2007/08/maps_for_scientists_choosing_1.html

Maps for Scientists: Choosing

If you’re creating a map, you’re best to start with an illustration in vector, not bitmap, format (unless of course it’s an aerial photograph)—you want to be able to scale it, make global changes to simple color schemes, hide and show layers containing place names, distributions, and the like, and keep the file size relatively small and the resolution high. All this is what vector graphics are designed for.

Most vector maps commercially available will be EPS files, though PDF is becoming more common; Illustrator can read them both. There are any number of map art vendors on the web; Map Resources and Digital Vector Maps are typical. Expect to pay $50 or more for a single map, and several hundred dollars for a CD with a selection of them. It’s worthwhile for a whole department or lab to invest in a full set of countries of the world, plus globes in a variety of projections, but this is often too expensive for an individual. There are very few places where you can get inexpensive quality vector maps online; here are two.

National Geographic’s website gives away world and country map, with placenames or just plain vanilla, in PDF format. Illustrator can convert these and open them, though you probably don’t own the fancy fonts that National Geographic uses, so it will substitute Arial or somesuch. It’s not too difficult to select and edit the fonts, though. The maps are plainly marked as copyrighted, though modifying them for non-commercial purposes would seem to be allowed under Fair Use provisions. They’re being given away explicitly for educational use, after all.

iStockPhoto has an enormous amount of artwork available very cheaply; a basic cartography search brings up a bunch of globes, and you can search more specifically for Mollweide and so on. Most cost around a buck. This is an excellent site for quickly finding quality images of any kind, especially generic advertising photography useful for illustrating brochures and blog postings; it’s worth bookmarking and buying a bunch of credits just in case you need a cheap photo in a hurry.

 

The next big decision is choosing an appropriate map projection. 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.

Regardless, rectangular world maps are generally bad. The traditional Mercator projection (which radically distorts sizes), and the Peters projection (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).

If you allow the poles to contract a little, and depart from a rectangle, you distort relative sizes far less. The Robinson projection was adopted by the National Geographic as a good general-purpose world map, and later replaced by the Winkel Tripel projection. 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).

 

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.) and Rhumb Lines and Map Wars: a Social History of the Mercator Projection by Mark Monmonier (University of Chicago Press, 2004).