.. _example_18:

(18) Volumes and Spatial Selections
-----------------------------------

To demonstrate potential usage of the programs
:doc:`gmtspatial </gmtspatial>`, :doc:`grdvolume </grdvolume>` and
:doc:`gmtselect </gmtselect>` we extract a subset of the
Sandwell & Smith altimetric gravity field [1]_ for the northern Pacific
and decide to isolate all seamounts that (1) exceed 50 mGal in amplitude
and (2) are within 200 km of the Pratt seamount. We do this by dumping
the 50 mGal contours to disk, then use gmtspatial that
returns the mean location of the points making up each closed polygon,
and then pass these locations to
:doc:`gmtselect </gmtselect>`, which retains only the
points within 200 km of Pratt. We then mask out all the data outside
this radius and use :doc:`grdvolume </grdvolume>` to
determine the combined area and (gravimetric) volumes of the chosen seamounts. Our
illustration is presented in Figure.

.. literalinclude:: /_verbatim/example_18.txt
   :language: bash

.. figure:: /_images/example_18.*
   :width: 500 px
   :align: center

   Volumes and geo-spatial selections.


.. [1]
   See http://topex.ucsd.edu/marine_grav/mar_grav.html.