QGIS Python – How to Merge Selected Features Using PyQGIS

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I have been looking for the answer to this question for some time now, but can't seem to find it! My problem is straight forward:
I have one layer
It contains lots of features (only polygons), say countrys
I want to merge some features together.
Using python.

I know that I can select features with the mouse and hit the "Merge selected features"-button, but that is to slow.
I can use python to select the features that i want, but how can i merge the selected features from python? (the features don't have a "common field" or something like that, what features are to be merged will be decided by in-data)

Help is greatly appreciated!

Best Answer

You can use QgsGeometry.combine( QgsGeometry ) for that.

Just loop over all your features and call

geom = geom.combine( currentFeature.geometry() )

with geom being a QgsGeometry

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I am assuming that you have pre-determined the combinations of admininstrative units that will generate each TTWA equivalent, and that this is in the form of a list like this:

Admin            TTWA
Southampton UA   1
Eastleigh        1
Test Valley      1
Gosport          2
Fareham          2
Portsmouth UA    2

In which case you need to transfer this list to the Admin boundary dataset using an attribute join (if it's not already part of that dataset). Then use the Vector > Geoprocessing Tools > Dissolve command on this dataset, specifying the TTWA attribute as the Dissolve field. This command will merge all admin polygons with the same TTWA value together across the whole dataset.

If you do not have such a pre-determined list of TTWA assignments, then you would need to use zone design techiques to generate TTWAs meeting a set of criteria. A couple of options to consider would be AZTool and PySAL

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What you are calling a Merge appears to be what I would use the Join Field tool for because it ...

Joins the contents of a table to another table based on a common attribute field. The input table is updated to contain the fields from the join table. You can select which fields from the join table will be added to the input table.

Best Answer

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