PP navigation that is downstream but not on the mainline

[pauldzy]

This is another known issue with the legacy navigation code that has moved forward into the NLDI navigator.

PP navigation does not contain a proper QA evaluation of whether the stop flowline is downstream of the start flowline other than the check that the start flowline hydroseq is larger than the stop hydroseq. Thus when the stop is not actually downstream of the start, odd results occur. Basically the navigator will return navigation results that show the downstream navigation from the stop flowline to the point whereby it intersects with the downstream navigation from the start flowline.

The typical scenario that occurs is a user clicks on a start flowline and then tries to click on a downstream stop flowline but accidentally snaps to a small tributary of the flow. Here is a code pen example that once was used to illustrate the open task.
http://codepen.io/pauldzy/pen/raNxbr

We can see the same results from NLDI
https://cida-test.er.usgs.gov/nldi/comid/19053304/navigate/PP?stopComid=19266232&f=json

I think we had this marked as a needed bug fix to better indicate to users that their input parameters were unworkable. Its probably not a large endeavor.

Paul