QGIS – First and Last Point of MultiLineString-objects in QGIS 3

pointpyqgisqgisvertices

I would like to identify the start- and endpoints within a set of polylines and return the coordinates of both points finally.

So far i have been able to iterate the features and found out that ArcGIS's export created a MulitLineString with a number of vertices (where the number corresponds to the length of the line).

Using feature.geometry().vertexAt(0) (where feature is the current feature within a for loop) returns the first point of every feature and the print-result looks like that <QgsPoint: Point (4800744.81731882505118847 2812251.2178244274109602)> but i didn't find a way to set up a "for vertex in vertices"-style iteration to get the last point of every line.

The Python2.x-style from Can vector layer get start point and end point of line using PyQGIS with

geom = feature.geometry().asPolyline()
start_point = QgsPoint(geom[0])
end_point = QgsPoint(geom[-1])

doesn't work anymore.asPolyline() seems to return an empty list.

I've tried to iterate the vertices with vertices(self) → QgsVertexIterator but cant get any results.
since i am pretty new to PyQGIS the QGIS Python API seems really confusing to me.

Best Answer

If you're input is a multilinestring, then you'll need to handle that by either iterating through all the parts or (blindly!) just taking the first part alone.

For QGIS <= 3.4:

To take the first part:

multilinestring = feature.geometry().get()
first_part = multilinestring.geometryN(0)
# first_part will be a QgsLineString object

To get the first/last vertices:

first_vertex = first_part.pointN(0)
last_vertex = first_part.pointN(first_part.numPoints()-1)

For QGIS > 3.4:

The API is much easier here. You can use code like this and it'll work for BOTH linestring and multilinestring inputs:

for part in feature.geometry().get():
    first_vertex = part[0]
    last_vertex = part[-1]

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It is a pure Python problem not limited to PyQGIS. I learn Python before using PyQGIS and the Help of PyQGIS.

In pure Python examine the attributes of feature with the dir() function:

dir(features)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__iter__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'close', 'isClosed', 'next', 'nextFeature', 'rewind']

And there is no get here, hence the error: QgsFeatureIterator' object has no attribute 'get' See log for more details when you try f1 = features.get(1).

In contrats, features has the __iter__ method which means that you can use a for loop, look at Understanding Python Iterables and Iterators

When Python executes the for loop, it first invokes the iter() method of the container to get the iterator of the container. It then repeatedly calls the next() method (next() method in Python 3.x) of the iterator until the iterator raises a StopIteration exception. Once the exception is raised, the for loop ends. (when Python executes the for loop, it first invokes the iter() method of the container to get the iterator of the container)

So for feature in features is the same as features.next(), features.next(),...

Therefore the answer of Underdark features.next() is a pure Python problem, not limited to the Help of PyQGIS (The same kind of problem existed with Avenue, even with the Help)

[GIS] Counting number of vertices of object on vector layer PyQGIS

Indentation.

The first part of your code is correct, but the rest can be greatly simplified if you just want the number of vertices:

layer = qgis.utils.iface.activeLayer()
feat = layer.getFeatures()

for feature in feat:
    if feature.geometry().isMultipart(): # new part for multipolylines
        vertices = feature.geometry().asMultiPolyline()
        print [len(v) for v in vertices]
    else:
        vertices = feature.geometry().asPolyline()
        n = len(vertices)
        print n

If you also want the coordinates of the vertices, then you can write (polylines only):

layer = qgis.utils.iface.activeLayer()
feat = layer.getFeatures()

for feature in feat:
    vertices = feature.geometry().asPolyline()
    points = []

    for v in vertices:
        points.append(v)

Best Answer

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