Understanding "Join Attributes by Location" in QGIS

Question

I wonder if there is somewhere an explanation for the parameters that can be set when running the Algorithm "Join Attributes by Location" in QGIS 2.18.

I had a look here:

• Documentation QGIS Testing | Join attributes by location
• Documentation for QGIS 2.8 | fTools Plugin

but there is no explanation. Any hints?

Answer 1

Join Attributes by Location concatenates the following set of methods to compare geometries:

• intersects if the intersection of both geometries is not empty

  

• contains if the second geometry is completely contained into the first one

  

• disjoint if the intersection of both geometries is the empty set

  

• equals if they are spatially identical

  

• touches if the only points in common between both geometries lie in the union of their boundaries

  

• overlaps if the intersection of both geometries results in a value of the same dimension of both geometries and is different from both the first and the second geometry

  

• within if the first geometry is completely contained into the second one

  

• crosses if the intersection of both geometries results in a value whose dimension is less than the maximum dimension of both geometries and the intersection value includes points interior to both geometries, and the intersection value is not equal to either the first or the second geometry

  

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References:

• A quick tutorial to SpatiaLite | 4.2 Evaluating relationships between geometries
• boundlessgeo/workshops/postgis/source/en/spatial_relationships
• GIS Geography | How Spatial Join Works in GIS

Answer 2

Don't think the QGIS docs has something as detailed as what is shown in the link in your comment (here is the link for English speakers). I would assume the terminology would be pretty much similar if not the same.

However, the tool uses the QgsGeometry Class which for each geometric predicate has the following basic description:

• intersects - Test for intersection with a geometry (uses GEOS)
• contains - Test for if geometry is contained in another (uses GEOS)
• disjoint - Test for if geometry is disjoint of another (uses GEOS)
• equals - Test for if geometry equals another (uses GEOS)
• touches - Test for if geometry touch another (uses GEOS)
• overlaps - Test for if geometry overlaps another (uses GEOS)
• within - Test for if geometry is within another (uses GEOS)
• crosses - Test for if geometry crosses another (uses GEOS)

To elaborate on the answers given above, the geometric predicates of QGIS (via GEOS) utilise the spatial predicate definitions of DE-9IM. The following link provides the detailed explanation of each of the common predicates: https://en.wikipedia.org/wiki/DE-9IM#Spatial_predicates

Answer 3

I found this a very easy to understand (with a nice image) the predicates:

https://docs.qgis.org/3.28/en/docs/user_manual/processing_algs/algs_include.html#geometric-predicates