Creating a Proportional Symbol map in QGIS 2.18

Creating proportional symbol maps in QGIS is made very easy in QGIS with two main methods for making them. You can use basic Single Symbol style with the  Size Assistant in the Data Defined Override,  or you can use the Graduated style and choose Size as the method of gradation. Below is a set of instructions on how to use either method to create proportional point symbols for your map.

The Data

  • Download the 1:10 million populated places (Simple) Shapefile from Natural Earth:
  • Add in the Stamen Toner Lite WMS using either the OpenLayers or QuickMapServices plugins. If you haven’t done that before, here is a quick guide:

Method 1: Graduated Symbols

This method works best when you want to use all the points in your dataset. QGIS allows you to vary the size of the data point based on the values in attributes of the data.

  • First, open the live layer styling menu by pressing F7.

Make sure the top menu is set to the Natural Earth Point data then change Single symbol to be Graduated.

There are now a few new options to play with:

  • Change Column to be pop_max
  • Change Method to Size
  • Change “Size from” to 3 and “to” to 20
  • Click the Classify button to see how this looks.

The map will automatically update to look like the map show below, zoomed in to the UK and Northwest France:

You have flexibility to change the number of classes and how you divide the classes, Equal Interval, Quantiles, Natural Breaks etc. You can also manually edit the classes by double click in the values column in the table.

Method 2: Data Defined Override

If you want a little more control over how the data is displayed this method allows you to quickly change maximum and minimum values without having to set each level in between.

  • First, open the live layer styling menu by pressing F7
  • Make sure the top menu is set to the Natural Earth Point data
  • Click on the Data Driven Override button  on Size

 

 

  • Click on the Size Assistant option at the bottom of the box.

You will now be looking at the Size Assistant dialog. The options here are very similar to using the method described above, allowing you to select the field to base the size on and having control over size of the points.

  • Set the Field to be POP_MAX
  • Set the Scale method to be Radius
  • Change the “Size from” to be 3 and “to” to be 20

As you can see in the example above the symbols automatically scale from the smallest to the largest values in the field selected. However where this method has a big advantage over the previous one you can set the upper limit to be different. The points will now automatically scale to this value without you having to manually update each range. This can be very useful if you are viewing only a portion of the data.

In the area we zoomed to in the previous method the largest cities are London and Paris, both having a population of below 10,000,000

  • Change the “Values from” to be 0 and “to10,000,000

You can now see int he map below there is a bigger range of point sizes than the previous method for the area of the map.

Blend Modes

A little added tip is to use the blend modes on your symbology, so you can see the basemap through the shapes and reveal the detail on any overlapping symbols:

  • In the Layer Rendering section of the style tab change the Layer blending mode to be Multiply. This will allow you to see the basemap through the points
  • Change the Feature blending mode to be Multiply. This will allow you to see overlapping points within the point dataset.

The Result:

4 Replies to “Creating a Proportional Symbol map in QGIS 2.18”

  1. Not sure if I ‘ve missed something, but there doesn’t appear to be any grid references in the EU referendum CVS file. Not sure if I can continue Creating 3D choropleth or prism exercise as i’m unable to project the data onto the map canvas. Can you help?

    1. Hi Dean,

      Apologies for the delayed response. You don’t need coordinates in the csv as you are just attaching the attributes to the polygons in the boundaryline data. You are doing a table join rather than a spatial join. I’ll add a ready made shapefile to the exercise so you can proceed or you can use the geopackage from the Dot Density Map exercise as it has all the info you need.

      I hope this helps,

      Tom.

  2. Hello!

    Is there a way to create dot-density maps, where a polygon attribute data defines the number of dots (e.g. 1 dot = 100 ) using QGIS 2.18?
    Hope you can help me on this.

    Thanks!
    Bless

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