Choosing a Map Projection

Choosing which projection to use is an important step in making a map, yet few people seen to spend much time on it. The Earth is a sphere and paper is flat, so any representation of the planet has to distort something. Distances, shapes, or areas: what gets distorted depends on what you want to use the map for, and you have to consciously decide what compromises you want to make.

Regardless, rectangular world maps are generally bad. The traditional Mercator projection (left) which radically distorts sizes, and the Peters projection (right), which distorts shapes and distances, should both be avoided. The Mercator is designed for sailors plotting a compass bearing, but isn’t much good for anything else. Because it overemphasizes polar area, it has to be cropped to not look silly, and the Southern hemisphere usually comes off worse in the cropping because, well, there’s less stuff down there (it’s really not an insidious plot by European mapmakers). The Peters projection is a political rebuttal to the Mercator, but in keeping relative land area equal it sacrifices everything else. Peters didn’t invent equal-area maps; there are many different one, going back to the 19th century, and the Peters version is by no means the best, though it’s certainly the most controversial; see the discussion in Monmonier (2004).

mercator_peters.gif

If you allow the poles to contract a little, and depart from a rectangle, you distort relative sizes far less. The Robinson projection (left) was adopted by the National Geographic as a good general-purpose world map, and later replaced by the Winkel Tripel projection (right). For most of us, as long as you avoid rectangles, it doesn’t make much difference which projection you use (there are plenty more available, all different solutions to fitting a sphere on a page).

robinson_winkel.jpg

Here’s another one. The Mollweide is one of a number of projections that preserves relative area, but in the process distorts shape. One way to reduce the distortion is to interrupt it at the oceans (assuming, of course, you’re not interested in oceans). These orange-peel interrupted maps come in many different variations, and should be used more widely. I’ve never seen one for marine biologists, carving up land masses so as to accurately portray the oceans, but it must exist.

Mollweide-interrupted.gif

If instead of portraying the whole planet you only want to focus on one area, a planar projection might be better. The usual ones we see are the round maps of the North or South poles, but they can be centered on any part of the world; they’re most accurate in the middle and become progressively more distorted as you reach the edges. In the past I’ve used Antarctic planar projections to depict the breakup of Gondwana and the circumpolar distribution of the ratites. If you’re buying a set of maps, make sure it comes with a good variety of planar projections (usually just categorized as “globes”), including one centered on your area of interest.

globes.gif

References: Map projections are of course well covered in Wikipedia and a little more accessibly on this page. Also check out Flattening the Earth: Two Thousand Years of Map Projections by John P. Snyder (University of Chicago Press, 1997 new ed.), Rhumb Lines and Map Wars: a Social History of the Mercator Projection by Mark Monmonier (University of Chicago Press, 2004), and XKCD on what your map projection says about you.



2 responses to “Choosing a Map Projection”

  1. Xan Gregg says:

    Not that I especially want to defend Mercator, but it is useful in one other situation: when you’re zoomed in very far, such as near city level. Then Mercator properly accounts for the difference in latitude degrees and longitude degrees so that, for instance, SW-NE roads appear at 45 degrees. Of course, many other transformations, such as Albers, have that property but Mercator is computationally simpler since it’s rectilinear. Notably, WGS-84 (aka Plate Carree aka equirectangular) does not have that property.

  2. Bob D says:

    Here’s one used by National Geographic “carving up land masses”.

    http://education.nationalgeographic.com/education/media/selecting-map-projection/?ar_a=1

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