GIS Communications Lab 4 - Choropleth Maps

 Welcome to Lab 4, discussing one of the most useful and most proliferated (in my opinion) types of thematic maps. The Choropleth map. These are a type of thematic map that use color ramps to provide valuable information about some type of enumeration unit. In the case of the below you will see an example of a couple like color ramps that I created, and one that was pulled from color brewer, a software that helps develop these color ramps for you. Then you will see a map of Colorado, with the counties as the enumeration unit. The counties are colored based on the percentage of population change from 2010 to 2014 in a diverging style based on the equal interval classification method. 

Before we get more into that lets look at some specific color ramps that I created and or worked with. So, to start, there were a set of starting values to choose from. These values are based off of an assigned value between 0 - 255 for Red, Green, and Blue. The combination of these values drives different colors, each of the individual patches in the color ramp has its own unique combination value derived from the RGB combination. 

All three of the color ramps are sequential schemes with various changes in hue, lightness, and saturation. Brewer, 2016 discusses these three things as the basis of understanding perceptual dimensions for color applications. One of the biggest differences between my linear and adjusted progression ramps to the color brewer lamp is the maintaining of the Blue-Green hue. Because the basic color from the lab given directions had slightly more blue added to the green base I kept that offset blend (ranking from most to least, green, blue, red) throughout the ramps. The sequential multi-hue color brewer ramp ends with a much lighter last step, whereas my ramps have a brighter final step. Though the linear progression and adjusted progression are very similar, the 2^nd to 3^rd and 4^th to 5^th  of the adjusted progression seem to be closer to one another than their respective steps in the linear progression. One of the biggest things that I learned after this is the adjustment of saturation to reduce the brightness of the lightest value on my two ramps. I can manipulate either the RGB number combination to get a lighter hue, or I can swap to a HSV (Hue, Saturation, Value) combination to more readily adjust singular aspects of the color, like its overall brightness. 

As a reminder, for the map below, I am working with the state of Colorado, with the data applying to the county level. The map is based on highlighting the percentage of population change over 4 years. 

 

Colorado’s population change is variable between +10% and – 13.5%. Because these highs and lows are relatively symmetric I utilized an Equal Interval approach. Additionally, looking at the histogram, the data only shows a slight positive skewness. Dealing with a somewhat normalized distribution and desire for fairly equal (3-5%) data classes on either side of 0, I decided upon the Equal Interval. One potential downside with equal interval is that it could leave some classes empty when dealing with skewed data. That is not the case here. All classes have multiple observations. I chose 7 classes because I wanted a distinctive 0 class, and each of the positive and negative classes represents approximately 3-5% of change. Now, I did look at using 5 classes, but this created too generic a distribution of class observations. That, and the range increased to approximately 3 – 7% spread. I didn’t think that breakout was as meaningful as the smaller 3-5 rundown. This method allows for a greater understanding of where the most and least change has occurred. 

Population change in this context is not good nor bad, but it is absolutely positive and negative. So I went with the Green – Purple change ramp. I think it was also complimentary to my light blue and dark gray background features. The purple and green are distinctive and have high enough hue / saturation / value to be a positive figure to the other ground features in the visual hierarchy. 

Please let me know if you have any feedback, thank you!

v/r

Brandon