Cartographic Skills Module 9: Flow Line Mapping

This week's lab adventure was creating a flow map, or a map that depicts movement (of people, goods, traffic, etc.) from one place to another. In this case, the dataset was numbers of immigrants to the United States from various parts of the world, and the map has two parts: flowlines showing total numbers of immigrants from each region, and a choropleth map of the U.S. showing the percentage of those immigrants that ended up in each state.

This map was more fun to make than I thought it was going to be when I first realized the whole thing needed to be done by hand in Adobe Illustrator (a personal nightmare). There are a lot of stylistic things you can do with this kind of map, starting with drawing the flow lines themselves and having complete control over what they look like and how to place them. For this assignment, we drew the biggest line first, tweaking it until it looked good, and then used Excel to calculate proportional widths for the remaining lines based on the width of the first one. That way the relative size of the lines reflects the numbers of people they represent. 

Once I had the lines ready to go, I made them partly transparent so as not to hide country or continent boundaries, and added drop shadows to make them pop out to the reader. (I also used a drop shadow to make the U.S. stand out, to draw attention to the choropleth data there and to the fact that it's the endpoint for the rest of the data.) I also used a little bit of a gradient pattern for the stroke of the flow lines, and finally, I used AI's text on a path tool to make the flow lines' labels match their curves.

One other thing I did with the lines, which wasn't really part of the assignment, was separate out Canada. The data we were using was totaled by continent, but that meant North America was going to either need two separate arrows or two converging arrows in order to not be misleading, and either way I wanted to get the proportions right. Fortunately, the Excel spreadsheet also broke down the immigration totals by country, even though we didn't need to use that part, so I was able to subtract Canada from the rest of North America and satisfy my compulsion for accuracy.