Proposed Features/Coastline-River transit placement
|Coastline-River transit placement|
|Status:||Proposed (under way)|
|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.
At Tag: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:
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.
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.
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).
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).
With significant tides the coastline should go upstream at least to a point where on waterflow is going downstream for a significantly longer part of the tidal cycle than it goes upstream due to raising tide.
Since water currents are difficult to assess the width of the river can be used to establish the lower limit more easily. A large river with a very shallow mouth will usually not fulfill the above conditions 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).
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 color will show inconsistencies in placement prominently. In the standard mapnik style the effects are only visible at zoom levels z=5 and lower where water polygons are not rendered. Applications like determining if a city is located at the coast will get inconsistent results.
The above rules will not restrict the mapper in a significant way and most likely very few rivers will need to be changed to comply with it. It will however help the mapper to make a reasonable decision on placement of the coastline closure and will help avoid extreme cases like above.
Please discuss this proposal, using the Talk:Proposed_Features/Coastline-River_transit_placement.