From Randall Munroe, what map projections say about you.

From Randall Munroe, what map projections say about you.

I type in Dvorak and am wearing a pair of Vibram FiveFingers right now, so I guess I should prefer the Dymaxion. (And indeed, I’ve always had a soft spot for all things Buckminster Fuller.)

Someone asked me why Google Maps and Google Earth use “such stupid” map projections. Well, they don’t, actually—for their purposes, they both use exactly the right projection. Google Maps uses Mercator (or close enough); yeah, it’s not a very good projection for comparing the relative size of tropical and polar areas on global maps (“it makes Greenland look bigger than Africa!”, people like to point out), but most people don’t use Google Maps that way.

For what most people use it for—navigation and getting bearings for places they’re going to—the “conformal” aspect of Mercator, such that an angle on the map matches an angle in real life, is much more important than having equal areas at a continental scale. It’s hard to find city-level maps, especially in higher latitudes, in any global projection other than Mercator (because it looks so wrong!), but if you can find one of, say, Anchorage, and do a comparison, you’ll see what I mean—the perpendicular intersections of the street grid are shown as askew.

Google Earth, on the other hand, uses two projections—internally it uses an equirectangular format (a “plate carrée”) that has the nice feature of a pixel in the digital map being directly translatable to a latitude and longitude. For displaying to the user, it uses perspective projection, which is what you learned in art class—it uses sight lines extended to a vanishing point to translate the 3D object of the globe into the 2D canvas of the screen. This projection fails in virtually every category cartography geeks like to compare projections by; but it looks like what you’d actually see if you were floating in a balloon or in orbit, so for the purpose—exploration of our Earth—it’s the right choice, too. (One thing it can’t do is show the entire Earth at once; a hemisphere is all you get.)

(I’m not speaking for the Maps or Earth teams, officially or un-.)

http://xkcd.com/977/