Mapping Hacks

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Chapter 3, Mapping Your World

#29 Plot Arbitrary Points on a World Map

HACK

Note that there’s nothing particularly Perl-ish about our script. The same

technique should be easy to implement in any language. In fact, in “Plot

Points on an Interactive Map Using DHTML”

[Hack #41], we’ll show off the

exact same technique in JavaScript, with a few added wrinkles.

Running the Code

Here’s a sample run of plot_points.pl, run on the Pathfinder image from the

flatplanet gallery and fed points for London, New York, and Tokyo. You can

hit Ctrl-D after you’re done entering points by hand. Figure 3-29 shows the

result:

$ perl plot_points.pl PathfinderMap.jpg three_cities.jpg

51 0

40 -74

35 140

Of course, if you have a file full of points, you can just use shell redirection

to dump the list right into the script. If the file contains other things, such as

labels for each point, that’s okay, so long as the first two decimal numbers

on each line are the latitude and longitude, respectively. For example, sup-

pose you have a tab-separated file of the world’s 7,000 largest cities, sorted

by population, in a file called cities.txt:

place name country current population latitude longitude

Mumbai India 12383146 18.96° 72.82°

Buenos Aires Argentina 12116379 -34.61° -58.37°

"Karāchi" Pakistan 10537226 24.86° 67.01°

Figure 3-29. London, New York, and Tokyo, plotted on an Equidistant Cylindrical world

map