No cartography education or body of knowledge is complete without the (seemingly) ubiquitous choropleth map- that which portrays a derived quantitative measure by some administrative unit. The map above displays the percent change in population for Georgia's counties, which includes both population gains, with positive percentage values, and population losses, which are shown with negative percentage values. The classification scheme used to group the counties according to their percent increase and decrease is the Natural Breaks method, slightly modified. This method gives a decent visual of where the county gains and losses were, without obscuring any patterns or trends, by maximizing the similarity of values within classes, and maximizing the differences between the classes. The colors chosen for each class use a diverging color ramp, where the lightness and saturation increase with the subsequent relative size of the population increase and decrease, which uses two different hues, in order to distinguish easily between gains and losses. Thus, the class directly in the middle represents counties with very little to no population change, and is the least saturated and lightest shade on the map. The legend itself gives indication of the value ranges for each class, with the average percent change at the bottom. The labels for "increase" and "decrease" are added with vertical orientation to the side of the classes, for an easy visual of the direction of the scale. The processes of normalizing derived data, evaluating thematic and data content, choosing effective colors and scale, and a plethora of other small decisions are integral to the construction of a functional and aesthetically pleasing choropleth map. These skills, in particular, form the foundation of an effective cartography skill set.
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