Here’s a fun little symbol design that I did back in 2013 and quite liked. At the time, a colleague of mine was mapping data from a bike rental program. Among many other things, he wanted to plot how many bikes were checked into and out of each of the city’s docking stations over the course of a day. I suggested the following:
The length of the red arc shows the number of bikes docked at the station, and the blue shows biked checked out of the station. In this case, many bikes left the station during this time period, and few entered.
There are plenty of ways to convey a data set like this. You could do a little chart with varying bar lengths, or you could do proportional circles that change in size based on activity. You could even just write the numbers near the stations.1 But, I like this solution because it carries a sense of movement and flow to it. The feather effect on the red and blue sections gives a sense of connection to the base map. Bikes are flowing from the surrounding area into the station (represented by the grey dot), and flowing out again. It feels more natural than a simple, static, proportional circle or bar chart. This symbol lives in the community, rather than sitting on top of it.
It’s also nicely non-specific about location. People who dock bikes at this station are coming from many different places, and people who take bikes from the station will have many different destinations. The symbol is drawn to look like it’s taking in flow from a wide area, and distributing it across a wide area. It’s less specific than, say, using arrows. It would be easy to draw a black dot with a red arrow coming in and a blue arrow coming out, to indicate bike check-ins/outs. But I think those arrows would feel too specific, suggesting bikes only came from one way and went out another. The symbol I designed above is not perfect: the red is on only one half of the circle and the blue on the other. But, I think it’s a little better than the alternative.
In the end, the colleague ended up going another direction, but instead of letting this symbol languish in permanent obscurity, I thought I’d put it up here, in case it provided anyone with some ideas or inspiration. It exemplifies the sorts of little details that cartographers spend a lot of time thinking about, and which map readers ultimately don’t think about at all.
1 That’s probably a good idea no matter what symbology you use.