.. _example_24:

(24) Data selection based on geospatial criteria
------------------------------------------------

Although we are not seismologists, we have yet another example involving
seismicity. We use seismicity data for the Australia/New Zealand region
to demonstrate how we can extract subsets of data using geospatial
criteria. In particular, we wish to plot the epicenters given in the
file ``oz_quakes.d`` as red or green circles. Green circles should only be used for
epicenters that satisfy the following three criteria:

#. They are located in the ocean and not on land

#. They are within 3000 km of Hobart

#. They are more than 1000 km away from the International Dateline

All remaining earthquakes should be plotted in red. Rather that doing
the selection process twice we simply plot all quakes as red circles and
then replot those that pass our criteria. Most of the work here is done
by :doc:`gmtselect </gmtselect>`; the rest is carried
out by the usual :doc:`coast </coast>` and
:doc:`plot </plot>` workhorses. Note for our purposes
the Dateline is just a line along the 180 meridian.

The script produces the plot in Figure. Note
that the horizontal distance from the dateline seems to increase as we
go south; however that is just the projected distance (Mercator
distortion) and not the actual distance which remains constant at 1000 km.

.. literalinclude:: /_verbatim/example_24.txt
   :language: bash

.. figure:: /_images/example_24.*
   :width: 500 px
   :align: center

   Data selection based on geospatial criteria.
