Colors: Bringing Maps to Life

 Choropleth Mapping using Colors in GIS

Basics In Color

In GIS, we use colors to display point, polygon,  vector, and raster data in a way in which the user can understand the map. Colors help identify terrain, natural vegetation, or even icons of interest on a map. Today we are going to talk about how a GIS user can manipulate colors to display Choropleth maps regarding population changes, land use, and unique color ramps. 

Working with RGB and HSV Colors

Tha image above shows RGB values and HSV values. These values are how the GIS user selects the hue and saturation of values to derive meaningful symbology to data on a map. As you can see, by changing these values we can alter colors such as keeping the hue (red for example) but altering the saturation of light to create shades of light red and dark red. By changing the hue into a divergent color ramp, we can have symbology that uses multiple colors and shades from light to dark. Working with the hue and saturation in how the map reader perceives the color of data is critical in trying to convey the message of a mp. 

Linear progression

Adjusted progression

ColorBrew

 In (Brewer, 2016) Chapter 3, the text references variation in lightness so that data values that are low are lighter and data values that are high are darker. This would be represented well with the Color Brewer color ramp, as the lighter color classes are easier to depict rather than the linear and adjusted color ramps.  The Color Brewer results vary from the adjusted progression and the linear progression because the intervals between color hues are more random and change more frequently. There is greater color differentiation when using the color brewer symbology and the colors are more profound than the other two-color ramps. 

For example, the 6^th class of the lightest shade of green for Color Brewer is significantly lighter than the linear and adjusted progression color ramps, with the interval for green in the RGB shading being less than the other two ramps. 

However, Color Brewer has great intervals in the red and blue shading of RGB, and the values are much higher than the values of red and blue for the other two-color ramps which helps the color appearance appear brighter. The linear and adjusted color ramps only focus on changing the intervals of the green shading; however, color brewer uses all 3 color shades to make the colors vary from class to class. 

https://colorbrewer2.org/#type=sequential&scheme=BuGn&n=3

https://en.wikipedia.org/wiki/Cynthia_Brewer#:~:text=ColorBrewer%20is%20an%20online%20tool,It%20was%20launched%20in%202002.

Altering Hue & Saturation to group Land Features together

I decided to keep the same hue for each section of groups for my land use classification. Therefore, red was going to be urban / residential land, blue was going to be water (reservoirs), green for cropland, a tan color for hardwood, a blue green for vegetation. In doing so, I adjusted the saturation to alter the lighting for each shade of color or hue to help the map reader identify the groups of hues but also differentiate the different features in each group. 

The values of RGB in changing the saturation to go higher for lighter colors and lower for darker colors made the color ramp pop in each group without hiding any features from the map. In (Brewer, 2016) on page 134, Dr. Brewer talks about saturation and how is a measure of vividness of a color, and the influences of said color is the intensity, purity, chroma, and colorfulness. These are derived from the perception of the colored feature, and we can use saturation to display a single hue ranging from desaturated cyans (greyish) to saturated hues full of color. 

This example represents the saturation of a light red to almost dark reddish-brown in a color ramp. I followed this process when displaying the colors for each group in my land use map, using this saturation technique to identify group of different colors in a way that the map reader can still understand. 

Look at the map below in how we can use a sequential color ramp by keeping a similar hue, but alter the saturation to provide a classification map of population data. 

Divergent Color Ramps

I used a 5-class symbology with the natural break’s classification method which then changed to a manual interval due to creating a class break as the middle class in the color ramp. I used a divergent color scheme with the Color Brewer RGB hue and saturation values from blue to red. I used a class break of 0.28% which is the average percent change of all the counties in Georgia. I used the negative range of 1.3% to this average as a white “neutral” zone to show that counties in this class experience no increase or decrease. I felt that 5 classes were enough to show counties with a decrease, a significant decrease, an increase, a significant increase, and counties with no percent change. 

The legend displays the 5 classes as percentage intervals with blue being positive and red being negative, representing an increase in population and a decrease in population. I used the to word to display the interval ranges. I also based the intervals on the natural break’s intervals, as well as labeling the middle class (my class break of the state percent change average) as no significant change so that the map reader can clearly see the counties with no change. 

Check out the video below on Color brewer!

APA Citations

Brewer, C. A. (2016). Planning Maps. In Designing better maps: A guide for GIS users (pp. 129–177). essay, Esri Press.