There are some use cases where you want to find all neighboring polygons of each of the polygons in a layer. With a little python script, we can accomplish this and much more in QGIS. Here is an example script you can use to find all polygons that share boundary with each of the polygons in a layer and also add their names to the attribute table. As an added bonus, the script also sums up an attribute of your choice from all the neighboring polygons.
To demonstrate how the script works, we will use a layer of country polygons and find countries that share the border. We also want to compute the total population of the country’s neighbors.
We will use the Admin 0 - Countries dataset from Natural Earth.
Download the Admin 0 - countries shapefile..
Data Source [NATURALEARTH]
Download the neighbors.py script
and save it to your disk.
ne_10m_admin_0_countries
layer by going to
.neighbors.py
script and click Open._NAME_FIELD
and
_SUM_FIELD
values to match the attributes from your own layer. If you
are working with the ne_10m_admin_0_countries
layer, you can leave those
as they are. Click the Save button in the Editor
panel if you made any changes. Now click the Run script button
to execute the script.ne_10m_admin_0_countries
layer
and select Open Attribute Table.NEIGHBORS
and SUM
. These
were added by the script.Below is the complete script for reference. You may modify it to suit your needs.
################################################################################
# Copyright 2014 Ujaval Gandhi
#
#This program is free software; you can redistribute it and/or
#modify it under the terms of the GNU General Public License
#as published by the Free Software Foundation; either version 2
#of the License, or (at your option) any later version.
#
#This program is distributed in the hope that it will be useful,
#but WITHOUT ANY WARRANTY; without even the implied warranty of
#MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
#GNU General Public License for more details.
#
#You should have received a copy of the GNU General Public License
#along with this program; if not, write to the Free Software
#Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
#
################################################################################
from qgis.utils import iface
from PyQt4.QtCore import QVariant
# Replace the values below with values from your layer.
# For example, if your identifier field is called 'XYZ', then change the line
# below to _NAME_FIELD = 'XYZ'
_NAME_FIELD = 'NAME'
# Replace the value below with the field name that you want to sum up.
# For example, if the # field that you want to sum up is called 'VALUES', then
# change the line below to _SUM_FIELD = 'VALUES'
_SUM_FIELD = 'POP_EST'
# Names of the new fields to be added to the layer
_NEW_NEIGHBORS_FIELD = 'NEIGHBORS'
_NEW_SUM_FIELD = 'SUM'
layer = iface.activeLayer()
# Create 2 new fields in the layer that will hold the list of neighbors and sum
# of the chosen field.
layer.startEditing()
layer.dataProvider().addAttributes(
[QgsField(_NEW_NEIGHBORS_FIELD, QVariant.String),
QgsField(_NEW_SUM_FIELD, QVariant.Int)])
layer.updateFields()
# Create a dictionary of all features
feature_dict = {f.id(): f for f in layer.getFeatures()}
# Build a spatial index
index = QgsSpatialIndex()
for f in feature_dict.values():
index.insertFeature(f)
# Loop through all features and find features that touch each feature
for f in feature_dict.values():
print 'Working on %s' % f[_NAME_FIELD]
geom = f.geometry()
# Find all features that intersect the bounding box of the current feature.
# We use spatial index to find the features intersecting the bounding box
# of the current feature. This will narrow down the features that we need
# to check neighboring features.
intersecting_ids = index.intersects(geom.boundingBox())
# Initalize neighbors list and sum
neighbors = []
neighbors_sum = 0
for intersecting_id in intersecting_ids:
# Look up the feature from the dictionary
intersecting_f = feature_dict[intersecting_id]
# For our purpose we consider a feature as 'neighbor' if it touches or
# intersects a feature. We use the 'disjoint' predicate to satisfy
# these conditions. So if a feature is not disjoint, it is a neighbor.
if (f != intersecting_f and
not intersecting_f.geometry().disjoint(geom)):
neighbors.append(intersecting_f[_NAME_FIELD])
neighbors_sum += intersecting_f[_SUM_FIELD]
f[_NEW_NEIGHBORS_FIELD] = ','.join(neighbors)
f[_NEW_SUM_FIELD] = neighbors_sum
# Update the layer with new attribute values.
layer.updateFeature(f)
layer.commitChanges()
print 'Processing complete.'
This work is licensed under a Creative Commons Attribution 4.0 International License