Thematic maps

In this session we will look at thematic maps. This is one of two broad types of cartographic representations (the other being terrain maps), encompassing any map where the main object is to communicate how a given property (the ‘theme’) varies in space, with the actual shape of the Earth being a secondary aspect (if present at all). Some common examples of thematic maps in archaeology are distribution maps (especially of sites) or density (choropleth) maps of finds. We will focus especially on point distribution maps, one of the most simple but effective genres of cartography in archaeology.

Objectives

By the end of this session, you should:

  • Be able to describe the fundamental data models of GIS
  • Be able to arrange and style vector geospatial data in QGIS
  • Produce an effective archaeological distribution map

Prerequisites

Practical

Data

Mapping point distributions

We will start by simply showing where the sites are in the world.

  1. Start a new project in QGIS.
  2. Add acheulean.csv into your map from the browser and the layer -> add layer menu
    • Q: Which one worked?
    • Q: Why do we need to tell QGIS what CRS this layer uses?
  3. This isn’t very useful on its own. Find relevent base layers from Natural Earth and add it to your map too.
    • For this practical, use at least one raster layer and at least one vector layer
    • Q: What kind of base layers are relevent here?
  4. Use the layers panel to arrange the layers of your map in a sensible order.
  5. Use the layer properties dialog to style your base layers. Remember to think about what you are trying to communicate with this map.
    • Q: How does the purpose of the map affect the choice of base layer styles?
  6. Use the layer properties dialog to style your point layer
    • Q: Apart from the location of sites, what other information might you want to show at this point?

Use Project -> Import/Export to save an image of your map for your portfolio.

Thematic mapping with points

Now we will pick a class of artefact to explore with our map.

  1. Using single symbol symbology, vary the size of the points according to the number of artefacts.
  2. Using graduated symbology, vary the colour of the points according to the number of artefacts.
  3. Using categorized symbology, vary the colour of the points according to the number of artefacts.
    • Q: Which do you think is more effective, and why?

Save an image of a map using your preferred symbology for your portfolio.

Thematic mapping with polygons

Let’s add a bit more information to our base layers, showing the countries in which Acheulean sites are found.

  1. If you haven’t already, download a country layer from Natural Earth Data and add it to your map.
  2. Adjust the symbology of the country layer appropriately.
  3. Using the select tool, select countries that contain an Acheulean site.
    • Q: In future sessions, we will look at more automatic ways to do this. Why might doing it by hand become inefficient?
  4. In the layer panel, export the selected features as a new layer, then add it to your map.
  5. Using single symbol symbology, adjust the appearance of the two country layers to highlight those containing Acheulean sites.
  6. Using inverted symbology, adjust the appearance of the two country layers to highlight those containing Acheulean sites.
    • Q: What other uses might inverted symbology have?

Save an image of a map using your preferred symbology for your portfolio.

Thematic mapping with lines

Finally, let’s explore how we can use data to vary the appearance of lines.

  1. If you haven’t already, download a river layer from Natural Earth Data and add it to your map.
  2. Adjust the symbology of the river layer appropriately.
  3. Use a data variable to adjust the width of the river layer. You will have to experiment to find the right variable and expression!

Save an image of your map for your portfolio.