Thematic MappingThematic Mapping&&

Data ClassificationData Classification

ObjectiveObjectives:s:

Thematic Mapping & Data Thematic Mapping & Data ClassificationClassification

What is a thematic mapWhat is a thematic mapQualitative vs. Qualitative vs. Quantitative Quantitative Data typesData typesHow do we map this How do we map this data?data?ConsiderationsConsiderations

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!