Using Command-Line Tools - Desktop GIS: Mapping the Planet with Open Source Tools - page 168

Joe Asks. . .
Where Does the GMT Data Come From?
GMT actually provides a number of datasets in five resolu-
tions ranging from fine to crude. When installing GMT, you can
choose which resolutions you want to include.
There are also tools available to convert other formats for use
with GMT. Use your favorite search engine to find tools applica-
ble to your situation.
Let's take a look at the other switches used to create the globe. The -B
switch defines the intervals for the boundary tick marks. In the globe
case, these are the lines of longitude and latitude. The arguments to the
-B switch indicate a gridline spacing of 30 degrees in the x (longitude)
direction and 15 degrees in the y (latitude) direction. Note how the x
and y settings are separated by a forward slash.
The -D switch selects the resolution of the dataset used in creating the
globe. The available choices are f , h , i , l , and c , which correspond to full,
high, intermediate, low, and crude. Some of these options may not be
available to you if you didn't install all the data sets with GMT. For the
globe, we used the low resolution data set.
To control the display of features, the -A switch allows you to specify
that features below a certain size not be drawn. In our example, we
specified that features with an area greater smaller 2,000 square kilo-
meters should not be displayed.
The fill color used for the countries is specified using the -G switch. The
color can be specified using RGB notation, a shade of gray, or a pattern.
In the globe, we used 187/142/46 to create a light brown color. We could
have specified a fill pattern using -Gp100/30 . This fills the land masses
with pattern number 30 at a resolution of 100 dpi. If we want to get the
highest possible resolution for the pattern, we can use a resolution of
0. Specifying -GP inverts the pattern. GMT has 90 predefined patterns
available for your use, and you can find examples of each in the GMT
Technical Reference. The same options apply for filling the water areas
in GMT, except we use the -S switch. There are a number of variations
for specifying fill colors, and these are well documented in the GMT
manuals and tutorial.