Mapping Hacks

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Chapter 3, Mapping Your World

#33 Georeference an Arbitrary Tourist Map

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See Also

• “Experiment with Different Cartographic Projections” [Hack #28]

• “Plot Arbitrary Points on a World Map” [Hack #29]

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#33

Georeference an Arbitrary Tourist Map Hack #33

Make any old map tell your story, using a simple JavaScript client to

georeference an image so you can later plot your own points, lines, and

shapes on it.

Back from a journey or a holiday, you can use the crumpled tourist maps

and street atlas pages at the bottom of your handbag or in the back seat of

your car to ell the story of your travels. Digital holiday snapshots add

accents and highlights to your stories; GPS traces provide a narrative struc-

ture for them. To get started, though, you need a base map which is

georeferenced—one for which you know the geospatial position of the top

left corner, the space represented by each pixel in the image, and the rota-

tion or skew of the image away from True North.

How can we georeference a random tourist map? If you know the precise

location of several points on the ground and can identify them on the map,

you can align them to fit on the map. It’s possible to do this by hand, adjust-

ing the projection of the points until they fit, but after we’d done this manu-

ally a few times, we were yearning for an automated solution—so we wrote

one.

Figure 3-38. Our new political map of the world, using NASA’s ...