How Spatial Join Works in GIS

Spatial Join

The spatial join tool inserts the columns from one feature table to another based on location or proximity.

Let’s say you have a set of land parcels. Each land parcel has a point inside of it.

By running a spatial join, you can transfer the point table columns into the land parcel layer.

So that means that if the points have the owner name, this field gets transferred to the land parcels.

Spatial join types

The spatial join tool uses geographic proximity to combine attributes. It moves the columns from the join features to the target features.

Before you run a spatial join, you have to select a spatial join type. That is, the proximity the spatial join will search for.

Here are the most common spatial join types.

Intersect

Two features touch at any location
Spatial Join Intersect

Within a distance

Two features are within a set distance
Spatial Join Within Distance

Completely within

The join feature is within the target feature
Spatial Join Completely Within

Identical

Both features match identically
Spatial Join Identical

Closest

The join feature is closest to the target feature
Spatial Join Closest

Depending on the spatial join type, it can affix one or several features into the target layer. In this case, you have to pick a join operation which include one-to-one or one-to-many.

Spatial join operations

Let's say you have multiple features that you want to combine into a target feature. In this case, you’ll have to specify that it's a one-to-many operation.

A one-to-one operation will join a single feature from the join features into the target features. This is usually the first record the spatial join tool finds.

one-to-one relationship

But a one-to-many operation will join all the features. It does so by creating multiple overlapping records. Each duplicate record contains the records from the joining features.

one-to-many relationship

For example, if you have 1 land parcel. This parcel has 3 points in it with 3 different owners. A spatial join will create 3 identical land parcels. But each record will have the land owners name in it.

READ MORE: Relate vs Join: Cardinality for Attribute Tables in ArcGIS

Spatial join examples

Spatial Join

Spatial join is one of the top 7 geoprocessing tools in GIS. It's just as common as clipping, buffering, dissolving and merging.

Here are some examples of practical uses of applying a spatial join:

  1. Students (points) reside in a school district (polygon). A spatial join will transfer the school district name to each student if they are within the polygon.
  2. Well logs (points) are within a specific watershed (polygon). By running a spatial join, you can affix the watershed attributes to each well location.
  3. Every county (polygon) is responsible for maintaining their own roads (lines). By running a spatial join, each road segment will add a column for the county it's in.

So that's all we have for spatial joining in GIS. What did we miss?

What would you like us to explain better? Please let us know with a comment below.

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