Activity 4 of 4

Visualizing GIS data

5 min read

In this tutorial, we are going to demonstrate how to map both nominal and interval data, using Environmental Justice Population and Tree Cover data as examples.

Nominal data refers to categories without any inherent numerical relationship.
Interval data refers to a numerical scale where the distance between numbers is known and consistent.

You can use this understanding to leverage cartographic conventions and software templates designed for each level of measurement.

Datasets

Mock researcher question: Are cooling resources in the city fairly distributed?

We are going to use the EJ_CRIT_DE field of the Environmental Justice Populations by Census Block Group, Boston, Massachusetts, 2020 to map nominal data. EJ_CRIT_DE refers to the EJ criteria description. Each census block group is assigned a category, e.g. English isolation; Minority and income, etc.
We are going to use the Percent_Tr field of the Tree Canopy Percentage by Census Tract (CT), Boston, Massachusetts, 2019 to map interval data. Percent_Tr refers to the tree canopy per census tract as a percentage of total land cover. These values are represented as percentages, e.g. 23.05%; 14.25%, etc.

Requirements

The instructor will demo next steps in QGIS. Participants are not required to follow along. If you wish to follow-along in class or later at home, you will need:

QGIS [Download instructions]
The sample data is available from the Harvard Geospatial Library (HGL), and also pre-packaged for the workshop. [Workshop data]

Nominal Data

Start a new QGIS project and add the Environmental Justice Populations by Census Block dataset (downloaded to your computer) to the QGIS project by either: (1) dragging the file with the .shp file extension directly into the QGIS window from your file browser, or (2) using the menu navigation Layer → Add Layer → Add Vector Layer and using the Data Source Manager to select the file with the .shp file extension, then selecting Add and Close.
Open the EJ Population layer properties by either double-clicking the layer in the Layers pane, or right-clicking the layer and selecting Properties.
From the Properties window, select Symbology from the menu.
Click where it says Single Symbol, and select Categorized. This is the QGIS template for symbolizing nominal data.
Underneath where it says Categorized, there is a field called Value. Click arrow to the right of this field, to open a drop-down menu. This lets you select which attribute or variable you want to symbolize.
Select EJ_CRIT_DE.
Click Classify.

Tip: QGIS will select colors at random to symbolize the data categories. To have more control over how your map looks, you can use a tool developed by cartographers called Color Brewer. Using Color Brewer, select the Nature of your data = Qualitative (nominal), and the number of classes you'd like (e.g. 7), to generate color schemes well-suited for mapping. The website will provide color codes you can copy-paste into QGIS. Double-click the colorful squares in QGIS to open the individual color properties.

In the QGIS Symbology Properties menu, select Apply and then OK.
Your map will now be symbolized by Environmental Justice Population criteria category. In the Layers pane, engage the toggle drop-down arrow to the left of the EJ Population layer to reveal the map legend.

Interval Data

Add the Tree Canopy Percentage by Census Tract (CT), Boston, Massachusetts, 2019 dataset to the same QGIS project by either: (1) dragging the file with the .shp file extension directly into the QGIS window from your file browser, or (2) using the menu navigation Layer → Add Layer → Add Vector Layer and using the Data Source Manager to select the file with the .shp file extension, then selecting Add and Close.
Open the tree canopy layer properties by either double-clicking the layer in the Layers pane, or right-clicking the layer and selecting Properties.
From the Properties window, select Symbology from the menu.
Click where it says Single Symbol, and select Graduated. This is the QGIS template for symbolizing interval data.
Underneath where it says Graduated, there is a field for Value. Click arrow to the right of this field, to open a drop-down menu. This lets you select which attribute or variable you want to symbolize.
Select Percent_Tr.
Click Classify. You will notice this populates your data classification with a graduated color ramp to represent density of the variable selected.

Graduated symbology tips

Here are some considerations for working with graduated symbology:

You will need to choose a classification Modes e.g. Equal Count, Equal Interval. To support your understanding, you can refer to mode documentation.

It is typical when exploring your data to try out various classification Modes.

The Histogram tab is a helpful tool for understanding the distribution of your data values. This tool is located next to the Classes tab. Click Load Values to display the diagram.

Once you have a better sense of your dataset using these exploratory tools, you will be better positioned to identify which classification Mode would be suitable for any published or rhetorical maps you make.

The choices you make for classification Mode will impact how readers interpret your data.

You will also want to consider color. You can use Color Brewer to select a color scheme. Let’s use green since we are representing tree canopy.

Select Apply and then OK.

💡 Discussion: Toggle the layers in the Layers pane using the checkboxes to compare the two maps. What do you observe? If you could plant 1,000 new trees, where on the map would you put them?