Generating chainage (distance) nodes in QGIS
Something that I need to do now and then is generate points along a line at supplied distance. I had never really looked into doing it in QGIS until this question poped up on gis.stackexchange.com. This is a quick blog post because I thought it was a pretty handy little thing to do.
In the development version there is a new method on QgsGeometry called interpolate. This method takes a distance and returns a point along a line at that distance. Perfect! We can then just wrap this in a loop and generate a point increasing the distance as we move along
from qgis.core import (QgsFeature, QgsGeometry,
QgsVectorLayer, QgsMapLayerRegistry,
QgsField)
from PyQt4.QtCore import QVariant
from qgis.utils import iface
def createPointsAt(distance, geom):
length = geom.length()
currentdistance = distance
feats = []
while currentdistance < length:
# Get a point along the line at the current distance
point = geom.interpolate(currentdistance)
# Create a new QgsFeature and assign it the new geometry
fet = QgsFeature()
fet.setAttributeMap( { 0 : currentdistance } )
fet.setGeometry(point)
feats.append(fet)
# Increase the distance
currentdistance = currentdistance + distance
return feats
def pointsAlongLine(distance):
# Create a new memory layer and add a distance attribute
vl = QgsVectorLayer("Point", "distance nodes", "memory")
pr = vl.dataProvider()
pr.addAttributes( [ QgsField("distance", QVariant.Int) ] )
layer = iface.mapCanvas().currentLayer()
# Loop though all the selected features
for feature in layer.selectedFeatures():
geom = feature.geometry()
features = createPointsAt(distance, geom)
pr.addFeatures(features)
vl.updateExtents()
QgsMapLayerRegistry.instance().addMapLayer(vl)
The above code might look a bit scary at first if you have never done any Python/pyqgis but hopefully the comments will ease the pain a little. The main bit is the createPointsAt function.
Cool! If we want to use this we can just stick it in a file in the .qgis/python folder (lets call it pointtools.py) and then run import pointtools in the python console.
So lets have a go. First select some objects then run the following in the Python Console
import pointtools pointtools.pointsAlongLine(40)
That will generate a point every 40 meters along then selected lines
Distance nodes along line in qgis-dev
To generate nodes along different lines, select the new features, then call pointtools.pointsAlongLine(40) in the Python console.
Simple as that!
(Who knows, someone (maybe me) might even add this as a core object function in QGIS in the future)
Filed under: Open Source, qgis Tagged: language python, Open Source, osgeo, pyqgis, python, qgis, qgis-tips, Quantum GIS, tips
