Happy Birthday Mercator

I recently read a Tweet celebrating Gerhardus Mercator's 500th birthday. This got me thinking about how Mercator has influenced our work on the Digital Coast. Not a subject that many people stay up late contemplating, I know, but stay with me. Even those of you who have no idea of the significance of a map projection have been greatly influenced by Mercator. The ability to take an almost spherical earth and place it on a flat surface such as a paper map or a computer screen wouldn't be possible without the work of Mercator and cartographers like him. Today, when we use or download data, we take for granted that everything lines up. It's magic, right?

Viewing the World As a Smashed Orange

Courtesy of Michael Kline, Dogfoose.com

I remember clearly the first time a professor demonstrated the map projection concept using a smashed orange peel. That aha moment when you realize that of course it has to work that way, and how could I have been so blind not to see it before? I have to admit that I was plenty scared when I saw the math calculations behind this map projection "slight of hand." Lucky for you, Digital Coast and your GIS software work the magic for you. No need to do any math problems, unless you really want to.

All flat maps distort reality. They can show one or more, but never all four:

  • true direction
  • true distance
  • true shape
  • true area

Does a 500 Year-Old Cartographer Really Matter?

Most online mapping sources in Digital Coast and elsewhere (Bing Maps, Google Maps, and Esri ArcGIS Online) have standardized their services on the Web Mercator coordinate system. Mercator projections are optimized for direction, which is ├╝ber important in navigation, and the main mapping issue of the day for Mercator.

But, in a Mercator projection, maps far from the equator are distorted because linear scale becomes infinitely high at the poles. Take a look at Greenland or Antarctica and see how stretched out they are. A good example of this distortion was probably imprinted early in your school-age brain. The Mercator world map that was in most U.S. classrooms showed Russia and the Warsaw Pact countries as large and menacing.

Look at those countries again in a different projection that holds area constant. Russia doesn't look quite so menacing when it's true area is presented.

If you need simple map and directions to Joe's Pool Hall, Web Mercator-based maps are just fine. At a street-level scale, the distortion isn't apparent. But, let's say you actually want to measure distance or area. Those numbers can't be calculated accurately on a Web Mercator map unless your program is doing some tricks behind the scenes to crunch the numbers.

So the take away message is that the world is round like an orange, not flat like a computer screen. Every map has distortion, so choose your errors wisely. To make it flat, you need math in the form of a map projection, like one from our birthday friend, Mercator. Most Web maps are optimized for direction, not shape, area, or distance calculations.

What else do I not know about Web Mercator projections that I should? Leave me a comment.


USGS Map Projections
University of North Carolina Learn NC
Microsoft Bing Map Projection
Web Mercator: Non-conformal, Non-Mercator by Noel Zinn

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