在同一图层中寻找相邻的多边形¶
注解
If you are using QGIS3, there is a simpler and better way to do this analysis using the aggregate function. See my post Find Neighbor Polygons using Summary Aggregate Function in QGIS
有些时候,你会想要寻找与图层中的某个多边形相邻的所有多边形。只要弄一点 Python 程式码,在 QGIS 中就能完成这件事情,甚至做得更多。这里有一个范例程式,让你可以寻找所有与指定的多边形分享边界的多边形,并且把它们的名字加入属性表格中。除此之外,此脚本也把所有邻接多边形的某个指定栏位进行加总运算。
内容说明¶
为了展示脚本如何运作,这边我们使用全球的国界图层,然后寻找共享国界的国家。我们也会计算与某个国家相邻的全部国家的总人口数。
取得资料¶
我们要使用 Natural Earth 提供的 Admin 0 - 国界 资料集。
下载 Admin 0 - 国界(Countries)shapefile
资料来源 [NATURALEARTH]
取得脚本¶
下载 neighbors.py 脚本
然后储存至硬碟中。
操作流程¶
选择
,载入ne_10m_admin_0_countries
图层。
此脚本使用 2 个栏位执行操作,分别是名称与用来加总的栏位。使用 识别图征 工具点选任何图征检察属性,在本例中,会使用 NAME 栏位当作名称,与 POP_EST 栏位当作人口估计值。
选择
。
在 Python 主控台 视窗中,按下 显示编辑器 的按钮。
在 编辑器 面板中,按下 开启档案 钮,找到刚才下载的
neighbors.py
脚本,按下 开启。
脚本载入后,或许需要依照图层属性,编辑
_NAME_FIELD
和_SUM_FIELD
的值,不过如果你也是载入ne_10m_admin_0_countries
图层的话,使用预设值即可。如果你有编辑过的话,请先按下 编辑器 面板中的 储存 钮。接下来,按下 执行脚本 钮,程式就会开始执行。
当脚本执行完毕后,右键点选
ne_10m_admin_0_countries
图层然后选择 开启属性表格。
你会发现多了 2 个属性,分别为
NEIGHBORS
和SUM
,这两个属性就是刚才的程式新增的。
以下放上完整的脚本供各位参考,你也可以依照个人需求编辑调整此档案。
################################################################################
# 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.'