This document discusses thematic mapping and provides an overview of four common thematic map types: proportional symbol maps, dot density maps, isoline maps, and choropleth maps. It explains what thematic maps are, the different types of information they can show, and considerations for making choropleth maps such as using appropriate enumeration units, normalizing data, classifying data, choosing color schemes, and using the proper map projections. The document concludes by explaining how to make a choropleth map using the Carto online mapping tool.

1. Thematic Mapping
with Carto!
Maptime Madison: December 7th, 2016

2. What is a thematic map?
● A map that shows the variation in one (or sometimes several) geographic
phenomenon across the landscape
○ i.e., numeric attributes across space
○ e.g., Population (numeric attribute) per state (across space)

3. What is a thematic map?
● Thematic maps show two information types:
○ Individual information-attribute values of individual objects in the landscape
■ Think points: cities, buildings, weather stations
○ Enumerated information-aggregated attribute values of a set of objects in a given region
■ Think polygons: countries, states, counties as enumeration units

4. Thematic map types
There are multiple thematic map types in cartography including:
● Proportional symbol
● Dot Density
● Isoline
● Choropleth
● Cartograms
● Dasymetric maps
● Flow maps
● Bivariate/multivariate maps
Let’s explore 4 common thematic map types.

5. A thematic map in which dot symbols
are scaled in proportion to the value
associated with the dot symbol.
● Enumerated and individual
information (enumerated in this
example)
● Phenomena shown through
symbol size
Proportional Symbol
Image Credit: Slocum et al. 2009

6. A thematic map in which the density of
dots within enumeration units are scaled
in proportion to the attribute values
associated with the units.
● Enumerated information only
● Phenomena shown through
arrangement and size
Dot Density
Image Credit: Slocum et al. 2009

7. A thematic map in which interpolation is
applied to points of known values to
create lines of equal attribute values.
● Individual (isarithm) and
enumerated information
(isopleth)
● Phenomena shown through
location
Isoline
Image Credit: Slocum et al. 2009

8. A thematic map in which enumeration
units are shaded with an intensity
proportional to the attribute values
associated with the enumeration units.
● Enumerated information only
● Phenomena shown through color
value and saturation
Choropleth
Image Credit: Slocum et al. 2009

9. Choropleth Considerations and
Best Practices
Let’s go into more on Choropleths before moving to Carto!

10. Enumeration Units
Remember, enumeration units!
○ Counties
○ States
○ Countries
Choropleth data MUST be
aggregated or summed to unit!
Image Credit: Slocum et al. 2009

11. Normalization
Choropleth’s require
normalized data because of
varying sizes and shapes of
enumeration units
○ Rates
○ Densities
○ Percentages
Image Credit: Slocum et al. 2009

12. Data Classes
The numbers of bins that you
place your data in!
More classes = more detail
Fewer classes = more generalized
No classes = ‘true’ to your data
Image Credit: Slocum et al. 2009

13. Data Classes
Image Credit: Daniel Huffman

14. Data Classification
The ways in which we bin the
data.
The maps on the previous
slides each had their own
classification scheme.
Image Credit: Slocum et al. 2009

15. Data Classification
Types: Equal Interval
For: evenly distributed or
uniform attributes
For: simple, easy to
understand legends
Image Credit: Daniel Huffman

16. Data Classification
Types: Quantiles
For: mapping contexts in
which the median value is
meaningful
For: comparison across
multiple attributes
For: convert to ordinal
level data to assign low,
medium, high
Image Credit: Daniel Huffman

17. Data Classification
Types: Jenks Optimal
Breaks
For: attributes with a clear
set of clusters in the
distribution that you want
to break up
Image Credit: Daniel Huffman

18. Color Schemes
Sequential: a series of colors with an
apparent increase from low-to-high in one
direction
Diverging: a series of colors with an
apparent increase in two directions away
from a Critical Value
Qualitative: a series of colors with no
apparent ranked order
ColorBrewer

19. Projections
Quick Review: When you take something
like a spherical globe and flatten it, there’s
going to be distortion
Projections allow us to partially preserve
some things, but not everything:
● Shape (kind of)
● Distance
● Direction
● Area
Image Credit: John Kryger

20. Projections
We need to preserve area in
Choropleths because we are
comparing shaded units
This type of projection is called
an Equal Area Projection
Unfortunately, online maps are
limited… Web Mercator Dun
Dun Dun
Image Credit: Slocum et al. 2009

21. Let’s Make a Map with Carto!
Now that you know the basics, it’s time to build a choropleth!

22. Carto Builder (Formerly CartoDB) Activity
www.carto.com
https://carto.com/academy/courses/beginners-course/your-first-ch
oropleth-map/ (Or Google Carto Choropleth Map and click on the first option)

23. References/Resources
● Slocum et al. (2009) Thematic Cartography & Geovisualization (Chapters 4, 10, & 14)
● colorbrewer2.org