[Tagging] Watershed or Drainage Basin relation draft proposal

Joseph Eisenberg joseph.eisenberg at gmail.com
Thu Sep 13 10:12:24 UTC 2018

So you believe the ridges are verifiable (and the network of waterways, I
assume), but potentially parts of the watershed would not be verifiable
because eg. terrain is too flat? I was thinking that in fairly flat areas
it is still possbile to see which way water flows in drainage channels, and
it's often possible to find the highest line throught he terrain when
surveying. Also, open topographical map sources and open source elevation
data (eg SRTM) would be the main way to determine this. Would it be ok to
map watersheds where they are verifiable?

Would these examples be verifiable?

Wolo river is 99% surrounded by steep ridges; good example?:
 see https://www.opentopomap.org/#map=13/-3.7955/138.9242

The Ibele river may be questionable in the flat valley
 see https://www.opentopomap.org/#map=12/-4.0620/138.7477

Uwe river is mainly surrounded by steep ridges. For the part in town
verification depends on seeing which way water flows in open drainage
ditches along streets; https://www.openstreetmap.org/relation/8687464
 see https://www.opentopomap.org/#map=11/-4.2105/138.7910

Tagi river is 95 surrounded by ridges:
https://www.openstreetmap.org/relation/8687463 ;

Lake Habema is 98% surrounded by ridges:
https://www.openstreetmap.org/relation/8688506 ;

I would be happy to include a warning in the proposal and wiki that not
every watershed can be mapped in OSM. Only those in terrain where the
dividing line is obvious and the direction of water drainage is clear. So
no watersheds should be mapped in wetlands, flat farmland, developed urban
areas, etc.

If the issue is the lines through flatter terrain, we could have the
watershed be a non-closed line which only connected ridges. (Personally I
feel there are places where the watershed is obvious, yet don't qualify as
a "ridge", which could also be included as part of the relation). If there
was no requirement to make a closed way from the segments, this would
remove the temptation to draw non-verifiable lines across flat land.

I value your opinion Christoph, because I hoped this relation might
encourage more complete mapping of ridges, watersheds and drainage basins,
thus making it easier to render good maps, eg

On Thu, Sep 13, 2018 at 6:14 PM Christoph Hormann <osm at imagico.de> wrote:

> On Thursday 13 September 2018, Joseph Eisenberg wrote:
> > Relations of type=watershed are currently used over 2000 times and
> > there is a descriptive Wiki page but no proposal. (
> > https://wiki.openstreetmap.org/wiki/Relation:watershed)
> >
> > It would be useful to have a relation that showed drainage divides
> > (aka watersheds) and drainage basins (the network of streams and
> > rivers draining into a water body or waterway)
> Watershed divides are an abstract non-physical concept that is generally
> not verifiable in practical mapping - there are cases where they are
> (because they evidently coincide with physical features like ridges)
> but there are huge parts of the world where they are not and you would
> only try to estimate them based on already existing data.
> In short: This is not something you can reasonably map in OSM.
> --
> Christoph Hormann
> http://www.imagico.de/
> _______________________________________________
> Tagging mailing list
> Tagging at openstreetmap.org
> https://lists.openstreetmap.org/listinfo/tagging
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