Sometimes, the biggest challenge in mapmaking is figuring out how to display two different variables at the same time, without turning the map into an unreadable mess.
For a few weeks now, I’d been struggling with a way to do exactly that. I wanted to make a map of New Bedford, Massachusetts (where I spent two long years when I was in the Coast Guard, back in the 1990s) that showed two things: where the city’s foreign-born residents tended to cluster; and where the city’s linguistically-isolated populations were.
(In this context, I’m using “linguistically isolated” to mean people who speak English poorly or not at all. I suspect linguists would argue with my use of the term in this way, but it’s my blog and I’ll do what I wanna.)
It made sense to me that these populations would overlap – that Census tracts with high percentages of foreign-born residents would also have higher levels of linguistic isolation among those communities. By displaying both variables at the same time, the map would be able to quickly confirm or refute that hypothesis – if they didn’t step all over each other, that is.
It’s the kind of problem webmapping is made for. A static map in ArcGIS or QGIS would have to display all the data points from both variables, all the time – which could get messy in a hurry. But because you can animate a web-based map, it’s easy enough to just switch data on and off as needed.
And that’s the approach I took. I used the fill color to represent the percentage of people living in each Census tract who were born in other countries, which is simple enough. Those colors are fixed. But when you mouse over each tract, the outline will change color, depending on how much of the foriegn-born population speaks poor or no English.
That way, you can clearly see both variables at once, and they don’t get in each other’s way. It’s easy to pick out patterns in the data – and to see that my hypothesis doesn’t really hold water. There doesn’t seem to be much, if any, of the geographic correlation I had expected to find. Oh well. Clearly, there’s more to understanding linguistic isolation patterns than simply knowing who was born where.
But I never would have known that without the map.