[Tagging] Watershed or Drainage Basin relation draft proposal

Warin 61sundowner at gmail.com
Thu Sep 13 10:53:18 UTC 2018

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 
> <mailto: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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