Incorrect and Unexpected Service Area Polygon

Question

Hi Everyone,

I have noticed an unexpected behaviour in Network Analyst (NA) Service Area Polygon function.The attached image shows the service area polygons at some arbitrary network distances from the highlighted facility. The distances are listed in the Breaks_NDist field values in the "Properties-Facilities" dialog box. As you can see there are 7 breaks. However, if you see the list of polygons, there are only six service area polygons generated by the NA. You will also notice that the polygon for the distance 131.18 ft is a very thin donut shape polygon unlike the others which are desired disc-shape i.e. no holes.

Curiously, the extent of the final polygon does reach to the final break distance i.e. 198.17ft in some directions along the network. Also, this happens to only some of the facilities.

Would anyone happen to know of the reasons for this behaviour, and way to solve this.

Thanks a lot.

Cheers,
Sanjay.

network_analyst_bug.jpg

ConnorWoods_0-1779307572955.png

SanjayRana · Answer

Hi Jay -

Many thanks for picking up the post. Actually, the distance to the edges of the outermost SA polygon from the facility is indeed the same as set out in the Breaks_ value. The distance to the inner and outer ring of the donut SA polygon also matches with breaks specified in the Breaks_ values. So, unless I am horribly wrong, I think that the specified distances are getting traversed. However, I have discovered that such donut shape SA polygons only occur when a pair of consecutive breaks have a gap of less than a certain amount, which is 2ft in my case. In this example, I changed the consecutive break values to be at least 2ft more than previous one and there was no donut shape SA polygon. Could it be the case that SA polygon generation part of the algorithm has some xy precision issue and some topological overlap/intersection type steps end up accidentally producing the donut polygon? Also, this would only happen when the SA polygon has a complicated shape i.e. with some risk of self-intersection.

Thanks,
Sanjay.
The service area polygons depend on how much the road network was traversed. So bring up the service area layer properties on the Line Generation tab, turn on Generate Lines and then turn on the Split lines at breaks. Solve it again. And now you can see how the lines are being traversed and what break values are present on them. Note: you may want to symbolize the lines on break values to see where they start/end. Examine the data to see why the final break value is not being reached. It is likely there is some restriction or disconnected edge that is not being traversed.

Jay Sandhu

JaySandhu · Answer

The service area polygons depend on how much the road network was traversed. So bring up the service area layer properties on the Line Generation tab, turn on Generate Lines and then turn on the Split lines at breaks. Solve it again. And now you can see how the lines are being traversed and what break values are present on them. Note: you may want to symbolize the lines on break values to see where they start/end. Examine the data to see why the final break value is not being reached. It is likely there is some restriction or disconnected edge that is not being traversed.

Jay Sandhu