Service Area: Why are some polygons less than my default break?

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04-17-2015 03:34 PM
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New Contributor II

Hi everyone!

I've been using the Service Area tool for some fun transit analysis.

I have a shapefile of streets and a shapefile of bus stops and I'm calculating the 800 metre walk shed from each of the stops. I've entered 800 metres as my default break and have no custom breaks for any specific "Facilities". I also have my Multiple Facilities Options set to "Overlapping".

Why are some of my service areas less than 800 metres? Most of them are 800 but arbitrarily one is 169m and one is 460m. There are no barriers of any kind and there's nothing fancy in the streets shapefile - no attributes like oneway worth considering.

Thanks for your help!!

Denis

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Esri Regular Contributor

It's not possible to tell if your network is well connected just by looking at the junctions.  You need to click on the edges with the Network Identify tool to assess whether or not it's actually connected to its neighbors.

One other idea: In your Service Area settings, have you set the polygon generation settings to use Non-Overlapping polygons?  If so, then the reachable area will be assigned to the closest bus stop if the area has more than one within the 800 meter break value.  If a particular stop is surrounded by several other reachable stops, it could be that the reachable area assigned to that particular stop never extends beyond your 169 meters because all along its boundaries it encounters areas that are closer to other stops.  The way to check if this is the case is to set your polygon settings to Overlapping.

Also, what happens if you run the Service Area on just one of the problematic facilities.  Do you get the same answer?  Try turning on line generation to see what streets are actually showing up as reachable.

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MVP Esteemed Contributor

could by chance, those values be area?

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New Contributor II

Unfortunately not. I've done measurements by hand and they correspond to the cost distance from the original "facility".

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Esri Regular Contributor

It's probably a connectivity problem in your network.  Suppose you have a small chunk of your network that's disconnected from the rest of the network.  If your stop snapped to a network edge in that disconnected portion, then it could be that it isn't possible to travel 800 meters.  The maximum distance you could travel was 169m.

Use the Network Identify tool (on the Network Analyst toolbar) to click around on the network edges in the vicinity of the problematic points.  In the pop-up Network Identify box, you can see what other edges are connected to the one you clicked on, so this is an easy way to identify connectivity problems.

In order for network edges to connect, there needs to be either an endpoint or a vertex on both features at the location where they touch, depending on your connectivity policy.  You can read more about network connectivity here: ArcGIS Help (10.2, 10.2.1, and 10.2.2)

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New Contributor II

OK, here's the junctions of my network dataset in dark blue. The red triangles are bus stops. The highlighted one on the left is one of the few that comes in with a radius under 800m. The highlighted one on the right is one of the majority of points that behaves as it should. Doesn't my network look normal?

Denis

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Esri Regular Contributor

It's not possible to tell if your network is well connected just by looking at the junctions.  You need to click on the edges with the Network Identify tool to assess whether or not it's actually connected to its neighbors.

One other idea: In your Service Area settings, have you set the polygon generation settings to use Non-Overlapping polygons?  If so, then the reachable area will be assigned to the closest bus stop if the area has more than one within the 800 meter break value.  If a particular stop is surrounded by several other reachable stops, it could be that the reachable area assigned to that particular stop never extends beyond your 169 meters because all along its boundaries it encounters areas that are closer to other stops.  The way to check if this is the case is to set your polygon settings to Overlapping.

Also, what happens if you run the Service Area on just one of the problematic facilities.  Do you get the same answer?  Try turning on line generation to see what streets are actually showing up as reachable.

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New Contributor II

Amazing! Your hint about turning on line generation was perfect. It made very clear that my network was not connected in the proper way. So here's my follow up question: How can I "flatten" my network dataset so that every  location at which lines intersect is considered a junction? Because I'm modelling walkability, pedestrians can (for the sake of argument) go anywhere along any road in this area.

Melinda - you rock!! Thank you so much.

Denis

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Esri Regular Contributor

Streets that are expected to connect have to have either a) and endpoint or b) a vertex on both street features at the location where they touch, depending on whether your connectivity policy is End Point or Any Vertex.  Please read this doc page to understand what those mean: ArcGIS Help (10.2, 10.2.1, and 10.2.2)

If your connectivity is Any Vertex and you just need to do a batch vertex insertion at locations where your streets touch, you can run the Integrate tool.  Make sure you make a back-up copy of your data first, as Integrate actually modifies the input features by moving them slightly.  You can set the tolerance to be small or tiny, though, so it won't move stuff much or at all and will instead just add vertices.  The one problem here is locations where you have an overpass and the crossing streets are actually not supposed to connect.  You might have to manually fix those areas.

If your connectivity is End Point, you may have more trouble.  If you split existing features down the middle so that they have end points, you risk messing up attributes that need to be carefully apportioned when the street feature gets split.  For instance, if you have a field called "walktime" and you split a street that has a value of 5 minutes in the walktime field, it would be incorrect for the two split parts of the street to both have a value of 5 minutes.  So, if you have End Point connectivity, you either need to split a lot of features very carefully, or you need to switch to Any Vertex connectivity and run the Integrate tool.

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New Contributor II

Integrate! Magical. My day is made. This is supremely useful and now my walksheds are much more accurate. I can't thank you enough Melinda!

PS: I recently found your GTFS app for ArcGIS and I will most definitely be downloading it for use when I get a free moment. It will be very useful for us.

Denis

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Esri Regular Contributor

Happy to help!

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