Use Hierarchy

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04-23-2012 08:35 AM
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New Contributor II
Hi,

I am performing network analysis with the ESRI streetmap North America network dataset. If I change settings like the cost attributes or restrictions or any other Attributes the performance is fine. But when I uncheck 'Use Hierarchy' and run the analysis it runs for a while and gives error message 'Error - Out of memory'.

When I test with network datasets that don't have hierarchy the tool runs fine.

Any suggestions?

Thanks!
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The out of memory can happen when you are running a very long distance route. So if you have a New York to Los Angeles route, then with the hierarchy the Network Analyst solver can exploit use the hierarchy of the roads (highways) to find the answer quickly. But if you turn off the hierarchy then it has to analyze every edge in the network. And in this North America dataset is very large (about 30+ million edges) and you can get out of memory. The other network datasets that did not have hierarchy and worked for routing are probably much smaller network datasets and thus can be used to solve for exact shortest path.

Jay Sandhu

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The out of memory can happen when you are running a very long distance route. So if you have a New York to Los Angeles route, then with the hierarchy the Network Analyst solver can exploit use the hierarchy of the roads (highways) to find the answer quickly. But if you turn off the hierarchy then it has to analyze every edge in the network. And in this North America dataset is very large (about 30+ million edges) and you can get out of memory. The other network datasets that did not have hierarchy and worked for routing are probably much smaller network datasets and thus can be used to solve for exact shortest path.

Jay Sandhu

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Thanks Jay for the clarification.

In case of UseHierarchy = True, the solver will calculate route with Primary roads only. Can we change that to include the next level in the calculation to exclude the detailed edges?

Thanks.



The out of memory can happen when you are running a very long distance route. So if you have a New York to Los Angeles route, then with the hierarchy the Network Analyst solver can exploit use the hierarchy of the roads (highways) to find the answer quickly. But if you turn off the hierarchy then it has to analyze every edge in the network. And in this North America dataset is very large (about 30+ million edges) and you can get out of memory. The other network datasets that did not have hierarchy and worked for routing are probably much smaller network datasets and thus can be used to solve for exact shortest path.

Jay Sandhu
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When hierarchy is ON, the route solver is not solving on primary roads only. It is looking at other local roads as well. But it favors using the hierarchy and searches less on the lower hierarchy road.

Are you getting bad routes with hierarchy ON? What is your use case for long distance routing that should avoid higher level roads?

Jay Sandhu
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I am calculating route from Bastrop, TX to Richmond, UT. ESRI output route calculates only Interstates whereas google maps is much shorter and quicker with a difference of 400miles and close to 8-10 hours difference.


When hierarchy is ON, the route solver is not solving on primary roads only. It is looking at other local roads as well. But it favors using the hierarchy and searches less on the lower hierarchy road.

Are you getting bad routes with hierarchy ON? What is your use case for long distance routing that should avoid higher level roads?

Jay Sandhu
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The differences can be due to the data being used. What version of ArcGIS are you using and what version of the streetmap are you using?

Jay Sandhu
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I am using ArcGIS 10 and I've tested with both 9.3 and 10 version streetmap data. Both versions give longer distance and time than google maps.

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When you solve in Network Analyst using streetmap data, the default is to minimize travel time. Bring up the properties of the route layer and on the analysis tab, change the impedance attribute from Time to Length and then solve again. You should get a smaller "length" route.

Jay Sandhu
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It is less when length cost attribute is selected but lot greater that google distances. Same with time too.

I've attached an image that compares google and esri routes to show the difference.
The esri route is with length cost attribute.

[ATTACH=CONFIG]14298[/ATTACH]
When you solve in Network Analyst using streetmap data, the default is to minimize travel time. Bring up the properties of the route layer and on the analysis tab, change the impedance attribute from Time to Length and then solve again. You should get a smaller "length" route.

Jay Sandhu
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Let me ask again: what version of the free streetmap data are you using?

Make sure you are using the version that came with ArcGIS 10. these files should be dated from 2010. You should not see this kinds of route with it. The older versions may not have good hierarchy information and you may sometimes get odd results.

You can always compare routes with "better" data. For example, in ArcMap, on the standard toolbar with the zoom in/out icons, there is a Find Route tool. You can use this tool to solve routes using data/service hosted on ArcGIS online using better street data. Help on this tool is here:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//007z00000002000000.htm


Jay Sandhu
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