Creating an Overlay Map with GMT

In our previous GMT examples we used a single command to create a simple globe and create an annotated map. In this example we will create a base map and then use the overlay feature of GMT to plot the location of volcanoes throughout the world.

The first step is to create the base map using pscoast:

pscoast -JN0/26 -R-180/180/-90/90 -W -G220/220/220 -S0/192/255 \
-N1 -B60g10 :."World Volcanoes": --PAPER_MEDIA=archA -K > world_volcanoes.eps

Here is a brief summary of the arguments used to create the base map:

(Many of these parameters are discussed in greater detail in the previous GMT posts—see links above.).

Use the Robinson projection centered at 0 degrees longitude. The map will be 26 cm wide
The region of interest is set to the entire globe
Draw coastlines
Fill the dry areas (i.e. land) with RGB color 220,220,220
Fill wet areas with RGB color 0,192,255
Draw national political boundaries
-B60g10 :.”World Volcanoes”:
Place annotation at 60 degree intervals on grid lines 10 degrees apart. Title the map “World Volcanoes”
Set the paper size to archA
More PostScript code will be appended later

The -K allows us to create an overlay by issuing additional commands.

If we were to run pscoast without -K it would give us a nice base map:

Now that we have the base map command complete (don’t forget the -K) we can add our overlay. For this we use a text file containing the locations of the volcanoes. The file format is simply longitude, latitude:


The file doesn’t contain any attribute information about the volcanoes, just the location.

To create the overlay, we use the psxy command:
psxy volcano_xy.txt -JN -O -R -Sc0.15c -G255/0/0 >> world_volcanoes.eps

The arguments are:

The name of the text file containing the locations
The map projection (Robinson). No additional parameters are required
The region of interest. With no parameters it will use those specified in our pscoast command
Set the symbol to a circle (c) with a size of 0.15 cm
Set the color for the symbol to RGB 255,0,0 (red)
The important argument—overlay on the previous map

Notice that we redirect the output using >> to append to our world_volcanoes.eps file.

This gives us the completed map:

With psxy you can also draw line and polygons. Check out the GMT documentation for more details and be sure to see the previous blog posts on creating a simple globe and creating an annotated map.

The volcano locations used in this post are available here.

January 16, 2009 · admin · No Comments
Tags:  · Posted in: GMT