.. _example_23:

(23) All great-circle paths lead to Rome
----------------------------------------

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 [1]_. This provides the inspiration for our next example which
uses :doc:`grdmath </grdmath>` to calculate distances
from Rome to anywhere on Earth and
:doc:`grdcontour </grdcontour>` 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.

The script produces the plot in Figure; 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 Example
:ref:`example_25`).

.. literalinclude:: /_verbatim/example_23.txt
   :language: bash

.. figure:: /_images/example_23.*
   :width: 500 px
   :align: center

   All great-circle paths lead to Rome.


.. [1]
   Pedants who wish to argue about the "other" arc going the long way
   should consider using it.