(Sorry, Lake Habema is https://www.openstreetmap.org/relation/8688509)
On Thu, Sep 13, 2018 at 7:12 PM Joseph Eisenberg <joseph.eisenb...@gmail.com> 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 > > On Thu, Sep 13, 2018 at 6:14 PM Christoph Hormann <o...@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@openstreetmap.org >> https://lists.openstreetmap.org/listinfo/tagging >> >
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