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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?

enter image description here

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What is your data structure like? Separate layers for points and lines? –  Simbamangu Feb 16 at 9:08
    
Gene is right, make pair then use a dfs algorithm to know how many edges. –  user27003 Feb 16 at 14:45

3 Answers 3

up vote 5 down vote accepted

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:

enter image description here

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

enter image description here

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:

enter image description here

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It depends on Your data structure and precision but I assume that You have separate layers for points and lines. If so, this is relatively easy with Python (either PyQGIS or arcpy) - example with pyqgis could look like this:

#point_layer - vector layer with points
#line_layer - vector layer with lines

iter_p=point_layer.getFeatures() #features from layer
iter_l=list(line_layer.getFeatures()) #changing iterator to list to avoid exhaustion

connections=0 #counter for number of intersections

for p in iter_p:
    geom_p=p.geometry().buffer(10,3) #create a small buffer around points, 10 is an arbitrary value, change it according to precision of Your data
    for l in iter_l:
        geom_l=l.geometry()
        if geom_p.intersects(geom_l):
            connections=connections+1 # if intersection is true increase counter by one
    if connections==2:
        #here a set of instructions for doing something - for example changing attrubute value for Tee, elbow or something else 
    #etc...for each significant value of intersections
    connections=0 #reseting counter
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Just checking the buffer isn't the best way. The lines might not connect to the points. –  Nathan W Feb 15 at 23:10
    
That's true but I've assumed that this is a more real-life problem and data structure is unknown - then it is safer to assume errors etc. - hence very small buffer. If the data is a topologically correct network then Gene answer is better –  mrz Feb 17 at 10:43

In Arcgis, the most simple way is to use "spatial join" : this will give you the count of lines that interesect your point. In order to make sure that your points are exactly located at the intersections, an integrate could be necessary.

On the other hand, you should be aware of your topology in order to know what you are counting. You assume here that all your lines are split at point location, but you should make sure of that. Otherwise, you could have points that intersect 2 lines on a Tee. A more robust method would thus consist in creating buffers around each point and counting the number of point intersections between your lines and your buffer, not just the number of lines that intersect.

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