Thematic Mapping
&
Data Classification
Thematic Mapping & Data
Classification
Objectives: What is a thematic map
Qualitative vs. Quantitative
Data types
How do we map this data?
Considerations
What is a Thematic Map?
Definition: “A map designed to demonstrate
particular features or concepts.”
In other words, a thematic map presents a
graphic theme about a subject.
Most maps you create are thematic maps! It is
not a reference map nor is it a topographic
map because it only displays one theme.
Types of Thematic Maps
Thematic maps can be divided into two groups:
1. Qualitative thematic maps: these show the spatial
distribution or location of kind. For example, a map
representing the spatial distribution of earthquakes.
Qualitative maps use nominal data:
The most basic form of measurement. It is just a label,
category or name. There is not much information you can
derive from nominal data. Mainly just distribution and/or
location of a phenomenon.
For example: The distribution of earthquakes
Types of Thematic Maps
2. Quantitative Thematic Maps: Display the spatial
aspects of numerical data. The map focuses on the
variation of a numerical theme from place to place.
A quantitative thematic map can use two levels of data:
a. Ordinal: Has order, but does not have equal distances. We,
at least, know that one thing we have measured is more or
less than another, but we do not know how much more or
less.
For example: A map displaying the distribution of earthquakes
categorized by high, medium, and low severity.
Quantitative Thematic Map
A quantitative thematic map can use two levels of
data:
b. Interval/Ratio: This is the very best type of data.
Ratio/Interval data has the most precision, and can
measure things in greater detail. It contains numbers,
therefore contains a wealth of information.
For example: A map displaying the distribution of the
magnitude of earthquakes.
Summary
• Thematic maps can be either:
qualitative or quantitative.
• Data to display on a thematic map can be:
Nominal: distribution of a phenomenon without a
measurement. Label, category. (Qualitative)
Ordinal: Ranked data. High, medium, low or how
much greater than. (Quantitative)
Ratio/Interval: how much different, measured by
numbers. (Quantitative)
Mapping data
There are many ways
to display qualitative
and quantitative
data.
Choropleth Maps
• Many quantitative thematic maps are called choropleth
maps. It has also been called shaded or area mapping.
• This is a a thematic mapping technique that displays a
quantitative attribute using ordinal/ratio/interval classes
applied as uniform symbolism over a whole spatial
feature. The boundaries of the spatial areas are
determined by the user.
Example: A map displaying population across Canada.
This could be mapped using province boundaries or
census boundaries depending on the level of data you
have.
The problem with this is….how does the GIS determine
where the breaks in data are? This is called data
classification. It is ultimately up to the cartographer to
determine which classification method best illustrates what
the intention of the map is….
Data Classification
In order to learn more about data classification,
please click on the link below for a PowerPoint
on:
Data Classification!