Using Command-Line Tools - Desktop GIS: Mapping the Planet with Open Source Tools - page 170

you want, but if there is a standard for the area you are mapping, you
should use it.
The remaining part of the -Jb switch is the size of the output. In this
case, we specified it as a scale of 1:12,000,000. This means that one
unit on the map represents 12,000,000 units on the ground (in this
case meters). If we just wanted output to fit on a page, we could specify
-JB-154/50/55/65/6.0i to get a 6-inch-wide image.
Map Extent
To set the map extent, we use the -R switch. In this case we already
determined that Alaska ranges from 172 to 230 degrees longitude and
roughly 51 to 72 degrees north latitude. To create the map covering this
area, we use -R172/230/51/72 .
Grid Lines
In this example, we not only want to draw grid lines but also want to
annotate them. This is done using -B10g5/5g5 . This tells pscoast to draw
grid lines 5 degrees apart for both latitude and longitude. The annota-
tion is drawn at 10 degree intervals for longitude and 5 degree intervals
for latitude. If you look at the documentation for pscoast , you will see
that the first number after the -B is the annotation interval followed
by the grid line interval. This notation gives you a lot of flexibility in
drawing and labeling gridlines.
Rivers
To make our map more interesting, we'll add rivers to it. GMT comes
with several levels of river detail that are specified with the -I switch.
The levels we are using are as follows:
• Permanent major rivers
• Additional major rivers
• Additional rivers
• Minor rivers
Note the -I options we specified in the pscoast command. One is required
for each river level we want to include on the map. The first two (major
rivers) are drawn using a pen width of 2 (2p), while the third and
fourth level are drawn with a width of 1 (1p). We use the same color
(0/192/255) for each river. If we wanted to include the intermittent
major rivers (fifth level), we would add -I5/1p/0/192/255 to the pscoast
command.