Thematic Mapping with Carto!

Maptime Madison: December 7th, 2016

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)

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

Thematic map typesThere 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.

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

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

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

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

Choropleth Considerations and Best Practices

Let’s go into more on Choropleths before moving to Carto!

Enumeration UnitsRemember, enumeration units!

○ Counties

○ States

○ Countries

Choropleth data MUST be

aggregated or summed to unit!

Image Credit: Slocum et al. 2009

NormalizationChoropleth’s require

normalized data because of

varying sizes and shapes of

enumeration units

○ Rates

○ Densities

○ Percentages

Image Credit: Slocum et al. 2009

Data ClassesThe 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

Data Classes

Image Credit: Daniel Huffman

Data ClassificationThe 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

Data Classification Types: Equal IntervalFor: evenly distributed or

uniform attributes

For: simple, easy to

understand legends

Image Credit: Daniel Huffman

Data Classification Types: QuantilesFor: 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

Data Classification Types: Jenks Optimal BreaksFor: attributes with a clear

set of clusters in the

distribution that you want

to break up

Image Credit: Daniel Huffman

Color SchemesSequential: 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

ProjectionsQuick 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

ProjectionsWe 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

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

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)

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