Spatial Join

A spatial join matches features by location instead of by a shared ID. Esri's ArcGIS Pro documentation says the Spatial Join tool "Joins attributes from one or more inputs to another input based on the spatial relationship." The features that receive attributes are the target features; the features that supply them are the join features. The result is written to a new feature class.

How matches are counted

The tool adds a Join_Count field: "The number of join features that match each target feature." There are two join operations:

• Join one to one: when several join features match one target, their attributes are combined with a merge rule such as Sum, Mean, First, or Concatenate. This is the default.
• Join one to many: the target feature is copied once per match, each copy carrying one join feature's attributes.

By default every target feature is kept, matched or not, which Esri calls an outer join. Turning that off keeps only the targets that matched, an inner join.

Match options

The relationship decides what counts as a match. Esri's list includes:

• Intersect, the default
• Within a distance, with a geodesic version for data that "covers a large geographic extent"
• Contains and Completely contains
• Within and Completely within
• Boundary touches and Share a line segment with
• Have their center in, which uses the centroid for polygons
• Closest

Esri also offers "Clementini" versions of Contains and Within. Under them, a feature lying entirely on the other feature's boundary "will not be matched."

Quirks to know

• "If a join feature has a spatial relationship with multiple target features, it will be counted as many times as it is matched."
• With Closest, when two join features are the same distance away, "one of the join features will be randomly selected as the matching feature."

Why it matters in mining claim research

Spatial joins answer everyday questions: which sections a mining claim touches, which mining claims fall inside a withdrawal, which cases sit in a township. BLM most commonly maps mining claim cases to quarter sections, so their polygons share edges with the PLSS grid and with each other. The match option decides whether a shared edge counts, and the counts change with it. The option used belongs in the notes for any result, and the matches should be checked against the legal descriptions. See Buffer Analysis and Geospatial Analysis.