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<H1><A NAME="SECTION0015230000000000000000"></A>
<A NAME="sec:example_23"></A><A NAME="24078"></A>
<BR>
7.23 All great-circle paths lead to Rome
</H1>
<P>
While motorists recently have started to question the old saying ``all roads lead to Rome'',
aircraft pilots have known from the start that only one great-circle path connects the
points of departure and arrival<A NAME="tex2html1445"
HREF="footnode.html#foot24079"><SUP>7.5</SUP></A>. This provides the inspiration for our next
example which uses <A NAME="tex2html1447"
HREF="../man/grdmath.html"><I><B>grdmath</B></I></A><A NAME="24997"></A> to calculate distances from Rome to anywhere on
Earth and <A NAME="tex2html1448"
HREF="../man/grdcontour.html"><I><B>grdcontour</B></I></A><A NAME="25002"></A> to contour these distances. We pick five cities that
we connect to Rome with great circle arcs, and label these cities with their names and
distances (in km) from Rome, all laid down on top of a beautiful world map. Note that
we specify that contour labels only be placed along the straight map-line connecting
Rome to its antipode, and request curved labels that follows the shape of the contours.
<P>
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<PRE>#!/bin/sh
# GMT EXAMPLE 23
#
# Purpose: Plot distances from Rome and draw shortest paths
# GMT progs: grdmath, grdcontour, pscoast, psxy, pstext, grdtrack
# Unix progs: echo, cat, awk
#
ps=example_23.ps
# Position and name of central point:
lon=12.50
lat=41.99
name="Rome"
# Calculate distances (km) to all points on a global 1x1 grid
grdmath -Rg -I1 $lon $lat SDIST 111.13 MUL = dist.grd
# Location info for 5 other cities + label justification
cat << END > cities.d
105.87 21.02 HANOI LM
282.95 -12.1 LIMA LM
178.42 -18.13 SUVA LM
237.67 47.58 SEATTLE RM
28.20 -25.75 PRETORIA LM
END
pscoast -Rg -JH90/9i -Glightgreen -Sblue -U"Example 23 in Cookbook" -A1000 \
-B0g30:."Distances from $name to the World": -K -Dc -Wthinnest > $ps
grdcontour dist.grd -A1000+v+ukm+kwhite -Glz-/z+ -S8 -C500 -O -K -J \
-Wathin,white -Wcthinnest,white,- >> $ps
# For each of the cities, plot great circle arc to Rome with psxy
while read clon clat city; do
(echo $lon $lat; echo $clon $clat) | psxy -R -J -O -K -Wthickest/red >> $ps
done < cities.d
# Plot red squares at cities and plot names:
psxy -R -J -O -K -Ss0.2 -Gred -Wthinnest cities.d >> $ps
awk '{print $1, $2, 12, 1, 9, $4, $3}' cities.d | pstext -R -J -O -K -Dj0.15/0 -Gred -N >> $ps
# Place a yellow star at Rome
echo "$lon $lat" | psxy -R -J -O -K -Sa0.2i -Gyellow -Wthin >> $ps
# Sample the distance grid at the cities and use the distance in km for labels
grdtrack -Gdist.grd cities.d \
| awk '{printf "%s %s 12 0 1 CT %d\n", $1, $2, int($NF+0.5)}' \
| pstext -R -J -O -D0/-0.2i -N -Wwhite,o -C0.02i/0.02i >> $ps
# Clean up after ourselves:
rm -f cities.d dist.grd .gmt*
</PRE>
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<DIV ALIGN="CENTER"><A NAME="fig:GMT_example_23"></A><A NAME="25008"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 7.23:</STRONG>
All great-circle paths lead to Rome.</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><IMG
WIDTH="518" HEIGHT="308" ALIGN="BOTTOM" BORDER="0"
SRC="img191.png"
ALT="\includegraphics[scale=0.5]{scripts/example_23}"></DIV></TD></TR>
</TABLE>
</DIV>
<P>
The script produces the plot in Figure <A HREF="#fig:GMT_example_23">7.23</A>; note how
interesting the path to Seattle appears in this particular projection (Hammer).
We also note that Rome's antipode lies somewhere near the Chatham plateau (antipodes
will be revisited in Section <A HREF="node145.html#sec:example_25">7.25</A>).
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<ADDRESS>
Paul Wessel
2010-01-14
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