# (35) Spherical triangulation and distance calculations¶

The script produces the plot in Figure. Here we demonstrate how sphtriangulate and sphdistance are used to compute the Delauney and Voronoi information on a sphere, using a decimated GSHHG crude coastline. We show a color image of the distances, highlighted with 500-km contours, and overlay the Voronoi polygons in green. Finally, the continents are placed on top.

```#!/usr/bin/env bash
#               GMT EXAMPLE 35
#
# Purpose:      Illustrate sphtriangulate and sphdistance with GSHHG crude data
# GMT modules:  coast, plot, makecpt, grdimage, grdcontour, sphtriangulate, sphdistance
# Unix progs:   rm
#

# set AWK to awk if undefined
AWK=\${AWK:-awk}

gmt begin ex35
# Get the crude GSHHS data, select GMT format, and decimate to ~20%:
# gshhs \$GMTHOME/src/coast/gshhs/gshhs_c.b | \$AWK '{if (\$1 == ">" || NR%5 == 0) print \$0}' > gshhs_c.txt
# Get Voronoi polygons
gmt sphtriangulate @gshhs_c.txt -Qv -D > tt.pol
# Compute distances in km
gmt sphdistance -Rg -I1 -Qtt.pol -Gtt.nc -Lk
gmt makecpt -Chot -T0/3500
# Make a basic image plot and overlay contours, Voronoi polygons and coastlines
gmt grdimage tt.nc -JG-140/30/18c
gmt grdcontour tt.nc -C500 -A1000+f10p,Helvetica,white -L500 -GL0/90/203/-10,175/60/170/-30,-50/30/220/-5 -Wa0.75p,white -Wc0.25p,white
gmt plot tt.pol -W0.25p,green,.
gmt coast -W1p -Gsteelblue -A0/1/1 -B30g30 -B+t"Distances from GSHHG crude coastlines"
# cleanup
rm -f tt.pol tt.nc
gmt end show
```