.. _example_01:

(1) Contour maps
----------------

We want to create two contour maps of the low order geoid using the
Hammer equal area projection. Our gridded data file is called ``osu91a1f_16.nc`` and
contains a global 1 by 1 gridded geoid (we will see how to make gridded
files later). We would like to show one map centered on Greenwich and
one centered on the dateline. Positive contours should be drawn with a
solid pen and negative contours with a dashed pen. Annotations should
occur for every 50 m contour level, and both contour maps should show
the continents in light brown in the background. This is how it is done:

.. literalinclude:: /_verbatim/ex01.txt
   :language: bash

The first command sets up a 2 by 1 :doc:`subplot </subplot>` layout. The subplot determines
the size of what map can fit so we use ? when specifying map widths in the
commands below.  This initial setup is followed by
two sequences of :doc:`coast </coast>`, :doc:`grdcontour </grdcontour>`,
:doc:`grdcontour </grdcontour>`. They differ in that the
first is centered on the dateline, while the second on Greenwich. We use the
limit option (**-L**) in :doc:`grdcontour </grdcontour>`
to select negative contours only and plot those with a dashed pen, then
positive contours only and draw with a solid pen [Default]. The **-T**
option causes tick marks pointing in the downhill direction to be drawn
on the innermost, closed contours. For the upper panel we also added -
and + to the local lows and highs. The labeling of the two plots with a)
and b) is automatically done by :doc:`subplot </subplot>`. You can find this illustration as

.. figure:: /_images/ex01.*
   :width: 500 px
   :align: center

   Contour maps of gridded data.
