6.1 Vector Analysis: Overlay Tools (Intersect, Union, Erase, Identity, Clip)
Key Takeaways
- Intersect computes the geometric intersection of all inputs, preserving attributes from all input features but retaining only overlapping areas.
- Union computes a geometric union of all inputs, requires polygon inputs exclusively, and preserves all features and their attributes.
- Erase removes portions of the input features that fall within the erase features, transferring no attributes from the erase layer.
- Identity computes a geometric intersection of the input features and identity features, retaining all input features while appending overlapping identity attributes.
- Clip extracts input features that overlay the clip features without combining attributes, functioning purely as an extraction mechanism.
Vector Analysis: Overlay Tools in ArcGIS Pro 3.x
Vector overlay operations form the backbone of spatial analysis in Geographic Information Systems. In ArcGIS Pro 3.x, these tools allow analysts to superimpose two or more feature classes to create a new output feature class. This new dataset combines the geometries and, depending on the tool chosen, the attributes of the inputs.
For the EAPA_2025 exam, you must have an intimate understanding of the geometric outcomes and tabular behaviors of the five primary overlay tools: Intersect, Union, Erase, Identity, and Clip.
1. Intersect
The Intersect tool computes a geometric intersection of the input features. Features or portions of features that overlap in all layers and/or feature classes will be written to the output feature class.
Geometric and Tabular Behavior
- Geometry: Only the overlapping areas are preserved. If no features overlap, the output will be empty.
- Attributes: The output contains attributes from all input feature classes.
- Inputs: Can accept points, lines, or polygons.
- Output Geometry Type: The output geometry type must be the same dimension as or lower than the input with the lowest dimension. For example, intersecting a line layer and a polygon layer results in a line or a point layer, but never a polygon layer.
Step-by-Step Workflow in ArcGIS Pro 3.x
- Navigate to the Analysis tab and click Tools to open the Geoprocessing pane.
- Search for and open the Intersect tool (Analysis Tools > Extract).
- In the Input Features parameter, add the layers you wish to intersect. The order of inputs does not affect the geometric output but can dictate attribute order.
- Specify the Output Feature Class in your project geodatabase.
- (Optional) Adjust the Attributes To Join parameter. The default is 'All', meaning all attributes are transferred.
- Run the tool.
Exam Tip: When a scenario asks you to find areas where multiple conditions co-exist (e.g., areas that are both forested AND have steep slopes) and you need the attributes of both layers for subsequent analysis, Intersect is the correct tool.
2. Union
The Union tool computes a geometric union of the input features. All features and their attributes will be written to the output feature class.
Geometric and Tabular Behavior
- Geometry: The full extent of all input features is preserved. Where features overlap, new intersecting geometries are created.
- Attributes: The output contains attributes from all input feature classes. For areas where a feature from one input does not overlap with a feature from another input, the attributes for the missing feature will be set to Null.
- Inputs: Requires all input feature classes to be polygons. This is a strict requirement.
- Output Geometry Type: Always polygons.
Edge Cases and Environment Settings
If you perform a Union and notice small, unintended sliver polygons, you may need to adjust the XY Tolerance environment setting. A larger XY tolerance forces vertices that are very close together to snap to one another, eliminating micro-slivers.
3. Erase
The Erase tool creates a feature class by overlaying the Input Features with the polygons of the Erase Features. Only those portions of the input features falling outside the erase features' boundaries are copied to the output.
Geometric and Tabular Behavior
- Geometry: Portions of the input features that overlap the erase features are removed.
- Attributes: Only the attributes of the Input Features are preserved. Attributes from the Erase Features are strictly ignored.
- Inputs: Input features can be point, line, or polygon. Erase features must be polygons (or have the same or higher dimension than the input).
Exam Tip: Erase is heavily utilized in suitability models to exclude restricted areas. For instance, if you are finding suitable development parcels, you would erase wetlands, steep slopes, and protected habitats from your study area.
4. Identity
The Identity tool computes a geometric intersection of the input features and identity features. The input features or portions thereof that overlap identity features will acquire the attributes of those identity features.
Geometric and Tabular Behavior
- Geometry: All input features are preserved. Where they overlap with identity features, new geometries are created based on the intersection boundary.
- Attributes: The output combines the attributes of the input features and the overlapping identity features. Input features not overlapping identity features retain only their original attributes (with nulls for the identity fields).
- Inputs: Input features can be point, line, or polygon. Identity features must be polygons.
5. Clip
The Clip tool extracts input features that overlay the clip features. Think of it as a cookie cutter.
Geometric and Tabular Behavior
- Geometry: Only the input features falling within the clip features are preserved. Their geometry is trimmed to the exact boundary of the clip features.
- Attributes: Only the attributes of the Input Features are preserved. Clip does not combine or join attributes.
- Inputs: Input features can be point, line, or polygon. Clip features must be polygons.
Tool Comparison Matrix
| Tool | Preserved Geometry | Preserved Attributes | Input Requirements |
|---|---|---|---|
| Intersect | Overlap only | Both inputs | Any geometry combination |
| Union | All inputs | Both inputs | Polygons only |
| Erase | Non-overlapping input | Input only | Erase features must be polygons |
| Identity | All input | Input + overlapping identity | Identity features must be polygons |
| Clip | Overlapping input | Input only | Clip features must be polygons |
An analyst is preparing data for a site suitability model and needs to exclude all areas that fall within a 100-year flood zone. The analyst wants to keep the original attributes of the study area but does not need any attributes from the flood zone layer. Which overlay tool should be used?
Which of the following vector overlay tools strictly requires all input feature classes to be polygons?
A GIS professional wants to extract road segments that fall within a specific city boundary. They do not need to append the city's demographic attributes to the roads; they simply want to cut the roads at the city boundary. Which tool is the most efficient choice?