Update field based on spatial query (QGIS)

Question

I was an intermediate user of MapInfo and have recently converted to QGIS. I would often use MapInfo to add a column in boundary file A saying which zone in boundary file B it is contained in. However, I cannot work out how to do this in QGIS.

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Sorry, this is the error message I received when I tried to run the function.

Answer 1

To add a column containing some attribute of a feature of another layer, with which there exists a topological relation:

1. Define a function: layer properties -> attributes -> field calculator -> function editor

2. Input code which does the work

Eg is_contained_in() returns the value from column of feature from B which contains feature A

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def is_contained_in(layername, column, feature, parent):
    layer = QgsMapLayerRegistry.instance().mapLayersByName(layername)[0]
    for feat in layer.getFeatures():
        if feature.geometry().within(feat.geometry()):
            return feat[column]

3. Run the script and go to tab Expression
4. Create a new field, a virtual one if you like, and type in an expression like this:

5. new field contained_in is added to the attribute table and hold the fid of a polygon of layer B

Tested with this situation (labels - red: layer B/fid, black: layer A/contained_in):

Answer 2

You can use the Join attributes by location tool.

Say you have the following layers:

• layer_a : the layer to which you want to attach attribute values from layer_b
• layer_b : the layer from which you want to take attribute values

Go to Vector > Data Management Tools > Join Attributes by Location.

Then, set the parameters as below:

The tool will create a third layer (named layer_c here) which will be layer_a features combined to layer_b attributes. You eventually have to remove columns that you don't need and the job is done!

Feel free to test Attribute Summary and Output table options depending on what you want to achieve.

If you are intersecting a lines or polygons layer with a polygons layer, the program might give you 'bad' results for overlapping features (i.e. a line intersecting 2 polygons or a polygon intersecting 2 polygons). In that case, the program will select attributes from the polygon that is intersected by the biggest part of the line / polygon.