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13. Spatial Join

Goal: Run a Spatial Join in ArcGIS Pro — the operation that combines two layers based on where they are.

→ For concepts, see Spatial Joins (roadmap).

Tool location

Geoprocessing → Spatial Join (Analysis Tools).

Parameters

Parameter Notes
Target Features The layer that gets the joined attributes
Join Features The layer providing the data
Output Feature Class Where to save
Join Operation JOIN_ONE_TO_ONE or JOIN_ONE_TO_MANY
Keep All Target Features Yes (left join) / No (inner join)
Field Map Choose which fields, and how to aggregate
Match Option INTERSECT, WITHIN, CONTAINS, CLOSEST, …
Search Radius Optional — for "within distance" matches

The match option

Option Meaning
INTERSECT Any geometric overlap
WITHIN Target fully inside join feature
CONTAINS Target fully contains join feature
CLOSEST Picks nearest join feature; can record distance
WITHIN_A_DISTANCE Match if within search radius
HAVE_THEIR_CENTER_IN Polygon centroid in target
CLOSEST_GEODESIC Closest, computed on the ellipsoid

One-to-one vs one-to-many

flowchart LR
    Tgt[Precincts<br/>22 features] --> Op{Match}
    Joi[Crimes<br/>1,200 features] --> Op
    Op --> One[One-to-one:<br/>22 output rows<br/>each summarized]
    Op --> Many[One-to-many:<br/>1,200 output rows<br/>one per match]

    classDef tgt fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    classDef joi fill:#fef3c7,stroke:#f59e0b,color:#92400e
    classDef out fill:#dcfce7,stroke:#10b981,color:#065f46
    class Tgt tgt
    class Joi joi
    class One,Many out
  • JOIN_ONE_TO_ONE (default) → one row per target. Multiple matches get summarized by the field map.
  • JOIN_ONE_TO_MANY → one row per match. Target geometry is duplicated.

Field map (the secret weapon)

In the Fields parameter, click each field and pick a merge rule:

Rule Use for
Count Total matches
Sum Add values
Mean / Median Average
Min / Max Extreme values
First / Last Just pick one
Join Concatenate text (e.g., comma-separated)

Classic patterns

Count points per polygon

"Crimes per precinct"

  • Target: precincts
  • Join: crimes
  • Match: INTERSECT
  • Operation: ONE_TO_ONE
  • Field map: Join_Count field is added automatically

Closest facility

"Closest fire station to each school"

  • Target: schools
  • Join: fire stations
  • Match: CLOSEST
  • Operation: ONE_TO_ONE
  • Tick Distance Field Name to record the distance

Tag points with the polygon they're in

"Add neighborhood name to each crime"

  • Target: crimes
  • Join: neighborhoods
  • Match: INTERSECT (or WITHIN)
  • Operation: ONE_TO_ONE
  • Field map: just keep NEIGHBORHOOD from join layer

Summarize Within (alternative)

For points-in-polygon counts/stats, Summarize Within is often easier than Spatial Join. Output is cleaner — a single attribute table with counts and stats.

Tool When
Spatial Join You need every joined field, custom field maps, ONE_TO_MANY
Summarize Within Just counts/stats, points-in-polygons

Pitfalls

Watch the search radius

With WITHIN_A_DISTANCE + a small radius, you can miss matches if the layers are in a geographic CRS. Project to a projected CRS first.

Geometry won't match

Two layers showing the "same" boundaries but loaded from different sources may not perfectly overlap. Use a small search radius to absorb tiny gaps.

Practice

Crime per precinct

  1. Project precincts (polygons) and crimes (points) to State Plane.
  2. Spatial Join: target = precincts, join = crimes, match = INTERSECT.
  3. In Field Map: keep INCIDENT_TYPE with merge rule Count.
  4. Symbolize precincts by the Join_Count.

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