Contents:
The QGIS NNPlugin can be used to join two vector layers (the input and the join layer).
All geometry type combinations are supported.
A feature from the input layer is joined to the nearest feature in the join layer.
The result of the join is a new vector layer with the same geometry type and coordinate reference system as the input layer.
Joining layers with different Coordinate Reference Systems (CRS) is supported. A warning is given when the user attempts to join layers with different CRS.
Self joins are supported. For self joins, each feature in the layer is joined to its nearest neighbour within the layer.
The result layer will contain all the attributes of both the input and join layers plus a new attribute “distance” that contains the distance between the joined features. The attributes from the join layer will get a prefix (the default “join_”, but this can be set by the user). If a join prefix is not used, attributes from the join layer that have the same name as attributes in the input layer will not be included in the output layer.
The user can choose to use an approximation of the input geometry (the centroid) to allow the use of indexes also for non-point layers.
The user can choose the prefix of the join layer attributes in the output layer.
Below is a table that cross tabulates input layer (rows) and join layer (column) geometry types, and indicates the usefulness of the plugin (OK or slow).
| Layer (row: input; column: join) | Point | Line | Polygon |
|---|---|---|---|
| Point | OK | OK | OK |
| Line | Slow! | Slow! | Slow! |
| Polygon | Slow! | Slow! | Slow! |
Multi geometries:
| Layer (row: input; column: join) | MultiPoint | MultiLine | MultiPolygon |
|---|---|---|---|
| MultiPoint | Slow! | Slow! | Slow! |
| MultiLine | Slow! | Slow! | Slow! |
| MultiPolygon | Slow! | Slow! | Slow! |
The implementation is naive, in that it loops through all the features of the input dataset, and for each feature finds the nearest neighbour in the join dataset.
For input layers with geometry type Point (and centroid approximations) a spatial index on the join layer is created, and the QgsSpatialIndex.nearestNeighbor function of this index is used to find the nearest neighbour for each input feature.
For input layers with other geometry types, the QgsGeometry.distance function is used to find the distances between the geometries. The feature of the join layer that has the shortest distance to the the geometry of the input feature is chosen as the nearest neighbour. This means that the geometry of each feature of the input layer has to be compared to the geometry of all the features in the join layer!
If the input and join layers have different Coordinate Reference Systems (CRS), the input geometry is transformed to the join layer CRS before the join is performed.
The current version is 1.1.0