Posted on September 12th, 2013

G-FAQ – Creating Maps for Color Blind People, Part II

Continuing with the theme started in last month’s Geospatial Frequently Asked Question (G-FAQ), I turn my attention to practical recommendations that can make your map design more appropriate for the color blind. In the August G-FAQ edition, we found out that nearly 1 in 20 people are impacted by some form of color blindness so this is a topic all cartographers need to incorporate in their map designs. In this edition of G-FAQ, I offer a set of recommendations for color blind map design and also suggest several color schemes based on a cumulative review of research on the topic.

As a quick recap, this two-part series on maps for the color blind addresses this set of core questions:

How does the human eye see color? What causes color blindness? What are the types of color blindness and how prevalent are each? How can I create maps that can be used by the color blind? What color combinations are appropriate for these maps?

The tips that follow for designing color blind-friendly maps have been gathered from a wide variety of resources on this topic as well as from my own personal experience. In order to make these recommendations more manageable, I divided the list into four broad categories: (1) tips for overall map design; (2) tips for map elements; (3) tips for color choices; and (4) tips specific to online maps.

Tips for Overall Map Design

This set of recommendations are high-level conceptual design ideas that should be kept in the back of your mind when you make maps that are color blind friendly. As such, these tips should be taken in combination with the more specific recommendations offered in the three sections that follow.

  1. First and foremost, never use color alone to indicate anything important on your map. Even if designing your map to be used by color blind individuals is not a goal, this is a consideration we should all bear in mind as you never know the lighting your map will be viewed in and if it will be reproduced as black and white only. There are many other ways to indicate significant map elements than color alone – in fact, I offer multiple suggestions below!
  2. Test your map to see if the color scheme and symbols you choose can be seen by color blind individuals. There are numerous tool out there for this purpose, for instance Color Oracle which I discuss here. Another quick test is to convert your map to black and white and see if the key information is still portrayed as intended.
  3. If you use color to portray any information on its own, then bear in mind that no single map will address every form of color blindness as there are far too many variations. The only way around this is to avoid portraying any critical information on your map with color only.

Tips for Designing Map Elements

Now that you have some tips for overall map design, let’s focus on suggestions for map elements. A map element is any of the lines, dots, symbols, icons, shaded regions, etc. that are displayed in your final graphic.

  1. The most important suggestion here is simply a restatement of Tip #1 above, never use color only to represent important map elements. Crucial map elements should be obvious even if the map was black and white.
  2. Incorporate recognizable icons/symbols to represent crucial map elements. For instance, if you are trying to show the location of a police department, you might represent them with a police badge. Or the location of escalators might be represented as a stair symbol rather than a dot only.
  3. If you are going to use dots or points to represent multiple map elements, vary the shape of the dot not just its color. You can use triangles, squares, crosses, stars, etc. for dots, not just ‘traditional’ circles. Varying dot size implies magnitude so it is not suggested.
  4. Similarly, if you use lines in your map, vary the line pattern by adding dashes, dots, etc. Varying line width also implies magnitude so it is not suggested.
  5. For a polygon or shaded area, you can add patterns or symbols to the inside to help to distinguish your various classes. Crosses, dots, squiggly lines, etc. are all excellent choices to add a visual clue besides color to your map classes.
  6. Consider annotating key map elements with labels. This helps the color blind and is also a good suggestion when your map contains a lot of information, different classes of data and/or multiple layers, as a crowded map can be hard to read otherwise.

Tips for Choosing the Color of Map Elements

Now that I have discussed some ways to differentiate map elements other than color, let’s look at some ways you can pick color schemes that are as color blind friendly as possible.

  1. One of the best ways to distinguish between colors is to use different intensities. The intensity of a color is referred to as its brightness, lightness or how vivid it is. If you chose all very intense colors (such as true red and true blue), then you have eliminated a visual cue that color blind people can detect.
  2. A simple tip is to lighten light colors and darken dark colors. This will also help to assure a noticeable difference in intensity between the colors you choose.
  3. Use highly contrasting colors. So stay away from picking subtle hue differences such as three shades of blue-green. Also avoid pastels. A high intensity difference between foreground and background colors is also suggested.
  4. Avoid the use of spectral schemes (i.e. violet, blue, cyan, green, yellow, orange and red) to represent sequential data. Many of these colors cannot be distinguished by the color blind and further the spectrum does not imply an order of magnitude.
  5. Avoid using red and green together as this is the most common combination that the color blind cannot distinguish.
  6. Some good color combinations are: blues and yellows; magenta-violet and yellow-reds; blue, green, yellow sequences; and for diverging data where the mid values and extremes are both important, red, orange, yellow, light blue and dark blue (center the yellow-blue transition on the pivot point in your data).
  7. If you use 5 or more colors, then it will be increasingly difficult to find ones that will be easily differentiated by the color blind. We suggest adding in symbols with your colors once you reach this threshold. I choose the threshold value of 5 based on color tables that show how the color blind see. In general, they see colors as either shades of blue, yellow, brown and pink (they can also see shades of grey). Here is an excellent chart to give you a sense of what I am speaking about. Use black symbols on light colors and white symbols on dark colors.
Screen Shot 2013-09-06 at 10.36.06 AMThere are three general classes of data: sequential, diverging and qualitative. Above are suggested color schemes for each of the classes. The values following the colored squares are Red, Green and Blue values that can be used to recreate each color.

Tips for Designing Online Maps

Online maps are those that appear on the Internet, and, given the technology, there are some special tricks that can make these maps more useable for the color blind.

  1. Add tooltips to colored items so that users can hover over top and find out exactly what they represent.
  2. Add the functionality to click on a colored point, line or polygon and query its true value. This is similar to a tooltip but its functionality is slightly different as it would require a click, not just a hover.
  3. If the look and feel of the map you are making is very important to you, consider adding a custom color blind friendly map that can be accessed by clicking on a link. That way both types of users have a map they can work with.

If you have a tip that I missed for designing effective maps for the color blind, please take the time to add a comment to this article. Hopefully you enjoyed this two-part GFAQ series and learned a bit along the way. Until next month, stay cool out there!

Do you have an idea for a future G-FAQ? If so, let me know by email at brock@apollomapping.com.

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Brock Adam McCarty
Map Wizard
(720) 470-7988
brock@apollomapping.com

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