Mapping Hacks

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Chapter 3, Mapping Your World

#30 Plot a Great Circle on a Flat Map

HACK

The following commands, using pscoast and psxy from GMT, were used to

generate Figure 3-31:

$ pscoast -JM18c -R-170/190/-75/85 -Bg30/g15 -A5000 -G192/192/192 -K > \

mercator.ps

$ psxy points.txt -JM18c -R-170/190/-75/85 -W8 -O -K >> mercator.ps

$ psxy points.txt -JM18c -R-170/190/-75/85 -Sa.75c -G0/0/0 -O >> mercator.ps

The call to pscoast draws the graticule (i.e., grid lines) and the base map of

the continents. We recommend reviewing

[Hack #28] to understand exactly

how these particular pscoast options do their magic. The first call to psxy

actually draws the great-circle arc into the same file. We give psxy the same

projection parameters we did pscoast, along with a filename, points.txt. The

points.txt file simply contains the following:

-122,38

0,51

The geographically savvy reader will recognize these as the longitude and

latitude coordinates of San Francisco and London, respectively. The

-W8

option to psxy tells it to make the great-circle arc 8 pixels thick. Finally, we

call psxy one more time, with the same projection parameters and filename

containing our points, but this time we give the

-S option to request that

symbols be drawn at each point, instead of a line connecting them. In this

case, the

-Sa.75c option draws us a star .75 centimeters wide at each end

point, coloring each one black via the

-G option.

The shortest ...