.. _example_17:

(17) Images clipped by coastlines
---------------------------------

This example demonstrates how :doc:`coast </coast>`
can be used to set up clip paths based on coastlines. This approach is
well suited when different gridded data sets are to be merged on a plot
using different CPTs. Merging the files themselves may
not be doable since they may represent different data sets, as we show
in this example. Here, we lay down a color map of the geoid field near
India with :doc:`grdimage </grdimage>`, use
:doc:`coast </coast>` to set up land clip paths, and
then overlay topography from the ETOPO5 data set with another call to
:doc:`grdimage </grdimage>`. We finally undo the
clippath with a second call to :doc:`coast </coast>`
with the option **-Q** (See the Figure :ref:`fig_ex17`):

We also plot a color legend on top of the land. So here we basically
have three layers of "paint" stacked on top of each other: the
underlaying geoid map, the land mask, and finally the color legend. This
legend makes clear how :doc:`grd2cpt </grd2cpt>`
distributed the colors over the range: they are not of equal length put
are associated with equal amounts of area in the plot. Since the high
amounts (in red) are not very prevalent, that color spans a long range.

For this image it is appropriate to use the **-I** option in
:doc:`colorbar </colorbar>` so the legend gets shaded,
similar to the geoid grid. See Appendix [app:M] to learn more about
CPTs and ways to draw color legends.

.. literalinclude:: /_verbatim/example_17.txt
   :language: bash

.. _fig_ex17:

.. figure:: /_images/example_17.*
   :width: 500 px
   :align: center

   Clipping of images using coastlines
