Automate saving features by results of join attributes by location

Question

In QGIS v3.8.3 I have one shapefile of all US Census Tracts and another of ~1,000 US Counties. There are multiple tracts in a county. So I did a many-to-one join in Join attributes by location to add a 'county' column to the Tracts file.

I'd like to export shapefiles for each county-based group of tracts. For example, one shapefile for the ten tracts in County 1, another shapefile for the twelve tracts in County 2...all the way to County 1,000. I'd prefer not to click Export > Save selected features as a thousand times.

Here's what I've tried and looked at so far, but I didn't think that any of them were specifically focused on batch exporting based on the value of a specific column (but I may be wrong, I'm not super Python-literate):

Batch save layers plugin

Exporting several files at same time in QGIS?

QGIS batch conversion

Automate SHP to JPG creation

Answer 1

The tool Split Vector Layer lets you batch export a layer based on the value in a column.

This tool outputs in whatever format you have chosen for Processing output file type, which by default is geopackage. To change this setting, go to the Processing options tab in the Settings menu. Change the "default output vector layer extension" to "shp". (For more details on this workaround see this answer.)

Answer 2

You could achieve this with a short Python script which chains together a couple of processing algorithms- Select by location and Save selected features. If we put these into a for loop, iterating over each feature in your County layer, selecting it and running Select by location (on selected features only) to select all census tracts which intersect the selected county feature and saving these as a new shapefile.

Firstly, create a folder in your local directory in which the output files will be saved.

Paste the following code into a new editor in the Python console (see image below).

Change the file path and layer names to match yours, and click run.

Disclaimer: I am not sure how long this will take to run on 1000 features.

import os

# change path to point to the folder you created earlier
path = 'C:/Users/Username/Desktop/Counties'

# change name strings below to match your layers
county_layer = QgsProject().instance().mapLayersByName('US_Counties')[0]
census_layer = QgsProject().instance().mapLayersByName('US_Census_Tracts')[0]
for f in county_layer.getFeatures():
    county_layer.selectByIds([f.id()])
    processing.run('native:selectbylocation', {'INPUT': census_layer,'PREDICATE':[0],
    'INTERSECT':QgsProcessingFeatureSourceDefinition(county_layer.id(), True),'METHOD':0})
    processing.run('native:saveselectedfeatures', {'INPUT': census_layer,
    'OUTPUT': os.path.join(path, 'County_{}.shp'.format(str(f.id()+1)))})
    # alternatively, you could use:
    # 'OUTPUT': os.path.join(path, '{}.shp'.format(f['Name']))
    # where 'Name' is the field in your county layer which contains the county name
county_layer.selectByIds([])
census_layer.selectByIds([])