On Monday 03 September 2018, Joseph Eisenberg wrote: > In my case, I've been debating whether to change the tagging of the > coastline in southwestern New Guinea (Papua, Indonesia) where many > large tropical rivers meet the Arafura sea among mangroves. The heavy > rainfall in this area means that the rivers have a pronounced > current. But the water is brackish and certainly tidally-influenced > far inland. Right now, it seems odd that many patches of mangroves > are made into "islands" by the use of natural=coastline, though > locally they would be considered part of the larger landmass of New > Guinea.
The sitaution at a mangrove coast is slightly different from elsewhere because the mangrove forest is typically mapped as land w.r.t. the coastline which makes the tidal channels in between look like wide rivers - which is however often misleading. For example https://www.openstreetmap.org/relation/3111444 is not a river despite being tagged as a riverbank polygon. This misunderstanding of the nature of mangrove coasts misinterpreting wide tidal channels as large rivers has led for example for some time in West Africa to a massive shortening of the coastline and a large fraction of virtual closing segments in the total coastline lenth - see: http://www.imagico.de/map/coastline_quality4_en.php This is now mostly fixed but the lure especially of armchair mappers to map this way is still there. > See: https://www.openstreetmap.org/#map=11/-4.8806/136.9339 ; the > coastline is quite noticable because it has also been tagged as the > administrative boundary. The administrative boundaries are generally a bad place to position the coastline because they are typically defined very differently from how OSM defines the coastline (and are also mostly very inaccurate geometrically). > What will be most helpful for data users and map renderers? Should > the coastline extend inland many kilometers, to where the mangroves > end? This will create a large number of apparent islands, and small > rivers will be entirely part of the "ocean" beyond the coastline. > Should it be down the the mouth of the river, to keep the coastline > as compact as possible? The coastline should be (a) a meaningful geometry on its own, i.e. the virtual parts of it (closing segments at river mouths) should be short and (b) be as easy to verify for the mapper as possible. Technically it is also important that the range of acceptable coastline positions is not too large so mappers do not move around the coastline a lot just to scratch their personal itch. > What about huge estuaries like the Saint Lawrence and the Rio De La > Plata? Should there be a vote on Imagico's proposal, or a new > proposal? I would be glad if anyone wants to reactivate the proposal or comes up with simpler rules for where to close the coastline. But IMO a hard requirement for this would be that these are physically based rules rooted in the observable reality and not based on political or other purely abstract considerations. Some newer examples of problematic closure placements (in addition to the ones in the proposal): https://www.openstreetmap.org/way/463191729 https://www.openstreetmap.org/way/474230093 -- Christoph Hormann http://www.imagico.de/ _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
