add new linear referencing functions for pg_featureserv about fwapg

[sebdalgarno]

Yes I think that is the best option. We will just have an R function that creates the Rkm column from Distance (in meters)

[smnorris]

I've generalized it a tiny bit more - rather than return distance, the function returns an index - multiply that index by the interval length to get Rkm.

[smnorris]

Proposed functions:

1. FWA_LocateAlong(blue_line_key, downstream_route_measure) - return a point geometry, basically just a wrapper around https://postgis.net/docs/ST_LocateAlong.html
2. FWA_LocateAlongInterval(blue_line_key, interval, downstream_route_measure) - return a table of point geometries located along the stream at set interval, starting at downstream_route_measure (default of 0).

@sebdalgarno, @joethorley - let me know if these names/parameters don't work (and any comments on the FWA_NearestStream -> FWA_IndexPoint name change in #27 are welcome too)

[sebdalgarno]

I think these names should work. We can always rename them in the fwapgr package, if need be.
In the FWA_LocateAlongInterval function, is it possible for you to return a 'Label' (or 'Distance'?) column along with the geometries in the returned table? i.e. some way for the user to know the order of the points...so at interval of 100m, Label would be 0, 0.1, 0.2, 0.3 etc. with default units in km starting at the most downstream point

[sebdalgarno]

actually this column could be called Rkm or RiverKm

[sebdalgarno]

if that seems too specific, perhaps there could be a third function FWA_Rkm that makes use of FWA_LocateAlongInterval and returns the Rkm labels...

[smnorris]

Sure, a third FWA_Rkm func is no problem.
Maybe easier is that FWA_LocateAlongInterval has a label / distance column (in metres) that you divide by 1000 in an FWA_Rkm R function?

[smnorris]

fwa_locatealonginterval is live. The default limit is low, so the server doesn't bog down on big features. Explicitly set your limit high if it is a long river. I think I have to adjust the return type in fwa_locatealong, the function is on the server but not showing up in the function list.

https://www.hillcrestgeo.ca/fwapg/functions/fwa_locatealonginterval/items.html?blue_line_key=354153927&limit=500

[smnorris]

Looks like the above request doesn't return a point at index=0, I'll fix.
index=0 does get returned if the starting point isn't 0 - my check on whether the location actually exists is too strict.

[smnorris]

Zero indexes are now included at measure=0 and fwa_locatealong() shows up in the list.
I'll close the issue for now but test out the functions and reopen if you find any problems.