[GIS] Why does QgsSpatialIndex.nearestNeighbor(point, 1) sometimes return 2 lines

nearest neighborpyqgispythonqgis

I'm trying to get the closest line to a point using PyQGIS in a plugin. The active code file is this: http://pastebin.com/TDzmsKVM

When I request the closest line (only 1)

nearestIds = self.spIndex.nearestNeighbor(self.feat2.geometry().centroid().asPoint(),1)

I sometimes get a list with two lines and the first one is not the closest but as far as I can see the second one is. Why is this?

Does QgsSpatialIndex use only boundingboxes for lines?
How do I get the real closest line always, not only most of the time?

Best Answer

The nearest points can return more then one according to the underlying code. RTree.cc (a underlaying file)

539  // report all nearest neighbors with equal greatest distances.
540  // (neighbors can be more than k, if many happen to have the same greatest distance).

The greatest distance I guess should be the smallest. But other then that it sounds like a truncation in the tree precision.

TL;DR

To get all features of a layer by the layer name you do not need to activate it. Just use

name = 'counties'
layer = QgsProject.instance().mapLayersByName( name )[0]
poles = layer.getFeatures()
for pole in poles:
    if is_north(pole):
        print('it is the north pole')

Active Layer

First of all, you do not need to care about the active layer. The active layer is the layer which is currently selected in the layer tree and therefore is nice for two things

letting the user select a layer to operate on
selecting a layer for quickly testing code from the python console

The second one is handy when you are developing, the first one is what all the maptools are based on: the currently selected layer (the active one) is modified, selected, deleted...

Methods to access layers

However, you seem to want to access layers not based on user input but based on something else.

Identifying layers

All layers have

An id that needs to be unique
A name that is not necessarily (but often) unique and can be changed by the user
A reference (also often referred to as pointer but in python the two terms are interchangeable) that is unique and stable for the lifetime of a layer during the application run time and can be assigned to a variable (e.g. vlayer is a reference to the currently active layer in your example).
Properties like the URI (table name etc.)

If you already have a reference to the layer (because you created/added it in your plugin) just use that one.

If you don't, think about what property you want to use to get access to the layer. Most likely something that is stable (the layer id is very stable for the lifetime of a project but always is different for different projects), the name is user adjustable, the table may depend on the data source.

Accessing layers

There is the map layer registry which lets you access layers. It manages references to all layers. This used to be called QgsMapLayerRegistry, but that is no longer available in QGIS 3, so QgsProject can be used instead.

registry = QgsProject.instance()

If you have the layer id go with

layer = registry.mapLayer( id )

If you have the layer name

layer = registry.mapLayersByName( name )[0] # The method returns a list since it can be ambiguous, 0 picks the first entry in the list

If you have something else you have to write your own code:

for lyr in registry.mapLayers().values():
    if lyr.dataProvider().dataSourceUri().table() == 'mytable':
        layer = lyr
        break

Or a shorthand that does the same and makes you feel more hacker'ish but will make the people who will have to maintain the code later on cost some extra time to decipher:

layer = (l for l in registry.mapLayers().values() if l.dataProvider().dataSourceUri().table() == 'mytable').next()

[GIS] Returning Multiple Nearest Neighbors

Following Joseph's suggestion, the answer lies in performing a distance matrix calculation using the PyQGIS algorithm designed for it. From here you can determine K-nearest neighbours and return a table of results. Exactly what I needed for my purposes.