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I created a simple service layer polygon in network analyst (ArcGIS Desktop 10.2) with no hierarchies set in the street network. Now I don't understand why is this polygon created along some particular streets?

Why isn't is properly spread out? For example, why doesn't the polygon cover the areas/streets I have marked in red? Obviously those marked areas are also within 1 km.

The impedance is 1 km and 2 km respectively.

The streets are also well connected individual line elements as you can see the junctions.

enter image description here

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  • oh yeah ofcourse, now its done. Commented Oct 20, 2014 at 9:57
  • It looks like a lot of intersections are not actually junctioned, although I am not sure if that's the problem. Commented Oct 20, 2014 at 11:06
  • I guess the junctions just depend on where the line elements are created so they need not necessarily be at all intersections. But in anycase that shouldn't be a reason for the service area polygon to form like this I think. Commented Oct 20, 2014 at 12:02
  • Junctions allow Network Analyst to know where turns are allowed. Imagine two situations... (1) Road A and Road B meet at an intersection -- you can turn from A to B, and should have a junction there. (2) Road A goes over Road B on a bridge -- you can't turn from A to B, so there should not be a junction at that intersection because it would be an invalid route. But it also looks like there are many N-S roads junctioned off that main E-W street that it's already driving on, so it SHOULD be able to turn off it... Commented Oct 20, 2014 at 12:13
  • Yes, actually every little N-S road is junctioning off the main E-W road. But for some reason the polygon simply doesn't go there. So does that mean wherever there is not a junction at an intersection in this network the two streets have elevation difference? Also there were no elevation fields while building the network dataset. This is actually a tiger street network for Portland. Commented Oct 20, 2014 at 12:58

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Per the comments, and as Erica originally suggested, your network isn't correctly set up for analysis as evidenced by a severe lack of junctions. This could result from either geometry issues with the data or how the connectivity policy is defined. I suspect both might be an issue in your case, as most junctions I see are at line endpoints, but there are some at apparent intersections and some endpoints do not have junctions. Your service area polygons aren't spread out because the analysis cannot reach those areas of the network.

The geometry issue: In a network when two lines cross that doesn't mean there is actually an intersection or junction. A turn (or transfer in multi-modal networks) can only be made at a junction. In order for a junction to occur both lines must have a vertex at a common point. Even if one line has a vertex snapped to the other edge, it won't work - they both have to have one. These issues must be addressed in the source line data from which the network is created.

The connectivity policy issue: The connectivity policy is a network dataset property and can be set one of two ways related to this issue.

  1. The connectivity policy for the street layer is set to End only: a junction can only occur at a line endpoint (aka node, or first/last vertex).
  2. The connectivity policy for the street layer is set to Any Vertex: Junctions can occur at any vertex, be they at the beginning or end of the line or anywhere in between.

This network connectivity policy is easiest demonstrated by creating a T intersection. Create a line with three vertices, then a perpendicular line with two - one of which snaps to the middle vertex of the first line. Now create your start point on one leg of the T and your stop on another. In case 1 above, the route will fail with an error message that the destination is unreachable because no connection can be made at the middle of the line. In case 2 the route will succeed, because a junction is allowed at a non-nodal vertex.

A functional network of the area you're looking at should appear more like this:

enter image description here

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  • thanks this descriptioon and the image are very helpful for me. I will have a look at the connectivity policy the network source layer to make suitable changes. Commented Oct 21, 2014 at 8:07
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    one little thing i would like to know is how did you create this image with all junctions from my image? haha Commented Oct 21, 2014 at 8:09
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    @Salman I didn't. I just happen to have a functional NDS of Portland and there was enough in your image to identify the area you were looking at. :) Commented Oct 21, 2014 at 19:02

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