.. _example_27:

(27) Plotting Sandwell/Smith Mercator img grids
-----------------------------------------------

Next, we show how to plot a data grid that is distributed in projected
form. The gravity and predicted bathymetry grids produced by David
Sandwell and Walter H. F. Smith are not geographical grids but instead
given on a spherical Mercator grid. The GMT supplement imgsrc has
tools to extract subsets of these large grids. If you need to make a
non-Mercator map then you must extract a geographic grid using
:doc:`img2grd </supplements/img/img2grd>` and then plot it using your
desired map projection. However, if you want to make a Mercator map then
you can save time and preserve data quality by avoiding to re-project
the data set twice since it is already in a Mercator projection. This
example shows how this is accomplished. We use the **-M** option in
:doc:`img2grd </supplements/img/img2grd>`\  [1]_ to pull out the grid
in Mercator units (i.e., do *not* invert the Mercator projection) and
then simply plot the grid using a linear projection with a suitable
scale (here 0.25 inches per degrees of longitude). To overlay basemaps
and features that has geographic longitude/latitude coordinates we must
remember two key issues:

#. This is a *spherical* Mercator grid so we must use
   --**PROJ_ELLIPSOID**\ =Sphere with all commands that involve
   projections (or use :doc:`gmtset </gmtset>` to change the setting).

#. Select Mercator projection and use the same scale that was used with
   the linear projection.

This map of the Tasman Sea shows the marine gravity anomalies with land
painted black. A color scale bar was then added to complete the illustration.


.. literalinclude:: /_verbatim/example_27.txt
   :language: bash

.. figure:: /_images/example_27.*
   :width: 500 px
   :align: center

   Plotting Sandwell/Smith Mercator img grids.


.. [1]
   You could also use :doc:`img2grd </supplements/img/img2grd>`
   directly - your only option under DOS