[Tagging] Watershed or Drainage Basin relation draft proposal

Joseph Eisenberg joseph.eisenberg at gmail.com
Thu Sep 13 11:05:25 UTC 2018

"In fact this does occur, a river can disappear into the sand!
And some lakes have no outflow."
Right, these are called an "endorheic basin"; an area where the water flow
does not reach the ocean: https://en.wikipedia.org/wiki/Endorheic_basin
In one of my examples above, the river disappears into limestone sinkholes
but then reappears later. This is one of the reasons that it can be
difficult to automatically calculate stream networks and watersheds, if
there are no waterway relations. The presence of endorheic basins and
vanishing streams in deserts and limestone areas is one of the reasons I
believe a watershed or drainage basin relation would be useful.

On Thu, Sep 13, 2018 at 7:55 PM Warin <61sundowner at gmail.com> wrote:

> I believe some waterways in Australia will flow away from wherever the
> rain happens to fall ...
> That is a produce not just of the flatness of the terrain but also to the
> quantity of rain - there can be 5 years of rainfall delivered in a single
> day.
> Someone has put in the Australian Great Dividing Range... fortunately it
> does not render as it is very rough data. And there is no motivation to fix
> it .. as it does not render most are unaware of it within OSM.
> On 13/09/18 20:12, Joseph Eisenberg wrote:
> 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
> I note the expatiation the a river will not stop in the middle of nowhere.
> In fact this does occur, a river can disappear into the sand!
> And some lakes have no outflow.
> 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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