Values in Tobreak in Service Area Analysis

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03-21-2019 10:17 AM
JoanieAppell
New Contributor II

I do fire station modeling for a countywide fire department.  Estimated drivetimes to an incident are a huge factor when deciding where to locate new fire stations.  To get those numbers for decision makers, I create a Service Area in Network Analyst using our existing stations and any potential station changes the Chiefs are considering. 

I create a new service area and set the default break to 20 (minutes) as this is higher than any actual drivetime that will be returned.  The resulting polygons provide me with a Tobreak value that is the maximum modeled drivetime encountered in that service area (e.g. 5.231).  In the last month or so, I have stopped getting the maximum modeled drivetime and instead the Tobreak field is just whatever Default Break parameter I set, regardless of the actual size of the service area.  

I'm currently using ArcGIS Desktop 10.6

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MelindaMorang
Esri Regular Contributor

I tested in 10.6, same as you're using.  Does your network dataset have a Service Area Index?  Check the network dataset properties on the "Optimizations" tab.  If not, turn on (check) the Service Area Index option, re-build the network dataset, and try again.

(Or just use ArcGIS Pro.)

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MelindaMorang
Esri Regular Contributor

Are you solving your service area against a local network dataset, or are you accessing the ArcGIS Online services to solve the service area through ArcMap?

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JoanieAppell
New Contributor II

I am using a local network dataset.  Visually, I'm getting the results I expect and know to be correct, it is just that darn Tobreak output that is no longer giving me the number I need.

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MelindaMorang
Esri Regular Contributor

You said the results have changed in the last month or so.  Did you do any of the following:

- Update your ArcGIS version

- Edit your network dataset

- Change your model or procedure

Something must have changed in order for the results to have changed.  Let's see if we can figure out what it was.

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JoanieAppell
New Contributor II

Thanks for replying, Melinda. 

  • There was no update to my ArcGIS version in the last month: however, today I updated to 10.6.1 to see if that would clear up the issue (it didn't). 
  • Our network dataset is a countywide dataset that is updated once a month. 
  • I first noticed the different data output after I started building a model for a fire station relocation project.  I have since tested the service area analysis outside of the model and have had coworkers test it using different data on their computers.  I have tried all different kinds of changes to the Service Area Properties, but I can't seem to get back to getting the maximum modeled travel time.

I agree that something has changed, but I can't figure it out for the life of me.  I've been running this same analysis for the last two years and have never had this issue until now.

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MelindaMorang
Esri Regular Contributor

Okay, sounds like you haven't identified an obvious change that caused the problem, so let's talk about what you're really trying to do and see if there's another way you can get the result you're looking for.

From what you've said, it sounds like you are trying to assign territory to the closest fire station.  In other words, you're not creating a 20-minute drive-time polygon around each fire station.  Instead, you're setting the break value to 20 minutes, but you want it to return a polygon showing a maximum of 20 minutes, but if there's an area where those 20-minute drive-time polygons would overlap, just assign the area to the closest fire station.  When this happens, the ToBreak generally will end up as something less than 20 minutes if your fire stations are pretty close together because there was nowhere that the fire station's service area got as far as the 20-minute break.  It always ran into the service area of another fire station first.  Do I understand this correctly?

I guess before speculating further, make sure your Service Area polygon generation settings are correct. Check the  information in the "The Polygon Generation tab" section under the "Multiple Facilities option" on the Service Area analysis documentation page.  If the output described above is what you want, you should be using the "Not overlapping" option.

Also, if you describe in more detail what you're really hoping to analyze, we might be able to find a better overall way to do it.  Feel free to elaborate.

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JoanieAppell
New Contributor II

You are understanding my analysis correctly.  I am having zero problem performing the analysis and getting the desired polygons.  The only thing I'm having trouble with is with the data in the attribute table after running the analysis.  

I'm now getting this output:

Attribute table showing ToBreak of 20

I want this output:

Attribute table showing maximum drivetime in ToBreak column

In the case of the bottom image, Deale is the only station showing 0 - 20 because they are the southernmost station and can hit that maximum drivetime of 20 minutes.  All of the other stations are providing me with their maximum drivetime before they hit territory where another station can get there faster.  E.g. our guys from Galesville should be able to get to all of their first due area within nine minutes from leaving their station.  That max drivetime number is the vital data I'm trying to get and is what is needed by my department's decision makers.  

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MelindaMorang
Esri Regular Contributor

Did you switch your polygon type from Generalized to Detailed at some point?

I just tried it, and Detailed gives me 0-20, but Generalized shows 0-[individual maximum].  This is probably a bug, which I will investigate further and log.

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JoanieAppell
New Contributor II

Unfortunately, when I set Generate Polygons to Generalized, I'm still getting the 0-20 result.  Changing between Rings/Disks or Generalized/Detailed does not change my result.  What version are you testing in?

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MelindaMorang
Esri Regular Contributor

I tested in 10.6, same as you're using.  Does your network dataset have a Service Area Index?  Check the network dataset properties on the "Optimizations" tab.  If not, turn on (check) the Service Area Index option, re-build the network dataset, and try again.

(Or just use ArcGIS Pro.)

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