Styling and symbology

NOTE

Multi-variable symbologies

1.1. Bivariate choropleth map of barangays showing population density and annual population rate

Level: Intermediate

What we want to achieve

1. A map of Quezon City barangays showing both the population growth rate (2000 to 2020) and the people per 100 square meters.

What we have

1. adm_brgy_quezon_city_pop2000-2020 - the barangay-level admin boundary data generated from a previous exercise with population from 2000 to 2020

Relevant QGIS knowledge/skills

• Blending modes
• Bivariate choropleth maps
• Expressions

Steps

1. Load the adm_brgy_quezon_city_pop2000-2020 layer. Make sure that it already has the fields for population from 2000 to 2020.

Attribute table of adm_brgy_quezon_city_pop2000-2020 showing population data from 2000 to 2020

2. Duplicate the adm_brgy_quezon_city_pop2000-2020 layer. Since we’ll be creating a bivariate choropleth map using blending modes, we need 2 vector layers to show. In this case, we’ll show the annual population growth from 2000-2020 and the number of people per 100sqm in the barangay. Name the two layers brgy_rate and brgy_popden.

NOTE

To duplicate a layer, right-click on the layer ‣ Duplicate layer.

brgy_rate and brgy_popden layers

3. Next, we make a choropleth map for the brgy_rate layer using a Graduated symbology using the following parameters.

   • Symbology: Graduated
   • Value:

     (pop2020-pop2000)/20

   • Classes
     • Low
       • Color: #e8e8e8
       • Values: -500.00 - 0.00
     • Moderate
       • Color: #dfb0d6
       • Values: 0.00 - 500.00
     • High
       • Color: #be64ac
       • Values: 500.00 – 5000.00

PRO-TIP

Notice the Expression icon () to the right of the Value parameter.

This means that we can directly use the Expression computing for the annual population growth from 2000 to 2020 ((pop2020 - pop2000)/20) without needing to create a new field on the layer.

Style parameters for brgy_rate layer

4. Your brgy_rate layer should look like:

Choropleth map for brgy_rate layer

5. Do the same for the brgy_popden layer but our parameters will be:
   • Symbology: Graduated
   • Value:

     100*pop2020/$area

   • Classes
     • Low
       • Color: #e8e8e8
       • Values: 0.000 - 1.500
     • Moderate
       • Color: #ace4e4
       • Values: 1.500 - 3.000
     • High
       • Color: #5ac8c8
       • Values: 3.000 - 15.000

Style parameters for brgy_popden layer

6. Your brgy_popden layer should look like:

Choropleth map for brgy_popden layer

7. Now that you have the two choropleth maps, create the bivariate choropleth effect by changing the blending mode of the top layer (brgy_popden) to Multiply.

Setting blending mode of brgy_popden layer to Multiply

Bivariate choropleth of the population growth (2000-2020) and population per 100sqm in Quezon City barangays

PRO-TIP

You can use the Bivariate legend plugin to create a legend for the map you created.

The Bivariate legend plugin

ANOTHER WAY TO CREATE BIVARIATE CHOROPLETH MAPS

There’s a plugin called the Bivariate color polygon renderer that adds an implementation of a bivariate color renderer directly for a polygon layer’s styling options.

Try to use this plugin and check what the pros and cons are of this approach vs the approach we took.

1.2. Using both size and color to communicate information about the districts

Level: Intermediate

What we want to achieve

1. Use circles to show population information about districts in Quezon City where the size pertains to the people per 1000 sqm and the color indicates the annual population growth from 2000 to 2020.

What we have

1. adm_dist_quezon_city_pop2000-2020 - the district-level admin boundary data generated from a previous exercise with population from 2000 to 2020

Relevant QGIS knowledge/skills

• Data-defined overrides
• Expressions
• Geometry Generators

NOTE

As with most things in QGIS, there are several ways to approach this problem in order to arrive at the same outcome. Here, I will show two options:

1. A straightforward option using geometry generators and data-defined overrides that do not require the creation of a new field or a new layer.
2. An option that requires creating a new field (pop_per_1000sqm) and a centroid layer then applying a graduated symbology with data-defined overrides.

Option 1 - Geometry generator approach

1. Load the adm_dist_quezon_city_pop2000-2020 and duplicate the layer.
2. Rename one base and apply a simple single symbol symbology.
3. On the other layer, selece Geometry Generator as its Symbol layer type.

Selecting Geometry Generator as Symbol layer type

4. Change the Geometry type to Point/MultiPoint and use the following expression to generate a centroid:

   centroid(@geometry)

Creating a point geometry using the centroid expression

5. On the Simple Marker, use the Assistant to override the Size parameter.

Selecting the Assistant

6. Use the following parameters:
   • Source

     1000*pop2020/$area

   • Values from 10 to 35
   • Output
     • Size from 500
     • to 3500

Symbol Size parameters for the Assistant

7. Your layer should now look like below:

District layer styled as differently-sized points

8. Next, let’s use the Assistant to override the Symbol Fill and Symbol Stroke Color parameters.

Symboll Fill Color parameters for the Assistant

Symbol Stroke Color parameters for the Assistant

Option 2 - Centroid approach

1. Add a new field to the adm_dist_quezon_city_pop2000-2020 layer to hold the number of people per 1000 square meters using the following expression:

   1000*pop2020/$area

Creating the pop_per_1000sqm field using the Field Calculator

2. Run the Centroid processing algorithm to create a new point layer (centroid) based on adm_dist_quezon_city_pop2000-2020.

Parameters of the Centroid processing algorithm

3. A new layer (Centroid) should be created.

Centroid layer

4. Apply a Graduated symbology and use the following parametres:
   • Value: pop_per_1000sqm
   • Method: Size
   • Size from: 500 to 3500 Map Units
   • Classes:
     • 10 - 15
     • 15 - 20
     • 20 - 25
     • 25 - 30
     • 30 - 35

Graduated symbology using size for the Centroid

5. Your Centroid layer should look like below:

Centroid with size-based Graduated symbology

6. Use the Assistant to override the Symbol Fill Color with the following parameters:
   • Source:

     (pop2020-pop2000)/20

   • Values from: 500 to 15000

Symbol Fill Color parameters for the Assistant

7. Your layer should look like below:

SOMETHING TO THINK ABOUT

How will you add a legend on the print layout to reflect this multi-variable symbology?

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