Spatial Joins
Included in this tutorial:
Setting up a spatial join
Executing a spatial join and inspecting the results
Software version in examples: ArcGIS Pro 2.8.2 (with an updated note for version 3.0.0)
Tutorial Data: The tutorial includes demonstration with sample data available here.
Credits: L. Meisterlin with Moses Levich, Thiago Lee and Erik Strand (2021)
A spatial join allows you to summarize data in one feature class layer based on its spatial relationship(s) to the features in a different layer.
This tutorial will demonstrate a point-to-polygon spatial join where point features are aggregated and summarized based on the polygons they intersect, although the same process applies to other geometry types and other spatial relationships.
Setting up a Spatial Join
Joins have direction. In our classic point-to-polygon example, the point features are the “join layer” which will be summarized within the boundaries of the polygon “target layer.”
To initiate a spatial join within the interface, right-click on the target layer’s name in the Contents Pane and click through Joins and Relates > Spatial Join or Add Spatial Join (the option is slightly different based on your version of the software, see below for more).
Here is a rundown of the options and parameters of the Spatial Join dialogue box:
The target features will be automatically selected (the Tracts feature class, in this example).
Select the join features layer (Points1, in this example).
Output:
Before version 3.0: Rename the output feature class and choose an output location.
Update starting with version 3.0: You will notice that the dialogue box does not include an output name or location options. The result of the spatial join will be temporary, existing within the current ArcGIS Pro project. To save the results of the join after it is executed, export the temporary layer to a new feature class or use the Spatial Join geoprocessing tool.
Join Operation: Specify how the join features are matched with target features in the case that multiple join features share the same spatial relationship with a target feature.
In the example below, “Join one to one” is chosen. If multiple join features have the same spatial relationship with a single target feature, the attributes from the join features will be aggregated using a Field Map merge rule.
Keep All Target Features: When left checked, all target features are kept, including the ones that don’t have any spatial relationship with join features. It is left checked for the example below.
Match Option: Here, the spatial criteria used to match Join and Target Features is specified. By default, Intersect is selected (any join features intersecting a specific target feature will be aggregated). The drop-down menu lists several other spatial relationship match options.
Search Radius: Join features within a distance from a target feature will be considered for a spatial join using a search radius parameter, depending on the Match Option chosen. In our example, the search radius option is left blank.
Field Map or Fields (depending on your software version): Here the attributes that will be in the output can be specified. Fields can be added, deleted, renamed, and reordered. And you can change their properties and how the input attributes are summarized into the output feature attribute table. Merge rules allow specification of how values from 2 or more input fields are merged into a single output value.
accessing and executing a point-to-polygon spatial join
Inspecting Results
Two new fields are added to the output feature class: Join_Count, which indicates the number of join features that match each target feature and TARGET_FID, which contains the target features identification numbers. If you select a different Join Operation, the FID for the join features will be added.
If fields were summarized (e.g., quantities were summed via the Field Map or Fields options) with your spatial join operation, those new summary fields would be represented in the attribute table as well.
new fields added to the attribute table after a spatial join