SPHDISTANCE(1gmt)					       Generic Mapping Tools						 SPHDISTANCE(1gmt)

NAME

       sphdistance - Calculate nearest distances from Voronoi construction of spherical data

SYNOPSIS

       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] ]

DESCRIPTION

       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.

       infiles
	      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.

OPTIONS

       -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 points].

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

       -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  sup-
	      plied  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[:ss.xxx][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). Alterna-
	      tively, 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 "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.  Option-
	      ally, 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.  Option-
	      ally, 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  pre-
       cision, 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 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

SEE ALSO

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

REFERENCES

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

GMT 4.5.7							    15 Jul 2011 						 SPHDISTANCE(1gmt)