Slippy Map Projections Explained | GISC

Recently, Dr. Sarah Battersby from the University of South Carolina gave a talk on Web Mercator as the UW Geography Department’s weekly Yi-Fu Tuan lecture. Dr. Battersby broached an important concern–that Web Mercator could in fact be reshaping the way that young people picture the Earth, reverting society’s default mental image of the world to one that was held before geographers succeeded in making print publishers self-conscious of Greenland looking bigger than Africa. She further argued that Web Mercator is useful for placing markers on a map, but we are using it in ways that go way beyond what it was originally intended for (three words–Google Maps choropleths). Battersby’s talk actually clarified a question I have had in my mind for a while: why can’t we just make tile sets with other projections? The answer I’m sure is obvious to the experienced web GIS programmer or strongly mathematically-minded person, but my brain took months to process it, so no doubt there are other noobs like me who don’t quite get what all the fuss is about. Hence this blog entry, my attempt at a basic explanation of the problems faced in building a slippy map alternative to EPSG:900913. First, the problem. Your computer screen is flat. The world ain’t. A projection is how we get a picture of the round Earth onto a flat piece of paper or collection of pixels on a monitor. Every projection distorts sizes, angles, and/or distances. The projection that looks most like the real deal is of course a globe, such as found in Google Earth. But an interactive globe has a few problems. For one, it relies on WebGL technology, which isn’t quite mature yet. For another, it heavily distorts distances and angles. And if you want to see the whole surface of the planet at once, you’re going to need a 2-D projection. A Mercator projection—what Google Maps sort of uses (we’ll get to that “sort of” in a minute)—happens when you roll up a piece of paper around a globe, touching it along the whole length of the equator, and flatten the globe onto it. This is called a cylindrical projection, because when the paper is rolled up around the globe, it looks like a cylinder. Cylindrical projections have a key advantage in that all meridians (longitude lines) are straight. Mercator works really great if you’re, say, Ferdinand Magellan looking for a compass bearing that will take you around Cape Horn, because all of the latitude and longitude lines and angles in between lay out nice and straight on the map like we experience them in real life. It also works well if you’re Google and you want a map image that you can neatly slice up into little squares that your server sends to a customer’s browser. North is always up, your hometown doesn’t look squished or slanted when you zoom in to it, and everybody’s happy. Well, not everybody. The trade-off is that since the real earth curves in toward the polls, in order to get those straight lines, you have to stretch and distort the surface more and more as you get closer to penguins or polar bears. The poles themselves are impossible to see, because as you approach them, the distance between latitude lines stretches out toward infinity. Source.

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