Mapping With Drupal - Alan Palazzolo [7]
For example, the Mercator projection has the property of preserving shapes and directions. However, it does this at the cost of enlarging all lands the further they are from the equator. Many people therefore get the impression that Greenland is about the same size as South America. Developing nations tend to be close to the equator, and their inhabitants often resent the Mercator projection, which makes Europe and North America appear much more significant in relative area than they are. Some of these objectors prefer the Equal-Area or Gall-Peters Projections, which are two different ways of representing the Earth that maintain the correct areas; the trade-off being distorting shapes—Iceland, for instance, looks like a stretched-out lens. Depending on what your map wants to communicate, it is important to choose a projection that will help your message. It is impossible to say what projection is best for your case, but it is important to consider what the projection communicates. No projection will be perfect, but one could be more appropriate for your audience than another.
In web mapping you are relying on the maps provided by Google, Bing, OpenStreetMap, or a similar service, and as a consequence there are only one or two projections available to use. The Spherical Mercator projection is the most common web mapping projection. This is probably what you will be using for your maps, but keep in mind the effect that the projection has on your map. If you need to map Antarctica, the Mercator projection assigns this continent an infinite area!
Some different map projections of the Earth are shown in Figure 2-1, Figure 2-2, Figure 2-3, and Figure 2-4. These are just a sample of projections to illustrate the impact that this choice has on a map.
Figure 2-1. A Spherical Mercator projection of NASA’s Blue Marble satellite imagery. Image from Learn NC
Figure 2-2. A Gall-Peters projection of NASA’s Blue Marble satellite imagery (image from Wikipedia)
Figure 2-3. An Ecker IV projection (image from Wikipedia)
Figure 2-4. A Mollweide projection (image from Wikipedia)
Spherical Mercator
The Mercator projection is one of the most common projections. A variant of it, Spherical Mercator, is used by all major web-based maps (including Google, Bing, MapQuest, and OpenStreetMap). It makes the assumption that the Earth is a perfect sphere (though it is not). As shown in the illustration (Figure 2-5), the method of flattening the globe is to spread out the globe onto a cylinder. Imagine a translucent world with a very bright lightbulb at the exact center and its image projected onto the cylinder. This means that areas close to the equator are fairly accurate, but as you get further toward the poles, the areas are very enlarged. This illustrates the problem with all map projections: it is impossible to have all four aspects of a map be accurate (direction, distance, area, shape). In fact, you can’t get more than two. Mercator is good for direction and shape.
Figure 2-5. Mercator Projection, courtesy of the U.S. Geological Survey
NOTE
If you are working with geographic data sets, you may come across numbers that refer to their data projection. Projections have been given identification numbers by the European Petroleum Survey Group (EPSG; now the International Association of Oil & Gas Producers). These identification numbers are often used by different tools to designate projections. The Spherical Mercator projection, temporarily given EPSG:900913, is now officially EPSG:3857. (If you are familiar with leet speak, you would notice that 900913 looks like the word Google). Another common projection is the projection that translates to latitude and longitude, which is EPSG:4326.
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