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Basic Pedshed Analysis - skewed results

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08-23-2022 08:16 PM
GISMac
by
New Contributor II

Hi,

I am new to Network Analysis but have been watching some tutorials in order to complete what I thought was a basic analysis.

What I want to do is use road network data (custom) and generate a Service area using a 800m distance. I will later update the road layer to include footpaths.

What I have done:

1. Create Network dataset using my roads layer. (no elevation involved).

2. Updated properties to turn on Indexes and add Walking as a Travel Mode (not that this should matter as it is 800m either way)

3. Build

4. Import the train station as a Facility

4. Run Service Area tool

 

The results are not correct. The area is skewed to the N/W and there is no reason someone cannot walk/drive 800m along the easterly roads (800m). There are no restrictions as far as I am aware. I have also run Topo checks (Topo) on the road data for the following:

  • Overlaps
  • Intersects
  • Pseudo-Nodes 
  • Self Intersect

As a test I drew a straight line from the station and re-ran, it picks this up! (TEST) which indicates it is the road data topo, but how!?

FYI I have also run a Repair Geometry on the roads.

 

Does anyone have any ideas on what is going on with the road data and why it doesn't seem to be seen as a network?

Thanks

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3 Replies
MelindaMorang
Esri Regular Contributor

My guess is you have some sort of network connectivity problem with the roads.  Likely your connectivity policy isn't configured correctly for your data.  Here's some information explaining connectivity: https://pro.arcgis.com/en/pro-app/latest/help/analysis/networks/understanding-connectivity.htm

For debugging your issue, the first step is to use the Explore Network tool to click one of those streets near your starting location and see what other streets it connects to.  You should see connected streets highlighted in the map, and you can also see the full list of connected streets in the tree in the Explore Network pane.

My guess is you'll discover that a lot of your streets are not connected, so your next step will be to determine if you need to adjust your connectivity policy or add vertices to the data at locations where they intersect.

If you need to add vertices, you can use the Integrate tool with a 0 XY tolerance, but make a backup copy first because the tool irreversibly edits your input data.

GISMac
by
New Contributor II

Hi Melinda,

Thankyou! I read the information provided in your links and figured out that it was due to my lack of knowledge regarding Junctions.

I needed to create a Junction dataset manually using the network start and endpoint vertices. After doing this and cleaning up a couple of dodgy lines right near the station it ran smoothly! 

Thankyou for providing the info, the videos etc that I had been using did not mention Junctions in much detail and I didn't realise how important they were for this to work 🙂

 

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MelindaMorang
Esri Regular Contributor

I'm glad you got it working.

I don't completely understand your solution regarding junctions.  Normally when the network dataset is built, it automatically generates junctions at locations where the network edges connect to each other, and you don't have to create these manually.  However, perhaps your network had some other connectivity problem that the manually-created junctions were able to resolve.  For instance, if you were using End Point connectivity, but your streets have vertices at intersections and not endpoints, you could add a junction source using a policy of Override, and this would allow connections at those intersections where there's an override point.  This works, although you could have fixed the problem by switching your edge connectivity policy to Any Vertex instead.  (But that's just a guess as to what the problem was.)

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