Introduction to Thematic Mapping

Francesca Giannetti

October 9, 2019

About this workshop

Present building blocks of geospatial visualizations, with an emphasis on humanities applications

Hands-on exercise modeling mini-workflow of GIS research question

What you will learn

Overview of GIS concepts

Greater familiarity with GIS data and a desktop mapping application (QGIS)

What is GIS?

A Geographical Information System allows us to interrogate, analyze and visualize data to understand the spatial relationships, patterns, and trends between geographic entities.

What is a Thematic Map?

Map of data showing one or more themes or variables (in a multivariate map, variables are related)

A thematic map makes a single argument based on its themes/variables

Two main types: qualitative and quantitative

What is a Thematic Map? (2)

“Thematic maps are an effective means of communication only if geographic information is conveyed […] in an easily understood manner, such that any underlying spatial pattern is obvious. In many cases, this implies that the cartographer must decide upon an appropriate level of detail for portraying the phenomenon of interest. More detail is not necessarily better.”

Scott, Darren M. “Aggregation.” In Encyclopedia of Geographic Information Science, edited by Karen K. Kemp, 7-8. Thousand Oaks, CA: SAGE Publications, Inc., 2008. doi: 10.4135/9781412953962.n5.

Thematic Maps: Qualitative

Types of qualitative (or categorical) data are names, types, or ranks. Different colors symbolize the different data values. In this example, the colors represent counties.

Example: New Jersey. (to accompany) Harriet E. Baker’s Book of Penmanship & Map. At Mr. Dunham’s School Windsor Vermont March 31, 1819.

Thematic Maps: Quantitative

These maps are based on the classification of quantitative (numeric) values. Features are grouped into different classes based on a numeric attribute. This example also shows counties. What does the shading represent?

Example: Hergesheimer, E. Map showing the distribution of the slave population of the southern states of the United States. Compiled from the census of 1860. Washington: Henry S. Graham, 1861.

Types of Quantitative Maps

Spatial Data

Data that defines a geographic location or boundary.

Types of Spatial Data

Vector (points, lines, polygons) vs. raster (imagery)


Vector data are described by one or more vertices or geospatial coordinates (latitude, longitude).

A point is described by a single pair of geospatial coordinates. Points are often used to reference simple locations.

A line is a set of vertices that are connected in sequence. This kind of data can record motion or connections.

A polygon is a set of vertices connected by lines, where the first vertex and the last vertex are the same so that they form a closed shape. Polygons commonly hold boundary information.

Vectors: Q & A

Which type of vector data would you choose to describe a railroad network?

Which type describes countries?

Which type might describe historic properties in New Jersey?


Raster data, like a digital photograph, are a grid of cells where each cell contains some kind of information. Unlike vector data, which automatically scale, a raster image can only be enlarged to a certain point before it becomes blurry. Some types include:

  • satellite imagery
  • temperature (heatmap)
  • georectified scans of historic maps and plans

Common spatial data formats: vectors

comma separated values, or CSV (point data in tabular format)

shapefiles: a proprietary format comprised of several different files. The .shp file contains the geometries; the .prj file contains the projection information; the .dbf file contains additional data associated with the geometries, etc.

Common spatial data formats: vectors (2)

shapefiles (cont.): the component files of a shapefile must be kept together for the shapefile to work properly.

geoJSON: an open format for spatial data often used for web maps.

KML (.kml or .kmz) files are used in Google Earth and Google Maps.

Common spatial data formats: rasters

Rasters are images. The most commonly encountered raster data formats are .png, .jpeg, .tiff, or .geotiff.

Data type and format questions

What data type and/or file formats would you need to explore the locations mentioned in a text?

… to explore rivers and floodplains in New Jersey?

… to explore census data about NJ counties?

Hands-on exercise

Handout and data

Further Reading