[Tagging] Watershed or Drainage Basin relation draft proposal

Joseph Eisenberg joseph.eisenberg at gmail.com
Thu Sep 13 10:42:55 UTC 2018

(Sorry, Lake Habema is https://www.openstreetmap.org/relation/8688509)

On Thu, Sep 13, 2018 at 7:12 PM Joseph Eisenberg <joseph.eisenberg at gmail.com>

> Christoph,
> 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?:
> https://www.openstreetmap.org/relation/8687500
> <https://www.openstreetmap.org/relation/8687500#map=13/-3.8438/138.8568>
>  see https://www.opentopomap.org/#map=13/-3.7955/138.9242
> The Ibele river may be questionable in the flat valley
> https://www.openstreetmap.org/relation/8687462
> <https://www.openstreetmap.org/relation/8687462#map=12/-4.0619/138.7478>
>  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
> <https://www.openstreetmap.org/relation/8687464#map=11/-4.2105/138.7912>
>  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 ;
> https://www.opentopomap.org/#map=13/-4.1419/138.6722
> 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
> http://www.imagico.de/map/water_generalize_en.php
> 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/
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