Feature Unions
This tutorial reviews a classic Boolean geometry operation: a Union. The tool separates overlapping polygons into distinct features representing the individual areas of overlap or non-overlap.
Included in this tutorial:
Accessing the Union Tool
The Union tool parameters
Running the Tool and Examining Results, with an Example
Software version in examples: QGIS 3.84 Prinzen
Tutorial Data: The tutorial includes demonstration with sample data available here.
Credits: L. Meisterlin (2025)
The result of a union operation will include all of the attributes from the input polygon layers. If multiple input layers overlap in a specific region, the resulting polygon feature will contain the attributes from each of those overlapping layers.
Accessing the Union Tool
To access the Union tool, you can click through Vector > Geoprocessing Tools > Union via the main menu.
Alternatively, you can search or navigate through the Processing Toolbox. There, you will find the Union tools within the Vector Overlay toolset.
accessing the Union tool within the main menu
accessing the Union tools within the Processing toolbox
Notice that the Processing Toolbox contains two relevant tools: Union and Union (multiple). This tutorial will review and demonstrate using the former (“Union”) tool. The “multiple” version of the tool is nearly identical but allows a user to include more than two input layers.
The Union Tool Parameters
The tool’s basic parameters require a user to specify the layers with features to be evaluated for overlaps. These are described below.
Input layer: A minimum of one input layer is required for the tool to process any overlapping geometries. If the input layer has an active selection within the current project, you can also optionally choose to process the selected features only with the checkbox.
Overlay layer: This optional parameter allows you to specify the “overlay layer” or a second layer of features to be compared to the input layer for overlaps. Again, if the overlay layer has an active selection within the current project, you can optionally choose to process the selected features only with the checkbox.
If you include an overlay layer: features from both layers will be split along the edges of their overlap. Attributes from the relevant overlapping features will be included in the output.
If you do not include an overlay layer: Only the overlapping geometry of the input layer’s features will be evaluated. Identical features representing the areas of overlap will be generated in the output, each containing their original attributes.
The Advanced Parameters include the helpful option to give a prefix to fields in the output’s attribute table if distinguishing the original source of different columns/fields could present a worry later in your analysis.
The Output options (under “Union” in the dialogue box) are standard. They include the option of whether and where to save the resulting output vector dataset (click the ellipsis [...] button to access saving and naming options) or create a temporary layer, as well as the checkbox option to add the resulting layer to the current map project.
the Union tool parameters, with example inputs
Running the Tool and Examining Results, with an Example
In this example, we will “unite” two polygon layers (see the image above for the tool’s inputs), each comprising a buffer polygons generated from the points in the Standard Tutorial Data package.
In the image below, we can see that the two layers (symbolized in pink and orange) are a mess of overlapping features.
example input layers of overlapping polygon features
After running the tool, the result is a single vector polygon dataset. Each polygon encompasses parts of the input and overlay layers, representing the areas of distinct overlap.
The image below includes the output geometry, with three features selected to show the limits of their perimeters. Again, for all features that include overlapping polygons from both the input and overlay layers, the attributes from both are included in the result.
resulting geometry of the example union operation, with three polygon features selected