qgis - Counting number of lines connected to point?

I have a water network which and I need to install elbows and Tees (T). When 2 lines connect to a point, I want to install an elbow, when 3 lines connect, I want to install a Tee.

How do I get ArcGIS or QGIS to determine how many lines are connected to a point and designate an elbow or a tee based on that criteria?

Answer

It is not a problem of geometry (buffer, etc.) but a simple problem of Graph Theory (nodes and connected edges) and Python has many modules for dealing with Graphs (PythonGraphApi, Graphs in Python, Python Patterns - Implementing Graphs, ...)

In your problem, you don't need a point layer, you only need the points/nodes of the polyline. They are the vertex/nodes of a Graph and the segments between two nodes are the edges of the same Graph. And you want to find the nodes with a degree 2 and 3 (number of edges incident to the vertex = when 2 lines connect to a point, I want to install an elbow, when 3 lines connect, I want to install a Tee)

The layer:

The corresponding Graph where we can see directly the desired points (2 and 3 lines connect - figure made with the NetworkX module):

I will use here the NetworkX module (pure Python module). The module can directly open shapefiles to generates a networkx.Graph (Networkx: GIS shapefile) but I do so in the Python console of QGIS:

We need to extract all the segments of the layer (from point to point): the LineString must be iterated over pairs to divide the polyline in segments.

def pairs(list):
    '''generator to iterate over pairs in a list '''
    for i in range(1, len(list)):
        yield list[i-1], list[i]

Creation of the Graph:

import networkx as nx 
# select the polyline layer
layer = qgis.utils.iface.activeLayer()
# create a Graph
G = nx.Graph()
# add the nodes and the edges to the Graph = segment point i -> point i+1
for features in layer.getFeatures():
    segment = features.geometry().asPolyline()
    # add the two end points of a segment as two nodes and an edge (point1 -> point2) to the Graph

    for seg_start, seg_end in pair(segment):
        G.add_edges_from([(seg_start, seg_end)])

Now we have the nodes and the edges of the Graph:

 print G.nodes() # = nodes of the polyline in (x,y) form
 [(198133,93258.8), (198757,93232.4), (197592,94296.2), (197579,93232.4), (198761,94274.3), (198146,94287.5)]
 print G.edges() # = all the segments of the polyline
 [((198133,93258.8), (198146,94287.5)), ((198757,93232.4), (198761,94274.3)), ((197592,94296.2), (197579,93232.4)), ((197592,94296.2), (198146,94287.5)), ((198761,94274.3), (198146,94287.5))]

We search the nodes which the degree is > 1

 for i in G.nodes():
    if G.degree(i) > 1:
        print QgsPoint(i), G.degree(i)
 (197592,94296.2) 2
 (198761,94274.3) 2
 (198146,94287.5) 3

And we obtain the desired points:

Result: