Symbolise layers by attributes

Before we actually make our map, let’s equip ourselves with some extra skills for symbolising layers

So far we’ve ignored information in the attributes table of vector layers, and used identical symbology for all features in a layer. Now it’s time to explore a powerful visualisation option in QGIS - the ability to symbolise features by attributes. For vector layers, symbols are drawn according to properties (columns) in their attributes table. For raster layers, symbols depend on the value within each grid cell

For all of these instructions you need to have the Layer Properties window open on the Symbology tab

Be aware that the layers in the screenshots below are different from those in our gibbon case study

Visualise categorical attributes

For vector layers

1. Select Categorized from the dropdown menu at the top (the default is Single Symbol)
2. Choose the column you want to symbolise from the Value dropdown menu; for example ThreatType from the ThreatEvidence layer
3. Click on the Classify button in the bottom left
4. Click Apply and OK to finish

For thematic raster layers

1. Ensure Render type is Paletted/Unique values
2. Double click on the Color or Label to manually edit the colour or label of each pixel value in your

Visualise continuous attributes

For vector layers

1. Select Graduated from the dropdown menu at the top
2. Choose the column you want to symbolise from the Value dropdown menu; for example GroupSize in one of the GibbonSightings layers
3. Click on the Classify button in the bottom left
4. Click Apply and OK to finish

For continuous raster layers

We don’t have an example of a continuous raster layer in the gibbon case study, but a DEM would be one example. These instructions are included for completeness, to help those working with their own data

1. Under Render Type, select Single band gray or Single band pseudocolor
2. Click Classify to see the result of the default settings
3. Experiment with the settings e.g. Interpolation, Color ramp and Mode until you’re happy with the result

Adjust layer transparency

It can be useful to make layers partially transparent, particularly if you want to reduce the visual impact of thematic raster layers, or allow layers such as a hillshade/terrain to be faintly visible without dominating your map

For vector layers

1. On the Symbology tab, click on the triangle beside Layer rendering to expand that section
2. Adjust the Opacity to below 100% (e.g. 60%) and click Apply to see the result

For raster layers

1. Switch to the Transparency tab
2. Adjust Global opacity to below 100% (e.g. 60%) and click Apply to see the result

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