Our lab exercises this week introduced us to some of the difficulties cartographers face when displaying data using proportional symbols. It can be an intuitive way to present data, yet it can also be a struggle to create a map that does not create unnecessary confusion. In the above example, we were required to use proportional symbols to communicate job gains (a positive number) and job losses (a negative number) by state. The first hurdle was dealing with the negative job numbers; directly symbolizing both positive and negative numbers from a single layer in ArcMap does not produce an acceptable result. We needed to export selections of states with positive and negative job numbers into two new layers. A new field was then added to the 'states with job losses' layer in which the field calculator was used to convert the negative numbers to positive. The resulting two layers were then used as the basis for the proportional map above.
The primary variable in creating this style of map is choosing how large the symbols should be and if Flannery's compensation should be applied. In this case, compensation was not applied. However, there was a discrepancy in size between the two layers. Experimentation with the minimum symbol sizes created proportional symbol progressions that were equivalent. Once I finalized the legend layout (converting it to graphics and editing it within ArcMap), I wanted to apply a type of transparency that is not available within ArcMap. I exported the layout to Adobe Illustrator and applied a 'multiply' transparency to create the transparency effect seen above. This minimizes the interpretive problems created by symbol overlap.
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