QGIS Atlas: Rule-Based Raster Symbology For Species Maps

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QGIS Rule-Based Symbology for Rasters in Atlas

Creating species distribution maps using QGIS and its Atlas feature is a fantastic way to visualize geographical data. If you're already using the Atlas with a polygon layer to represent quarter-degree grid cells for species observation records, you're on the right track! Now, let's dive into how to use rule-based symbology for rasters within your Atlas project. This approach allows you to dynamically change the raster symbology based on attributes, making your maps much more informative and visually appealing.

Understanding Rule-Based Symbology

First off, let's break down what rule-based symbology is and why it’s super useful. Instead of applying a single, uniform style to your entire raster layer, rule-based symbology lets you define specific rules that determine how different parts of the raster are displayed. These rules can be based on attribute values, expressions, or even the spatial location of the raster cells. This is particularly handy when you want to highlight different categories or values within your raster data.

Why is this important for species distribution maps? Think about it: you might want to show areas with high species richness in one color and areas with low richness in another. Or maybe you want to highlight areas where a particular species is present or absent. Rule-based symbology lets you do all of this dynamically, and when combined with the Atlas feature, it means each map in your series can have its symbology automatically adjusted based on the data associated with each grid cell.

To get started, open your raster layer properties in QGIS. Navigate to the Symbology tab. Instead of the default singleband gray or color option, choose “Rule-based.” You’ll see a window where you can add, edit, and manage your rules. Each rule consists of a filter (an expression that defines which raster cells the rule applies to) and a symbol (the color, transparency, and other visual properties that will be applied to those cells).

For example, let's say you have a raster layer where each cell represents the number of species observed in that area. You could create a rule that says: "If the cell value is greater than 10, color it dark green." Another rule might be: "If the cell value is between 5 and 10, color it light green." And so on. The key is to use expressions that accurately reflect the criteria you want to use to differentiate your raster cells.

Setting Up Rule-Based Symbology for Rasters

Alright, let's get practical! Here’s how you can set up rule-based symbology for your raster layers in QGIS:

  1. Open Layer Properties: Right-click on your raster layer in the Layers panel and select “Properties.”
  2. Navigate to Symbology Tab: In the Layer Properties window, go to the “Symbology” tab.
  3. Choose Rule-Based: In the “Render type” dropdown menu, select “Rule-based.”
  4. Add a New Rule: Click the green plus (+) button to add a new rule. This will open the Rule Properties dialog.
  5. Define the Filter: In the Rule Properties dialog, you'll see a field for the “Filter.” This is where you write the expression that determines which raster cells will be affected by this rule. You can use the Expression Builder (the epsilon symbol) to help you create your expression. For example, if you have a field named “SpeciesCount” in your raster attributes, you might write an expression like "SpeciesCount" > 10. This rule will apply to all cells where the value of “SpeciesCount” is greater than 10.
  6. Define the Symbol: Below the Filter field, you'll see options for defining the symbol. This is where you set the color, transparency, and other visual properties for the raster cells that match your filter. You can choose from a variety of color ramps and adjust the settings to get the look you want.
  7. Add More Rules: Repeat steps 4-6 to add more rules for different categories or values in your raster data. Make sure your rules are mutually exclusive, so each raster cell only matches one rule. You can reorder the rules by dragging them up or down in the list. The rules are evaluated in order, so the first rule that matches a cell will be applied.
  8. Apply and Test: Click “Apply” to see how your rules look on the map. If you’re not happy with the results, you can go back and adjust the rules until you get the desired effect.

By following these steps, you can create a dynamic and informative raster symbology that adapts to the data in each Atlas map.

Integrating with Atlas

Now for the magic – integrating this with the Atlas Generator! The key here is to ensure that your raster symbology rules are dynamic and respond to the Atlas feature's current feature. This usually means referencing fields from your Atlas coverage layer (the polygon layer representing your grid cells) within your rule expressions.

How does this work in practice? Imagine your polygon layer has a field called “SpeciesID” that contains the ID of the species for each grid cell. You want to display a raster layer showing the habitat suitability for that species. Your raster layer might have a field called “SuitabilityScore.” You can create a rule that says: "Only show the raster cells where "SuitabilityScore" > 0.5 AND "SpeciesID" = attribute($atlasfeature, 'SpeciesID'). "

Here’s a step-by-step guide:

  1. Ensure Atlas Coverage Layer is Set Up: Make sure your Atlas coverage layer (the polygon layer) is correctly set up and linked to your Atlas configuration.
  2. Access Layer Properties: Open the properties of the raster layer you want to symbolize.
  3. Navigate to Symbology Tab: Go to the “Symbology” tab and select “Rule-based” as the render type.
  4. Create Dynamic Rules: When defining your rules, use the attribute($atlasfeature, 'fieldname') function to reference attributes from your Atlas coverage layer. For instance, if your coverage layer has a field named “SpeciesID,” your rule might look something like this: "RasterField" > 0.5 AND "SpeciesID" = attribute($atlasfeature, 'SpeciesID')
  5. Test Your Rules: Before generating the entire Atlas, test your rules by manually iterating through the features in your coverage layer. You can use the “Preview feature” option in the Atlas configuration to see how the map will look for each feature.
  6. Generate the Atlas: Once you’re satisfied with your rules, generate the Atlas. QGIS will automatically apply the correct symbology to each map based on the attributes of the current Atlas feature.

By using this approach, your raster symbology will dynamically update for each map in the Atlas, showing only the relevant information for that specific area or species.

Practical Examples and Use Cases

Let's solidify this with some real-world examples. Suppose you're mapping the distribution of different tree species across a forest. Your raster data represents the density of trees, and your Atlas coverage layer represents different forest management units.

Example 1: Highlighting Areas with High Species Richness

  • Scenario: You want to highlight areas within each forest management unit that have a high density of a particular tree species.
  • Raster Layer: Tree density raster.
  • Atlas Coverage Layer: Forest management units with a "UnitID" field.
  • Rule: `"Density" > 75 AND