Cartography Update 3

Proportional Symbols - Proportional symbols are a good method to use when trying to convey data in relation to different size shapes. The most convenient shape to use is the circle. The circle is hailed as the most stable shape when increasing size. However, the average person viewing a map based on circles will lose up to 50% of the geometric size of the circle when looking away leading to over/under compensation of circle size. This can lead to problems if you choose the incorrect way to size the circles for your map. There are essentially two methods used to create these symbols; Mathematically (poor choice) and Perceptual (good choice).
If the map is done through mathematics to come up with circles that are doubled in area size, then it becomes very difficult to decipher which circle is which (See map below).

Especially in the higher classes, it is impossible to tell which circle is which when looking on the map.


Now if the map is based on the perceptual circles, which actually look twice as large, it becomes much easier to tell which circle is which on the map (See map below).

This allows for the viewer to decipher which circle is which and even switches around the circle arrangements somewhat on the map itself. (Side note, the map above is a bivariable map. Meaning that it displays two forms of information. A choropleth, which shows population per state, and the proportional symbols which show number of EPA sites per state).

Flow Map - Flow maps are used to represent a movement of some object from one place to another. These maps can be very difficult to do. The first step to do is to determine the width of the line for each data group you represent. You can this by:
1.) Sort the data into several groups based on a certain geographic trait
2.) Then calculate each groups percent contribution to the total number
3.) visually represent each group by plotting them on a graph
4.) using natural breaks, group each dataset into a cluster of groups
5.) using a maximum line width (usually 80-90pts.), multiply the percentage of each cluster by the maximum line width then divide by the number of elements in that cluster
6.) Repeat for each cluster, then design the legend based on the numbers calculated from step 5

The flowing is an example of immigration to Canada from surrounding countries