Given a point and few lines, how should I go about finding the nearest line to the point?
I am aware of Finding nearest line to point using ArcGIS Desktop (ArcObjects/ArcPy)?, but that is in ArcGIS and uses its functions.
The "duplicate" question doesn't answer this question properly, it's just a pointer to MMQGIS.
You can use QgsSpatialIndex class for finding nearest objects. First you will need to create a new object of this class. Then add the required features to the index. Then you should be able to use QgsSpatialIndex.nearestNeighbor (QgsPoint point, int neighbors) method to retrieve the nearest ones.
Here is sample that I managed in python console.
lineLayer = iface.activeLayer()
provider = lineLayer.dataProvider()
spIndex = QgsSpatialIndex() #create spatial index object
feat = QgsFeature()
fit = provider.getFeatures() #gets all features in layer
insert features to index
while fit.nextFeature(feat):
spIndex.insertFeature(feat)
pt = QgsPoint(-0.00062201,0.00001746)
QgsSpatialIndex.nearestNeighbor (QgsPoint point, int neighbors)
nearestIds = spIndex.nearestNeighbor(pt,1) # we need only one neighbour
print nearestIds
Edit:
To get the actual QgsFeature object from the python list, you can do this,
featureId = nearestIds[0]
fit2 = lineLayer.getFeatures(QgsFeatureRequest().setFilterFid(featureId))
ftr = QgsFeature()
fit2.nextFeature(ftr)
# ftr now contains the QgsFeature object for the id
Almost 10 years later it is probably worth adding an update to @vinayan's answer:
By storing the features geometries inside the QgsSpatialIndex() using QgsSpatialIndex.FlagStoreFeatureGeometries flag, you can get the real closest features, not just the approximate ones by their bounding box. This adresses the comment by Håvard Tveite. Also, nowadays it is much more efficient to bulkload the index at once; this is much faster than adding features one by one after the creation of the index in a loop. Additionally you can get the nearest geometries directly from the index, which is much faster than getting the whole feature via a QgsFeatureRequest(). If you still want to get the whole feature, you can now also use directly .getFeature(id), which is shorter. Apart from that, the syntax is now for PyQGIS 3. The code:
layer = iface.activeLayer()
Bulkloading the index is much faster than adding features one by one
Storing the geometry in the index allows finding the exact nearest neighbor, also for not single-point-features
index = QgsSpatialIndex(layer.getFeatures(), flags=QgsSpatialIndex.FlagStoreFeatureGeometries)
QgsSpatialIndex.nearestNeighbor(QgsPointXY(), int neighbors)
pt = QgsPointXY(-0.00062201,0.00001746)
nearestIds = index.nearestNeighbor(pt,1) # we need only one neighbour
Or, not stated in the docs, if you store the geometry in the index as done here, you can also: QgsSpatialIndex.nearestNeighbor(QgsGeometry(), int neighbors)
geom = QgsGeometry.fromPointXY(QgsPointXY(-0.00062201,0.00001746))
nearestIds = index.nearestNeighbor(geom,1)
note that in reality, if both layers are the same, the nearest feature will always be itself
in that case you should request 2 nearest neighbors and remove the one with the same id as your current source feature
print(nearestIds)
for nearestId in nearestIds:
# get the geometry of nearest feature efficiently directly from the index without a slow feature request
nearestGeom = index.geometry(nearestId)
print(nearestGeom)
# or get the whole feature via a short getFeature() request
nearestFeat = layer.getFeature(nearestId)
print(nearestFeat)
Meanwhile, there is an easy way to do that using QGIS expressions with overlay_nearest() function:
overlay_nearest ('line_layer', $geometry)[0]
Since QGIS 3.8 there is also a native processing algorithm: Join Attributes By Nearest
