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: ArcGIS Pro 3.5.2
Tutorial Data: The tutorial includes demonstration with sample data available here.
Credits: L. Meisterlin (2025)
The result of a union operation will include the attributes from all the input polygon layers based on which features overlap. If multiple input layers overlap in a specific region, the resulting polygon feature will contain the relevant attributes from each of those overlapping layers.
Accessing the Union Tool
accessing the Union tool through the main menu
To access the Union tool, you can click through the Analysis tab (on the Main Menu ribbon) > Tools section > Overlay Features: Union.
Alternatively, you can search or navigate through the Geoprocessing Toolbox to find the tool at Analysis Tools > Overlay > Union.
The Union tool parameters
The tool’s basic parameters require a user to specify the layers with features to be evaluated for overlaps and which attributes should be maintained in the results. These are described below.
Input features: A minimum of one input layer is required for the tool to process any overlapping geometries. Additional layers can be included here.
If you include more than one 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 include only one layer: Only the overlapping geometry of the input layer’s features will be evaluated.
Output Feature Class: Choose the name and location (and file type or format) for saving the output vector dataset.
Attributes to Join: This dropdrop down menu allows you to choose which attributes to combine in the output, which is particularly relevant for results that represent the areas of overlap between input features.You can choose to maintain All Attributes (which is the default option), only the feature IDs (which is useful when the inputs have many attribute fields), or all attributes except for the feature IDs (to avoid confusion later in your analysis).
The optional Gaps Allowed checkbox allows you to toggle whether only contiguous collections of features will be considered and generated.
the Union tool parameters
the Attributes to Join options expanded
Running the Tool and Examining Results, with an Example
In this example, we will “unite” two polygon layers, each comprising buffer polygons generated from the points in the Standard Tutorial Data package.
In the images 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
zooming into the input layers
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 images below include 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
zooming into the output layer