Service Area polygon appears to fill in between the roads for areas it should not reach

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01-31-2018 03:38 PM
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New Contributor II

In ArcMap 10.5.1 using Network Analyst, I have created a service area polygon of 1000m along a road network, with a trim of 50m. In the image you will see where the road is linked around, the inside is filled in (red circles). However I believe there should be a hole in this area as it is outside of the trim and not reachable within the parameters.

Process: After adding the Network Dataset I used the Network Analyst toolbar to create a new service area. I added Facilities, which was one single central point. Within Service Area layer properties I set the Impedance to 1000m, in the direction away from the facility. I chose to Generate Polygons - Detailed with a 50m trim. I also chose to generate a line.

What am I doing wrong here?

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Esri Regular Contributor

This is as designed. If you can reach the area from all sides then it is "covered" or "served" by that facility. The actual output of a service area solver is all the lines that can be reached from the starting facility. Turning that set of lines into a polygon needs a purpose. The main purpose of a polygon is for demographic analysis to answer questions like how many people live within 5 minutes of this facility. For that you need to intersect the service area polygon with a population polygon and get percentage covered to get population.

But if you want holes like you show in your picture, then I suggest you use the service area lines and then run the Buffer tool with the Dissolve All option. See screen shot below for the difference between Service Area polygon and Buffer.

Jay Sandhu

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Esri Regular Contributor

This is as designed. If you can reach the area from all sides then it is "covered" or "served" by that facility. The actual output of a service area solver is all the lines that can be reached from the starting facility. Turning that set of lines into a polygon needs a purpose. The main purpose of a polygon is for demographic analysis to answer questions like how many people live within 5 minutes of this facility. For that you need to intersect the service area polygon with a population polygon and get percentage covered to get population.

But if you want holes like you show in your picture, then I suggest you use the service area lines and then run the Buffer tool with the Dissolve All option. See screen shot below for the difference between Service Area polygon and Buffer.

Jay Sandhu

View solution in original post

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New Contributor II

Thanks Jay, very helpful and clear.

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MVP Esteemed Contributor

And there you go: a picture is worth a 1,000 words....

Isn't that the power of GIS? Visualization of complex analyses?