Mapping Hacks

Plot Arbitrary Points on a World Map #29

Chapter 3, Mapping Your World

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139

HACK

Manila Philippines 10232924 14.62° 120.97°

Dilli India 10203687 28.67° 77.21°

...

If you wanted to plot just the 50 largest cities, you couldn’t feed it directly

into plot_points.pl, because (a) you’d get all 7,000 that way and (b) the pop-

ulation figure appears before the latitude and longitude in each line. But you

could feed it through the Unix head(1) and cut(1) commands, and pipe that

into plot_points.pl:

$ head -50 cities.txt | cut -f4,5 | perl plot_points.pl PathfinderMap.jpg

biggest.jpg

If you don’t have such a file of your own, you can get ours from http://

mappinghacks.com/data/cities.txt. The map thus generated looks something

like Figure 3-30. Isn’t the Unix shell neat?

Why Use Any Other Projection, if This One Is So Simple?

Well, sadly, even though the Equidistant Cylindrical projection looks fine at

a global scale, and would even look all right for places near the equator,

where lines of latitude and longitude really are about the same length, all

that stuff near the Poles is pretty badly distorted. On the maps we’ve been

looking at, that doesn’t really matter much, but the Equidistant Cylindrical

projection definitely won’t make for such great maps of smaller regions far-

ther away from the equator.

The sinusoidal projection offers one alternative. Instead of treating latitude

and longitude as equal, distances ...