Desktop GIS - the book

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.).

-JN0/26

Use the Robinson projection centered at 0 degrees longitude. The map will be 26 cm wide

-R-180/180/-90/90

The region of interest is set to the entire globe

-W

Draw coastlines

-G220/220/220

Fill the dry areas (i.e. land) with RGB color 220,220,220

-S0/192/255

Fill wet areas with RGB color 0,192,255

-N1

Draw national political boundaries

-B60g10 :."World Volcanoes":

Place annotation at 60 degree intervals on grid lines 10 degrees apart. Title the map "World Volcanoes"

--PAPER_MEDIA=archA

Set the paper size to archA

-K

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:

7.25,27.08
131.6,34.5
-67.62,-23.3
-90.876,14.501
...

To create the overlay, we use the psxy command:

psxy volcano_xy.txt -JN -O -R -Sc0.15c -G255/0/0 >> world_volcanoes.eps

volcano_xy.txt

The name of the text file containing the locations

-JN

The map projection (Robinson). No additional parameters are required

-R

The region of interest. With no parameters it will use those specified in our pscoast command

-Sc0.15c

Set the symbol to a circle (c) with a size of 0.15 cm

-G255/0/0

Set the color for the symbol to RGB 255,0,0 (red)

-O

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.