Utility Network Analyst find path issues

JasonBarnes · ‎06-13-2012

I'm trying to calculate the distance from multiple dams downstream to a selected major confluence using NHD. I'm manually placing the flags for the two points (dam and confluence) and solving via the Find Path option in the Utility Network Analyst. For many of my points the selected path generated is the obvious and wanted route, however for some I'm getting very strange results. I've found that not only does the Find Path disregard line flow direction (see attached image) it is also choosing a path longer then the wanted main stem river segment.

For instance I'm trying to Find Path from the Boysen Dam down to the Yellowstone River. I would like to calculate the river miles along the Bighorn River using the Find Path tool. The output trace meanders on and off the Bighorn River going upstream of flow in some cases and also selects a longer path to the confluence. The path along the Bighorn River when I select it manually is 476 KM whereas the length for the Find Path output is 487.8 KM.

I was thinking that by setting a weight based on Stream Level (included in NHD) that I could force it to take the lower level route. The only problem is the weights in my Analysis Options is grayed out, and many of the Stream Level values for the stream segments are NULL. Do weights have to be initiated when originally setting up the Geometric Network?

Any thoughts to why the Find Path chooses a longer and against flow path? Why I can't seem to add weights to my analysis?

Thanks for all help.

JasonBarnes · ‎06-14-2012

In fact, can anyone enlighten me on how the Find Path in the Utility Network Analyst works? You would think it would want to find the least cost path (shortest distance if no other cost is present) between the points. I just tried to calculate the length from the Anderson Ranch Dam to the confluence of the Snake River along the Boise River using the medium resolution NHD. It took over 100 barriers to force the Find Path tool to stay along the Boise opposed to straying off on one of the many side canals or tribs - despite increasing the total length by up to 1.5 times. It's almost seems it does what it can to divert off of the main stems and instead find a less obvious route.

Anyone have any tips or information that can explain this behavior or tips to help increase productivity? Is it perhaps how the NHD network was setup in the first place?

Thanks.