Mapping Hacks

Plot a Great Circle on a Flat Map #30

Chapter 3, Mapping Your World

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143

HACK

Figure 3-32 was made using the same GMT commands as Figure 3-31,

except that the

-JM18c parameter for generating a Mercator projection was

replaced with

-JG-60/45/18c, which draws an orthographic projection 18

centimeters wide, centered on 60º W and 45º N.

Great Circles on a Gnomonic Projection

There are other ways to visualize great circles on a flat map, of course. If we

want a map that depicts the shortest distance between two points as a

straight line, we can turn to the Gnomonic projection. Basically, any straight

line on a Gnomonic projection, regardless of origin or bearing, represents a

great-circle arc on Earth’s surface. Although the Gnomonic projection dis-

torts the shape and area of landmasses something fierce, the property of

showing any great-circle arc as a straight line makes it quite useful for the

purposes of air navigation. Figure 3-33 shows the great circle connecting

London and San Francisco on a Gnomonic projection of the world.

Figure 3-33 was generated with GMT in the same fashion as Figures 3-31

and 3-32, using the

-JF-75/60/60/18c option to plot the Gnomonic projec-

tion centered on 75º W and 60º N, with a radius of 60º.

Figure 3-32. Great-circle arc from San Francisco to London on an orthographic

projection