Network Analysis - road connectivty

[BridgetWright]

Hey,

I am new to network analysis so the following problem may be very simple!

I am trying to create a 'nearest facility' network analysis dataset.  When I add stops on one road, the system seems to be able to solve the route. However, when I put a stop on a new road, the system cannot establish the connectivity. I am not sure if this is some sort of topological error.... I am pretty sure I set the roads to snap to each other..... Is there something I may be overlooking with endpoint connectivity? ..... I would also like to note that this network dataset is only for pedestrians so I presume the whole process should be less complicated than if it were for vehicles??
I have read a lot of documentation and tutorials and there seems to be little direction on how to tackle errors.....


I am working off a student version of ArcMap and I was just wondering is some of the functionality for network analysis limited on the student version? :confused:

[MichaelRice]

I assume that, when you say "the system cannot establish the connectivity", you mean that the Route solver is unable to find a path, correct?

If so, this could be for several different reasons, the two most likely of which would be the following:

• You have connectivity issues in your dataset. If the network is small enough, you can try troubleshooting this possibility by using the Network Identify tool on the Network Analyst toolbar to click on the starting edge (or junction) and try finding a simple path to the destination by progressively following the adjacent elements shown by the identify dialog. If there is no such path, then there are problems with your connectivity.


• The edge (or junction) on which you placed one of your stops is restricted or has a negative cost attribute value. Check to see which restrictions attributes you have turned on for your analysis, and, using the Network Identify tool, check to see whether the starting/ending edge (or junction) is restricted for any one of these attributes. Also, check to see if its associated cost attribute is negative. If either of these is the case, this is why you cannot find a path.

Let me know what you find for the two possibilities above, and we can proceed from there. Also, if you could provide the exact error and/or warning messages you are receiving, that would be helpful in figuring out the problem as well.

[BridgetWright]

Hey, thanks for your reply.

I have redrawn my pedestrian route shapefile and thus created a new network dataset. I ensured that all routes connected at endpoints or edges ( I am wondering is my error caused by the fact that I snapped to edges and not just endpoints...). The route solver is able to find some paths. I tried using the network identify tool - not sure if I was using it right - I was just hovering over the junction or edge and clicking and highlighting - this did not establish the connectivity.

On closer examination, there appears to be no topological errors - so i am not sure why the route solver cannot make the connection.

Also, I am not sure how the network would have negative cost attribute values - I have not included any barriers in the network dataset....

[BridgetWright]

Please find attached a jpeg image of the error message that appears when I am using the route solver tool. This attachment is related to the last post.

[MichaelRice]

I am wondering is my error caused by the fact that I snapped to edges and not just endpoints

Perhaps you are correct. A couple of things to note:

1. You must establish the appropriate connectivity policy for your network dataset. If you intend for lines to connect to one another at midspan positions (and not just at their endpoints), then you must set your connectivity policy to be 'Any Vertex' (and not 'End Point') when creating the network dataset.


2. Assuming you have set the appropriate connectivity policy, lines will only connect to each if they share a coincident vertex. That is to say, it does not suffice for them to simply cross or touch in geographic space. They must literally have coincident vertices where they cross/touch in order to establish connectivity. This can be achieved by using the 'Integrate' geoprocessing tool, as necessary.

Let me know if this helps. If not, we can go from there.