Cartography: Module 9 – Flow Line Mapping

     This week continues to examine thematic mapping by way of the flow line map. Flow maps illustrate the movement of phenomena between various locations. The widths of the flow lines are proportional to the value of the data they represent. Typically these maps are used to depict the flow of people or commodities (distributive flow maps) but they can also be used to map networks (network flow maps), radial phenomena (like migration – radial flow maps), continuous phenomena (continuous flow maps), and other information (like telecommunications). This map includes another thematic map type – the choropleth – as well.

     I made a distributive flow map of immigration to the United States by region (continent) from a base map provided as lab content. My map was entirely designed and edited in CorelDraw. The flow lines are of proportional widths to the data. The thicknesses were calculated using a specific formula entered into Excel. I used these calculated values to manually generate flow line width (within the Object Properties tool of Corel).

A distributive flow line map depicting immigration
from continental regions to the United States. 

Design Considerations

     The drop shadow and transparency were the main effect tools used. In tandem, these tools helped the flow lines lift off the page without obscuring geographic information. To simplify the map I placed the name of the immigration region within its flow line. Oceania and the Unknown region had such small flow lines that I opted for a placement above the flow line (whilst trying to keep typography guidelines in mind). The continents are somewhat transparent to create a figure-ground relationship with the choropleth map of the United States (one in which the choropleth becomes the central focus). 

     A contiguous legend is included for the choropleth map and the number of immigrants from each region is represented by its own legend of sorts. I used arrows of the same color as the region they represent to display the raw immigration data. The arrows are of decreasing length from the most immigration for a region (Asia) to the least immigration (Unknown). Here, the drop shadow is used to emphasize this information. 

     The maps are not to scale although attention was paid to keeping them in correct proportions (by using the Shift key while resizing). The projections of both the continents and the United States are also noted. Again, I employed the drop shadow tool for the title. As the map contains a lot of information, I did not want the title to be drowned out. The use of the drop shadow helped to make the title noticeable while not overburdening or detracting from the map.