Provided by: gmt_4.5.11-1build1_amd64 
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 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[: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). 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
"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 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. Software, 23 (3), 416-434.