Proposed Features/Coastline-River transit placement
|Coastline-River transit placement|
|Definition:||boundary between the river and the ocean at a river mouth|
|Rendered as:||as before|
At the mouth of larger rivers which are mapped with river bank polygons the coastline needs to be closed across the river at some point. There is currently no clear rule where this should be done. This proposal suggests to establish some limits to the range across the length of the river where this closure can happen while still leaving room for the local communities to decide on the exact placement based on the local situation.
In the documentation of natural=coastline it is specifically said that there is no fixed rule for placement of the closure segment. Mapping practice has mostly established that the upper limit of the tidal section (i.e. the part of the river where the water level changes with the tides) is also the upper limit of possible placements of the closure segment in areas with significant tidal variation. Apart from that placement varies, in particular there are a few cases where the coastline is closed very far downstream, for example:
There has been fairly elaborate discussion more recently in the context of some of the more prominent cases where placement of the coastline was disputed:
- Rio de la Plata: https://lists.openstreetmap.org/pipermail/tagging/2020-August/thread.html#54434
- Chesapeake Bay: https://lists.openstreetmap.org/pipermail/tagging/2020-November/thread.html#56310
It is proposed to establish the following limits for placement of the closure segment. Within these limits the closure may be placed according to the individual local situation. In any case the closure should always be a straight line segment with no in between nodes connecting two nodes at the water line on both sides.
The upper limit
In case of significant tidal influence at the river mouth the coastline should be closed no further upstream than the range of the tidal influence (i.e. where the water levels vary practically noticeably with the tides).
In case of no or insignificant tidal variation the coastline should not extend significantly above the sea level with the river (significantly not being precisely defined but i'd say at maximum a meter, less where sea state is typically calm).
The lower limit
With no or insignificant tides the coastline should go upstream at least to a point where the river current is clearly the dominating current in the water under normal weather conditions (i.e. no storm).
Since water currents are difficult to assess the width of the river can be used to establish this lower limit more easily. A large river with a very shallow mouth will usually not fulfill the above condition if it is more than about 20-30 kilometers wide (with for example 15000 m3/s discharge and an average depth of 5 meters you get a velocity of 0.15-0.1 m/s which will no more dominate in comparison to typical coastal ocean currents).
With significant tides the coastline should go upstream at least to a point where, during one tidal cycle, the volume of water going downstream is significantly higher than the volume of water going upstream during raising tide. Significantly here should be interpreted as noticeable without doubt for the local observer.
The distinction in mapping between the world ocean as the main reservoir of the global water cycle and the inland waterbodies can and should be considered one of the most fundamental aspects of mapping in OpenStreetMap. While the exact drawing of the line in this classification can be considered arbitrary it is no different in that regard than other classifications we perform in OpenStreetMap like the distinction between natural=wood and natural=scrub or water=lake and water=river. Performing this classification consistently in different parts of the world is of fundamental importance for the usefulness of the data and the social cohesion in the mapper community. Strongly varying placement of the coastline can result in problems for many users of the data, for example all maps that render ocean and other waterbodies in different styling will show inconsistencies in placement prominently. Applications like determining if a city is located at the coast will get inconsistent results.
There has always been consensus that the coastline tag is meant to document exclusively physical geography and is meant to be used independently of the the culture specific differences in understanding of terms like coast, ocean, sea, river or inland water and similar terms in different languages. Therefore the above proposal is built exclusively on verifiable aspects of the physical geography.
As of late 2020 for the most part the vast majority of coastline-river transits around the world are mapped within the bounds suggested by this proposal. The above rules will therefore not restrict the mapper in a significant way and very few river mouths will need to be changed to comply with it. It will however help the mapper to make a reasonable and informed decision on placement of the coastline closure, it will help avoid extreme cases like above and will help avoiding conflicts within the mapper community about this.
Please discuss this proposal, using the Talk:Proposed_Features/Coastline-River_transit_placement.