WCAG AA accessible polygon colors

[Steve_Graham]

To make a polygon map accessible that displays some data attribute field: we "can not use color alone to convey information", we need to also include something else like text or hatching, or alternatively without those, I have seen that we can use the contrast between colors. It seems to me that looking at the contrast checkers, that we can only get 3 colors to have the required 3:1 contrast ratio. Am I interpreting this correctly? Is it impossible to have 4 colors that all have at least a 3:1 contrast ratio? If so what are the options for things that are categorical like land use types? It also seems like all the examples of accessible maps of color schemes online are not accessible when the colors are put in a contrast checker. Am I misunderstanding the tools or requirements? Or are the accessible colors, only accessible for a subset like red green color blindness?

[KlaraSchmitt]

Hello @Steve_Graham,

There is variability in accessible maps. The visual hierarchy and context of a map may be closely tied to its purpose and intended audience, but in terms of options that you may consider applying:

• Consider textures and patterns to the polygon’s fill. For instance: https://codepen.io/geospatialem/pen/mdaEQpb
• Provide different outlines to polygons, such as dashes and solid lines
• Where possible, choose colors that can be discerned via monochrome
• Where possible, provide both fill and outlines to discern between polygons
• Where labels overlap, a halo on labels may help increase visibility. Keep the halo subtle and blend it into the basemap background, so that the label fill color adequately contrasts with surrounding map content—4.5:1 for regular text, 3:1 for large text.

You may also consider having multiple layers with the same data, but different representations of symbology to increase access of information while following WCAG principles.

Additional resources that may help:

• Minnesota's Map Design Guide
• Minnesota's Accessibility Design Guide for Interactive Maps
• ArcGIS Blog Designing Maps for Color Readability
• 7 Map Design Ideas for Accessibility That Enhance Readability

[Steve_Graham]

Thank you for this. I am thinking that maybe you are agreeing that we can use colors with differences in contrast of 3:1 in your monochrome item. I believe this only allows for 3 different colors to be used? Anything beyond this for polygons would mean adding a different fill patterns. I think in order for a polygon border alone to help, we would also need a label on the polygon, because otherwise the contrast difference would not be enough to tell which is which - is that what I should understand from this?

[JessicaMccall]

Hi @Steve_Graham - I agree with @KlaraSchmitt and I would agree that you can use more than 3 colors in a mapping application. I do not think that all colors chosen for polygons have to maintain a 3:1 color contrast ratio to each other, but I would consider that prominent labels or features probably should have that 3:1 contrast ratio to have readability. 

While there are no specific designated guidelines for accessibility in cartography, there are multiple ways that you can display the data and you can provide options for people who are consuming the data. This could include some of Klara's suggestions regarding color, but it might also mean that you can provide narratives, tables, stories from StoryMaps, well-designed pop-ups, and other features that contribute to the overall consumption of the data. As an example, if you have a data set that includes 8 different kinds of features, you should try to ensure that those features can be distinguished from each other and maybe provide that same feature layer styled in different ways to support better understanding. 

I like what the team at the City of Philadelphia did on their Disability Representation Map to add several layers of the same data to be viewed in different ways to support people with a vision impairment.  Labels are in different layers to be able to be turned on and off to reduce clutter and they have used a halo on the labels to have more contrast. 

[Steve_Graham]

Thank you very much for your response. My original question is still about how many colors we can use without adding other things like labels. It seems to me that if the colors have a 3:1 contrast ratio, then that counts as being accessible without adding something else to the polygons. I think that means we can only use 3 colors with enough contrast without having to add another distinguishing characteristic.

Thank you for your input on some options and examples of what to do to keep a map accessible, especially when beyond 3 colors, if that is in fact the limit.