Graduated symbols are the best way to visualize absolute numbers like population size. Usually the area of the symbols is made proportional with the number to be displayed. This is especially important if the values are far apart.
QGIS 1.8 offers four methods to display graduated symbols. All can be found in the dialogue ‘Layer properties’, on various tabs. The next table summarizes advantages and disadvantages of these four methods. ‘Old symbology’ is not discussed here.
|1 Style / Advanced / Size scale field||
|2 Overlay / Pie chart||
|3 Diagrams / Pie chart||
|4 Overlay / Proportional SVG||
None of these methods is perfect. This article only describes methods 1 and 2 in more detail.
Method 1 Style / Advanced / Size scale field
This method only works with point layers. In this example a text file is used with data on the Dutch provinces. A shapefile with province borders is used as background (download sample data).
First open the shapefile with province borders. Then add the data with ‘Layer / Add delimited text layer’. The attribute table is shown on the next image.
Open the ‘Layer properties’ of this layer.
Use the ‘Advanced’ button to select the field to be displayed. In this case ‘bev2011w’ (square root of population size). Change the units into ‘Map units’. Change ‘Size’ to set circle size and ‘Color’ to set circle colour. The result can be checked by clicking the ‘Apply’ button. The result is shown on the next image.
Method 2 Overlay / Pie chart
This method works with both point and polygon layers. Select the layer containing the data, open the ‘Layer properties’ dialogue and go to the ‘Overlay’ tab. If this tab is missing, first activate the plugin ‘Display diagrams’. Tick the option ‘Display diagrams’ and select ‘Pie charts’ as diagram type. Add the attribute ‘bev2011′. Double click on the field to set the color. As ‘Classification attribute’ the same field should be selected. This field determines circle size.
Now click on ‘Find maximum value’ and enter the maximum circle size (‘Size’). The result can be tested by pressing the ‘Apply’ butten. Currently, only one classification type is available. The name is misleading: symbols are not scaled linearly at all. The result would have been very different from the result of the first method.