On How to Read a (Good) Map

January 27, 2014

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Just as you shouldn’t trust everything you read or see on television, you should never blindly trust information just because it is on a map.

All maps posit arguments. Maps present information about how something is. Just as there are no unbiased arguments, there are no unbiased maps. Someone has decided which data to collect. A cartographer, or team of cartographers, has decided how to abstract the data and make it information. Moreover, they have decided how to present and highlight certain elements of a map over others. All of these decisions result in a map that is not objective or neutral. It is designed. What this means is that if the data are false or poor, the message of the map will not be accurate. When certain data sets are abstracted out of the map, you may be missing some key information that might impact how you interpret an environment. (A prime example of this is sidewalks. Why aren’t sidewalks included on most city maps? Surely they are just as important in many cases as roads for getting from point A to point B.)

Sometimes designers highlight certain map elements over others to get you to see a geographical issue or controversy from a particular light. So never trust a map blindly. In sum, if you are going to trust a map, make sure you take the time to critique and verify its usefulness.

Read on for my suggestions on how to approach a map you encounter for the first time and how to find the signs of good mapmaking, as in this great NASA map of the California Rim Fire.

Credit: Robert Simmon, NASA Earth Observatory.

Whenever you are critiquing a map you need to ask yourself four things:

  1. Who made the map?
  2. What is the purpose of the map? That is, what is the map attempting to communicate?
  3. Who is the intended audience? (It is important to remember that the map may not have been designed for you, but a more specialized audience.)
  4. Does the map effectively achieve its communication goals? Does it present an interesting story or argument?

This map by NASA Earth Observatory is a stellar example of map abstraction and effective design. The map accompanied a brief article about the rim fire that engulfed parts of Yosemite National Park in late summer of 2013. (Top: False-color image of the Rim Fire burning on August 30, 2013 as captured by the ASTER instrument on NASA’s Terra satellite. Credit: Jesse Allen, NASA Earth Observatory.) 

The purpose of the map is fairly clear: to give map readers an idea of just how extensive this fire is and how quickly it spread from its inception. The intended audience is the United States public, as NASA is a government-funded institution. So how well does the map achieve its communicative goals?

On its own, without the accompanying article, you can still get information from the map, which is a good sign. Notice how the colors imply heat – yellow to red. Thus, even if you didn’t know this was a map of fire, you would get a sense that it was not mapping rainfall or hydration. The colors used are also very intuitive. The cartographer knew that humans tend to see order in yellow-to-red color variation. We naturally presume that yellow occurs before red. This is used to magnificent effect here. Without even looking at the legend you can see that the map is showing a progression of some sort. Finally, by drawing the boundary of Yosemite National Park, one begins to understand the sheer scale and rapid spread of this fire. It shows the danger that the fire posed to one of the nation’s most well-known national parks.

Tying this back to the definition of a map – abstraction of a spatial environment – we can see that this map is not a true representation of reality. Take a second to think about what is missing from this map, what has been removed to make the information clearer for the map reader. Elevation is shown, as are major water bodies. Yet, roads and trails are missing. Homes and built-up areas are also absent. In fact, signs of life are conspicuously absent on this map. They have been removed to help more clearly communicate the main story of the map – the spread of the fire. The map does not begin to highlight the social, human, and animal suffering affiliated with this fire. This is not a fault with the map, as it was not the goal to present such information to the map reader. However, another cartographer making a map of this event might highlight these aspects instead of the physical geography. Unlike in math, in cartography there are not right or wrong ways of doing things; there are only good and bad decisions. For the intended purpose of this map, many good decisions were made.

Reprinted from NASA Earth Science Week Blog.


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