.. _example_28:

(28) Mixing UTM and geographic data sets
----------------------------------------

Next, we present a similar case: We wish to plot a data set given in UTM
coordinates (meter) and want it to be properly registered with overlying
geographic data, such as coastlines or data points. The mistake many
GMT rookies make is to specify the UTM projection with their UTM data.
However, that data have already been projected and is now in linear
meters. The only sensible way to plot such data is with a linear
projection, yielding a UTM map. In this step one can choose to annotate
or tick the map in UTM meters as well. To plot geographic (lon/lat) data
on the same map you simply have to specify the region using the UTM
meters but supply the actual UTM projection parameters. Make sure you
use the same scale with both the linear and UTM projection.

Our script illustrates how we would plot a UTM grid (with coordinates in
meters) of elevations near Kilauea volcano on the Big Island of Hawaii
and overlay geographical information (with longitude, latitude
coordinates). We first lay down the UTM grid using the linear
projection. Then, given we are in UTM zone 5Q, we use the UTM domain and
the UTM projection when overlaying the coastline and light blue ocean.
We do some trickery by converting the UTM domain to km so that we can
add custom annotations to the map. Finally, we place a scale bar and
label Kilauea crater to complete the figure.

.. literalinclude:: /_verbatim/example_28.txt
   :language: bash

.. figure:: /_images/example_28.*
   :width: 500 px
   :align: center

   Mixing UTM and geographic data sets requires knowledge of the map region domain in
   both UTM and lon/lat coordinates and consistent use of the same map scale.
