.. _example_02:

(2) Image presentations
-----------------------

As our second example we will demonstrate how to make color images from
gridded data sets (again, we will defer the actual making of grid files
to later examples). We will use :doc:`grdraster </grdraster>` to extract 2-D grid files
of bathymetry and Geosat geoid heights and put the two images on the
same page. The region of interest is the Hawaiian islands, and due to
the oblique trend of the island chain we prefer to rotate our
geographical data sets using an oblique Mercator projection defined by
the hotspot pole at (68W, 69N). We choose the point (190, 25.5) to be
the center of our projection (e.g., the local origin), and we want to
image a rectangular region defined by the longitudes and latitudes of
the lower left and upper right corner of region. In our case we choose
(160, 20) and (220, 30) as the corners. We use
:doc:`grdimage </grdimage>` to make the illustration:

.. literalinclude:: /_verbatim/example_02.txt
   :language: bash

The first step extracts the 2-D data sets from the local data base using
:doc:`grdraster </grdraster>` that may be adapted to reflect
the nature of your data base format. It automatically figures out the
required extent of the region given the two corners points and the
projection. The extreme meridians and parallels enclosing the oblique
region is **-R**\ 159:50/220:10/3:10/47:35. This is the area extracted
by :doc:`grdraster </grdraster>`. For your convenience
we have commented out those lines and provided the two extracted files
so you do not need :doc:`grdraster </grdraster>` to try
this example. By using the embedded grid file format mechanism we saved
the topography using kilometers as the data unit. We now have two grid
files with bathymetry and geoid heights, respectively. We use
:doc:`makecpt </makecpt>` to generate a linear color
palette file ``geoid.cpt`` for the geoid and use
:doc:`grd2cpt </grd2cpt>` to get a histogram-equalized
cpt file ``topo.cpt`` for the topography data. To emphasize the structures in the
data we calculate the slopes in the north-south direction using
:doc:`grdgradient </grdgradient>`; these will be used to
modulate the color image. Next we run
:doc:`grdimage </grdimage>` to create a color-code image
of the Geosat geoid heights, and draw a color legend to the right of the
image with :doc:`psscale </psscale>`. Similarly, we run
:doc:`grdimage </grdimage>` but specify **-Y**\ 4.5i to
plot above the previous image. Adding scale and label the two plots a)
and b) completes the illustration.

.. figure:: /_images/example_02.*
   :width: 500 px
   :align: center

   Color images from gridded data.
