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PROGRAM:

NAME

sphtriangulate - Delaunay or Voronoi construction of spherical lon,lat data

SYNOPSIS

sphtriangulate [ table ] [ ] [ ] [ ] [ unit ] [ nfile ] [ d|v ] [ ] [ [level] ] [
-b<binary> ] [ -d<nodata> ] [ -h<headers> ] [ -i<flags> ] [ -:[i|o] ]

Note: No space is allowed between the option flag and the associated arguments.

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 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.

REQUIRED ARGUMENTS

None.

OPTIONAL ARGUMENTS

table One or more ASCII (or binary, see -bi[ncols][type]) data table file(s) holding a
number of data columns. If no tables are given then we read from standard input.

-A Compute the area of the spherical triangles (-Qd) or polygons (-Qv) and write the
areas (in chosen units; see -L) in the output segment 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 to skip the last (repeated) input vertex at the end of a closed segment if it
equals the first point in the segment. [Default uses all points].

-Lunit Specify the unit used for distance and area calculations. Choose among e (m), f
(foot), k (km), m (mile), n (nautical mile), u (survey foot), or d (spherical
degree). A spherical approximation is used unless PROJ_ELLIPSOID is set to an
actual ellipsoid, in which case we convert latitudes to authalic latitudes before
calculating areas. When degree is selected the areas are given in steradians.

-Nnfile
Write the information pertaining to each polygon. For Delaunay: the three node
number and the triangle area (if -A was set); for Voronoi the unique node lon, lat
and polygon area (if -A was set)) to a separate file. This information is also
encoded in the segment headers of ASCII output files. Required if binary output is
needed.

-Qd|v Append d for Delaunay triangles or v for Voronoi polygons [Delaunay]. If -bo is
used then -N may be used to specify a separate file where the polygon information
normally is written.

-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[level] (more ...)
Select verbosity level [c].

-bi[ncols][t] (more ...)
Select native binary input. [Default is 2 input columns].

-bo[ncols][type] (more ...)
Select native binary output. [Default is same as input].

-d[i|o]nodata (more ...)
Replace input columns that equal nodata with NaN and do the reverse on output.

-h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
Skip or produce header record(s).

-:[i|o] (more ...)
Swap 1st and 2nd column on input and/or output.

-r (more ...)
Set pixel node registration [gridline].

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ or just +
Print an extensive usage (help) message, including the explanation of any
module-specific option (but not the GMT common options), then exits.

-? or no arguments
Print a complete usage (help) message, including the explanation of options, then
exits.

--version
Print GMT version and exit.

--show-datadir
Print full path to GMT share directory and exit.

ASCII FORMAT PRECISION

The ASCII output formats of numerical data are controlled by parameters in your gmt.conf
file. Longitude and latitude are formatted according to FORMAT_GEO_OUT, whereas other
values are formatted according to FORMAT_FLOAT_OUT. 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 FORMAT_FLOAT_OUT setting.

EXAMPLES

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

gmt sphtriangulate testdata.txt -Qv | psxy -Rg -JG30/30/6i -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

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

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