Cartographic Skills: Flow Maps

This week we created a Flow map depicting the flow of immigration to the United States. The type of flow map we created is a distributive flow. Distributive flow maps show the movement of people, commodities or ideas. In the map we created, we are showing the movement for the immigration of people into the United States.

We created our flow map entirely in Adobe Illustrator. In creation of our map we started with a pre-made base map in a Winkel Tripel projection which is a compromise between shape, area, distance and direction but a good choice for creating flow maps on a global scale. 

I started by adding bright colors to each continent and then created flow lines using the pen tool to draw lines from each continent to the USA. I used bright vivid colors for each continent and corresponded the same color to the flow lines. This makes it clear which continent the flow line is related to. I also carried the same colors into the lines within the map legend, corresponding to individual immigration total amounts. 

We created proportional line widths based off the total number of immigrants from each continent. So the larger line widths represent higher amounts and narrow line widths represent lower amounts. We used 2007 immigration statistics from the US Department of Homeland Security.  Based off a formula for creating line widths for total amount of immigrates to the US, we calculated higher to  lower amounts in reflecting our line widths. The formula is: Width of line symbol = (maximum line width) x (SQRT value / SQRT maximum value).

Using the Eclipse tool, I created an ellipse shape with blank stroke and matching color background. I layered this behind the world map in order to center an Inner Glow behind the map. This created a sun like effect for further creation of drop shadows for each continent. I adjusted each continent to have their drop shadow offset towards the center of the ellipse, giving the effect that sun is projecting center and not left or right of the map. Offsetting the drop shadow in this way also gives and effect to the projection of the map, in that it shows a more roundness or globe effect to the map.