topic Re: Inconsistent behavior in Network Analyst when adding origins in ArcGIS Network Analyst Questions

Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 25 Jul 2014 20:21:18 GMT

Hi all,

I noticed some strange behavior when solving a series of OD cost matrices in Network Analyst. The crux of the problem is that some origins and destinations are not being included in the solve. The strange part is that they are already set to be coincident with the network features I'm snapping to, so the search tolerance parameter shouldn't be an issue (I've verified this by running the Near tool, and confirming that NEAR_DISTANCE equals zero). Here's an image of a simplified example with two origin points:

After adding the origins to the network with a search tolerance of 15 meters, I get this:

The origin on the right is unlocated, despite being (seemingly) identically positioned relative to the relevant network features as the origin on the left.

The two ways I've found that I can get the origin on the right to locate are to:

Increase the search tolerance arbitrarily. At a tolerance of 500 meters, the origin will locate in the correct spot.
Maintain the shorter search tolerance but uncheck "Exclude restriction portions of the network" from the Network Locations tab in the OD Cost Matrix Properties window. (To be clear, the origin will always locate at a search tolerance of 500 meters with our without excluding restricted portions of the network.)

Neither of these two things should have any effect. As I've already mentioned, the origin is located on the feature. Additionally, there are no relevant network restrictions (that I'm aware of ... and I built the network). Besides, when it does locate, the origin gets placed in the correct spot, so that particular link can't be restricted.

Any ideas for getting to the bottom of this? I'd rather not just take one of the "solutions" above without fully understanding what's going on.

Thanks for any help.

Re: Inconsistent behavior in Network Analyst when adding origins

DanPatterson_Retired — Fri, 25 Jul 2014 20:25:46 GMT

If you go the other way...same result? Have you ruled out breaks in the network?

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 25 Jul 2014 20:29:58 GMT

I'm not sure what you mean by "go the other way." At this point I'm not actually solving the analysis layer, just trying to add origins and destinations.

For the same reason, breaks in the network shouldn't be an issue here - I can see that both origins are directly on top of the features I want them to snap to, yet one does and the other doesn't.

Re: Inconsistent behavior in Network Analyst when adding origins

DanPatterson_Retired — Fri, 25 Jul 2014 20:35:07 GMT

It is like it isn't part of the network, ie, a line segment sticking out in space, which may be why when you increase the tolerance, the point is found. Otherwise I have never seen problems that weren't associated with network connectivity (excluding solve bugs of course)

Sheldon, you might also want to move this to the Network Analyst are in Places even though you have tags

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 25 Jul 2014 20:50:15 GMT

Thanks, I've moved it.

I'm pretty sure this isn't a connectivity problem. If I use the same pair of origins with the same parameters on a route layer, network analyst successfully locates the stops and finds a route between them.

Re: Inconsistent behavior in Network Analyst when adding origins

DanPatterson_Retired — Fri, 25 Jul 2014 21:03:17 GMT

Sheldon
I can find nothing in the help files other than the search tolerance examples that you gave (ie the ? in unlocated locations). Is there any chance that you performed some modifications after initial setup since this link allows for updating locations after changes have been made. I am out of ideas, unless it is a bug...maybe someone else can weigh in...good luck

Re: Inconsistent behavior in Network Analyst when adding origins

MelindaMorang — Fri, 25 Jul 2014 22:36:48 GMT

Hi Sheldon. From your descriptions, it really sounds like the network edge is restricted.

- When you use a small search tolerance, the point can't find a non-restricted network element to locate on, so it shows up as unlocated.

- When you increase the search tolerance a lot, it finds another network edge that isn't restricted and locates on that network edge as well. Note that the point always shows up in the location of the original input no matter which network edge it's getting located on. The point doesn't move to show you that it located on a different edge.

- When you uncheck Exclude restricted portions of the network, it is now able to locate on the restricted edge that it's right next to, so it still works with the small search tolerance.

Try using the Network Identify tool (on the Network Analyst toolbar). Click on that network edge that's causing problems. You should be able to see the restriction values for it for any restrictions in your network. There must be something happening that you didn't expect.

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 25 Jul 2014 23:18:42 GMT

Thanks, Melinda.

Using the Network Identify tool, there's nothing obvious that pops up that would indicate a restriction on the two network elements that are close by. I see their length, travel time, source feature, and their connectivity to other elements. Everything seems normal. The properties are similar for the links proximate to the unlocated origin and the located origin. I didn't define any restriction attributes for this network dataset.

The output of a Route analysis shows that the origin is snapping to junction immediately to the right along the network. The properties of those links are also similar.

Re: Inconsistent behavior in Network Analyst when adding origins

MelindaMorang — Fri, 25 Jul 2014 23:26:39 GMT

Hmm...then I'm not sure what's going on. Are you able to share your data? If so, I could take a look.

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 25 Jul 2014 23:39:09 GMT

Sure. Let me try to put together something small and reproducible.

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 25 Jul 2014 23:57:53 GMT

OK - I've uploaded a FGDB to Dropbox containing a small network dataset with two origins.

Here are the steps to reproduce the error:

Add network_debug_ND to ArcMap.
Create a new OD Cost Matrix layer.
In 'Network Locations' uncheck "origins" and make sure "roads_small" is set to shape.
Check "Exclude restricted portions of the network."
Set search tolerance to 15 meters.
Add the "origins" feature class to the Origins network layer.

The origin on the right will not locate. Expanding the search tolerance or allowing location on restricted elements will fix the problem.

Thanks for any help.

Re: Inconsistent behavior in Network Analyst when adding origins

MelindaMorang — Sat, 26 Jul 2014 00:25:12 GMT

Hi Sheldon. I see the same behavior as you. It's rather baffling. We're going to look into it further on our end.

However, you might be able to work around the problem on your end. I noticed that you have a junctions source in your network called "origins". Origins should really be part of your network analysis layer and not built into the network itself. When I remove the origins junction source, your origin points load correctly into the OD Matrix.

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Sat, 26 Jul 2014 00:30:23 GMT

Thanks for looking.

Ultimately I'm using your Add GTFS to a Network Dataset tool to calculate travel times between transit stops. So, the "origins" in my case are doing double duty - they get created as, and are part of, the network dataset, and I use them again as origins. I have about 7,500 transit stops in my network. Only about 70 - randomly peppered throughout the network - don't locate when I run the OD Cost Matrix. The rest seem fine.

Re: Inconsistent behavior in Network Analyst when adding origins

MelindaMorang — Sat, 26 Jul 2014 00:39:59 GMT

Oh, I see. Is there some reason why you don't want the points to locate directly on the origins? If you're assessing travel time between stops, then having the stop located on the stop junction features wouldn't be a bad thing.

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Sat, 26 Jul 2014 00:42:25 GMT

No, I suppose there isn't. I'll try that on the next full run. But still, the origins ‌should ‌snap to the street features. I'll be curious to learn if it's a bona fide bug.

Re: Inconsistent behavior in Network Analyst when adding origins

JaySandhu — Thu, 31 Jul 2014 22:05:19 GMT

Sheldon,

This does look like an issue that we need to fix. Thanks for reporting it. And just to reiterate, the best way to create/maintain street data is not to include the origins/destination type locations as junctions in the network as they can cause ambiguity with location snapping especially if they are not connected to the street features.

Regards,

Jay Sandhu

Re: Inconsistent behavior in Network Analyst when adding origins

SheldonBrown — Fri, 01 Aug 2014 02:08:54 GMT

Thanks, Jay.

As I mentioned above, in my actual network dataset I don't include the origins and destinations as junctions per se. It just happened the be the case that in the analysis I was conducting, they were coincident.