Why won't overlap lines in a service area draw?

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06-03-2015 09:36 AM
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New Contributor II

Hi,

I am trying to get the intersection of two 5 minute service areas from a Network Dataset in Network Analyst - rendered as a double line in the output. Even though I set my output Layer Properties / Line Generation setting to "overlapping" ("Generate Lines" is checked) I still get a single line in the overlap areas of the two firehouse service areas. This seems straight forwards enough - I tried the opposite "Not Overlapping" in case I was reading it backwards - but I'm not coming up with the simple intersection of the lines for these two firehouses 5 minute service areas.

I'm running ArcMap 10.1 on Windows 7.

My road network file is in good condition - I have 2:

One is a 2D file in a separate network dataset which generates lines by a "speed" field.

The other is a 3D file - I generated from the 2D with the use of a terrain. That 3D file seems a bit problematic as I'm getting a shorter 5 minute service area than with the 2D - There really aren't any overpasses in the study area so I don't think using the 2D is that much of an issue.. But one problem at a time. Can anyone in the community see where I'm going wrong here?

Thanks,

Jim

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MVP Frequent Contributor

I bet Melinda Morang (ESRI Staff/Network Analyst expert) knows the answer.

(when she logs on she will get a notice that her name is mentioned and will see your post).

Chris Donohue, GISP

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Esri Regular Contributor

[Chris, I get automatic notification of all new Network Analyst posts even if my name isn't tagged, but thanks for drawing my attention to this anyway.]

Hi James.  What you're describing should work.  When the SA lines are set to Overlapping, you should get duplicates of lines that are traversed more than once if they are reachable from different facilities.

For instance, in the simple test case below, the middle area is reachable by both facilities, and the highlighted line is in the Lines output twice, each one marked with a distinct FacilityID.


The question is why you aren't getting this expected answer.

Do you have more than one Service Area layer in your map?  Are you perhaps adjusting and solving the wrong one and then looking at the results for a different one?  Sometimes that happens if you forget to switch the drop-down layer selector in the Network Analyst window.

Are you sure that the reachable area from your two fire stations actually overlaps?

If it's not any of that, then I don't know what the problem would be.  Are you able to share your data?  If you could wrap up your network and your layer (with all its settings) in a map package and share it, I could take a look at it.

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New Contributor II

Hi Melinda,

Thanks for responding. I'm not very experienced with NA so bear with me.

Yes, I set up a clean MXD for each of the two NDs I have - one generated with a 2D road feature class; one generated from a 3D road fc that I converted from the 2d with the appropriate tool and a terrain.

Having said that - unfortunately, I'm not authorized to send out data. However, I'm very familiar with the area and given that the speed field I'm using is almost exclusively a 25mph set of line segments (this is a military installation) and given you should cover approx 2.1 miles in 5 minutes at 25mph - I'm certain the two firehouses service areas should overlap.

The 2D bases ND was created by someone prior to my being here - it generates a service area that extends further out than the results I get from the ND I built off of the 3D road file.

I'll attach a screen grab of the ND Properties attribute tab for each ND. Can it be I have to recreate a ND? Or is the ND(s) good - but my analysis settings are awry? I don't know if you can make it out - but:

  1. the ND properties attribute tab for the 2D ND 1) has its "add" button still active (3D is grayed out)
  2. The 2D ND attributes tab shows Time as a cost in Minutes with a small blue icon next to it - while the 3D ND has Speed (field Name) as a cost in minutes.

Again, I hope the ND has been properly built to start with

Thanks in advance.

Jim

Service Area - 3D Roads.JPGService Area - 2D Roads.JPG

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Esri Regular Contributor

Okay, so which network is giving you the solution you expect?  2D or 3D?  I wasn't quite sure from your screenshot which is which.

Regardless, it appears that the two network datasets don't have the same cost attributes.  The one with the "Add" button not grayed out looks like it has several attributes that make sense.  "speed" is a descriptor which is probably used to calculate the "Time" attribute, which has units of minutes.

The other network (the one where the Add button is grayed out for some reason) seems to have an attribute called speed with units of minutes.  That doesn't make sense to me.  Assuming this is the network you set up yourself, then I think you just didn't set up your cost attributes correctly.  Instead of a 5-minute travel time, your service area is solving for 5 some-nonsensical-units.

Since you're new to Network Analyst, I encourage you to work through the Network Analyst tutorials.  Hopefully these will clarify how to set up cost attributes and use them in an analysis.

ArcGIS Help (10.2, 10.2.1, and 10.2.2)

I recommend starting with Exercise 1 and then doing Exercise 5.  Those will be the most relevant for your workflow.

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