Discrete vs Continuous Abrupt vs Smooth Phenomena
Discrete-Continuous / Abrupt-Smooth Phenomena
Figure 5.1 from Thematic Cartography showing phenomena and appropriate ways of representing them

Typically, data are collected data at discrete locations (weather stations, cell phone towers) or aggregating over regions (blocks, counties, states). The actual phenomena being modeled is continuous.

Type of visualization used depends both on the nature of the underlying phenomenon and the purpose of the map.
Compare these visualizations?

Visual Variables for Qualitative Phenomena:

Visual Variables for Quantitative Phenomena should reflect ordinal level of measurement:

Here is an example of using colour to map life expectancy in the US from mapoftheunitedstates.org

For the same data, the second is from measureofamerica.org. Which one is more readable?

Here is an appropriate use of a wide variety of colors showing data from facebook on the favorite American Football teams for various counties.

We can also use pictographic symbols
Here are the issues:

Comparison of choropleth, proportional symbol, isopleth, and dot mapping:
Figure 5.10 from Thematic Cartography


Isopleth (contour map)

Proportional symbol

Dot mapping

Selecting visual variables for choropleth maps
Visual Variables
Colour Visual Variables
Figure 5.11 from Thematic Cartography

Here are some other ways of displaying data from Information Graphics -  A Comprehensive Illustrated Reference

Here is a New York times example for the 2009 Afghanistan election

Here is an image showing crime statistics compared to the average over time for the US from CommonGIS via the Thematic Cartography and Geovisualization book.

Materials on this page were adapted from Visualization and Visual Analytics class at UIC by Prof. Andrew Johnson.
© Last revised: Oct 20th, 2016