How can I switch line direction in QGIS?

Question

Something that is hopefully pretty easy.

I would like to be able to swap the direction of a polyline/line in QGIS.  I made a custom tool to do this in MapInfo a couple of years ago, however I can't seem to find anything for QGIS.

Does anyone know of a tool to do this?

If one doesn't exist then I'm happy to have a go at making it, just didn't want to if one has already been made.

Nathan W · Accepted Answer

Ok here is the Python that I used to do it:

layer = qgis.utils.iface.mapCanvas().currentLayer()
for feature in layer.selectedFeatures():
   geom = feature.geometry()
   nodes = geom.asPolyline()
   nodes.reverse() 
   newgeom = QgsGeometry.fromPolyline(nodes)
   layer.changeGeometry(feature.id(),newgeom)

Before running the above code:

Select the layer you want to edit; 
Toggle Editing on;
Select the features in this layer you want to reverse.

Run the above python code by:

Going to Plugins > Python Console;
Copying and pasting the code into the window;
Hit Enter 2x to run.

It should output "True" for each feature whose direction was swapped.  If it outputs "False" then it didn't swap that feature's direction.  This will happen if the layer doesn't have Editing toggled on.

Pretty easy!

I have wrapped it up in a plugin called Swap Line Direction and it's available in the QGIS plugin installer.

This plugin is no longer available (as of 11/16/2015) in the plugin installer but you can build your own pretty easily with the "Plugin Builder" plugin.

I'll have a look at how easy it is to intergrate with fTools.

Mapperz · Answer

If you have the GRASS plugin use the v.flip option - http://grass.osgeo.org/wiki/GRASS_AddOns#v.flip

Denis Rouzaud · Answer

Following Nathan's answer, you can create a python action in the layer where you want to swap lines:

layer = QgsMapLayerRegistry.instance().mapLayer("_your_layer_id_")
r = QgsFeatureRequest([% $id %])
f = QgsFeature()
if layer.getFeatures(r).nextFeature(f):
  geom = f.geometry().asPolyline()
  geom.reverse()
  geom = QgsGeometry.fromPolyline(geom)
  if layer.changeGeometry([% $id %], geom):
    qgis.utils.iface.messageBar().pushMessage("Line swaped", QgsMessageBar.INFO, 2)
    qgis.utils.iface.mapCanvas().refresh()
  else:
    qgis.utils.iface.messageBar().pushMessage("Cannot swap line. Turn editing on.", QgsMessageBar.WARNING, 3)
else:
  qgis.utils.iface.messageBar().pushMessage("Cannot edit this feature.", QgsMessageBar.CRITICAL, 3)

You will be able to swap lines by one click on them.
It is much user-friendly!

GreyHippo · Answer

This plugin will switch the direction of a polyline.

http://plugins.qgis.org/plugins/DigitizingTools/

AndreJ · Answer

This plugin reverses the line direction too:

https://hub.qgis.org/projects/swapvectordirection

You have to select a feature, and turn the layer into edit mode before clicking on the plugin icon.

eclipsed_by_the_moon · Answer

Based on Nathan's post I've created a processing script to reverse the direction of features:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# define Interface
##QGIS tools (QGIS 2.x)=group
##Reverse vector direction=name
##Input_layer=vector
##Only_selected_features=boolean true

from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4 import *

from qgis.core import *
from qgis.gui import *
from qgis.utils import *

# get input parameters from GUI
inlayer = processing.getObject(Input_layer)
selected = Only_selected_features

# check 'Only selected features' option
if selected is True and inlayer.selectedFeatureCount () == 0:
    raise RuntimeError('No features selected on Layer '' + inlayer.name() + ''.')

# check if input layer is editable
if not inlayer.isEditable():
    inlayer.startEditing()

# reverse vector direction
def reverse():
    geom = feature.geometry()
    nodes = geom.asPolyline()
    nodes.reverse()
    newgeom = QgsGeometry.fromPolyline(nodes)
    inlayer.changeGeometry(feature.id(),newgeom)

if selected is True:
    for feature in inlayer.selectedFeatures():
        reverse()
else:
    for feature in inlayer.getFeatures():
        reverse()

# refresh input layer
inlayer.dataProvider().forceReload()

grego · Answer

(Using QGIS 3.10)
Following @Nathan W 's answer above, here's a solution if your line is a LineStringZ (i.e. a line with an x, y and Z value) - as are most Google Earth KML files (even if that Z value = 0, which is often the case.)
You might get this error when the nodes.reverse call is made in the code below:

TypeError: index 0 has type 'QgsPointXY' but 'QgsPoint' is expected

1   layer = qgis.utils.iface.mapCanvas().currentLayer()
2   for feature in layer.selectedFeatures():
3      geom = feature.geometry()
4      nodes = geom.asPolyline()
5      nodes.reverse() 
6      newgeom = QgsGeometry.fromPolyline(nodes)
7       layer.changeGeometry(feature.id(),newgeom)

The solution to that is to use QgsGeometry.fromPolylineXY() instead in line 6:
1 vLayer = qgis.utils.iface.mapCanvas().currentLayer()
2 for feat in vLayer.selectedFeatures():
3     geom = feat.geometry()
4     pointXY_nodes = geom.asPolyline() # >>> returns list of QgsPointXY objects: [<QgsPointXY: POINT(-117.60560599 ..
5     pointXY_nodes.reverse() 
6     newgeom = QgsGeometry.fromPolylineXY(pointXY_nodes)
7     vLayer.changeGeometry(feat.id(),newgeom)
8     print("Geom reversed!")

How can I switch line direction in QGIS?

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