area) from
              a  separate  file [Default acquires this information from the ASCII segment headers
              of the output file].  Required if binary input via -Q is used.

       -Q     Append the name of a file with pre-calculated Voronoi  polygons  [Default  performs
              the  Voronoi construction on input data].  For binary data -bi you must specify the
              node information separately (via -N).

       -R     west, east, south, and north specify the Region of interest, and  you  may  specify
              them  in  decimal  degrees  or  in [+-]dd:mm[][W|E|S|N] format.  Append r if
              lower left and upper right map coordinates are given instead of w/e/s/n.   The  two
              shorthands  -Rg  and  -Rd stand for global domain (0/360 and -180/+180 in longitude
              respectively, with -90/+90 in latitude). Alternatively,  specify  the  name  of  an
              existing grid file and the -R settings (and grid spacing, if applicable) are copied
              from the grid.

       -V     Selects verbose mode, which will send progress  reports  to  stderr  [Default  runs

       -:     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.


       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.


       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


       To  construct Voronoi polygons from the points in the file testdata.txt and then calculate
       distances from the data to a global 1x1 degree grid, use

       sphdistance testdata.txt -Rg -I1 -Gglobedist.grd

       To generate the same grid in two steps using sphtriangulate separately, try

       sphtriangulate testdata.txt -Qv > voronoi.d
       sphdistance -Qvoronoi.d -Rg -I1 -Gglobedist.grd


       GMT(1), sphinterpolate(1) sphtriangulate(1) triangulate(1)


       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.