Tag:natural=coastline
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It has been proposed that this page or section be merged with Coastline. (Discuss) |  |
+/- natural=coastline
Used to mark a coastline.
Used combinations in
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The natural=coastline tag is used for identify the coastline, being the boundary between the sea and the land, and also between the sea and rivers.
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How to use
- see also: Coastline
For smaller islands a single area can be drawn around the perimeter of an island, to loop back on itself. For larger landmasses the coastline is defined as a series of linear ways connected end to end. These ways should create a continuous interlinked coastline.
The direction the ways are drawn is very important, they must be drawn so that the 'land is on the left side and water on the right side' of the way (when viewing in the direction of the way arrows). If you regard this as tracing around an area of land, then the coastline way should be running anti-clockwise. Obviously this applies to all the ways forming the complete coastline polygon, such that one way ends on the same node where another way starts. It's easy enough to reverse the direction of a way in Potlatch, JOSM, and all good editors.
The coastline is drawn as a series of linked ways about the entire coastline of an island in the sea or indeed a whole continent at the Mean high water spring (MHWS); for non-tidal waters (tide less then 30 cm) at Mean Sea Level (MSL). The direction of the way should be such that land is on the left side and water on the right. Realistically speaking the vast majority of our natural=coastline ways will be based on PGS data for a long time to come, and some will be corrected based on Bing or Yahoo Aerial Imagery (variable tide level), but if you get the opportunity to fix up the data while out surveying, then the high tide position is the one to go for.
There are discussions about where the transition between the sea to a wide river (tagged with waterway=riverbank) should occur; there is a view that this should ideally be at the top of the tidal section or the river, but it is also accepted that the junction may be closer to the mouth of the river with the tidal section of the river tagged with tidal=*. The important thing is to ensure that the coastline and riverbank share two nodes to create a seamless junction between the river and the sea. See Proposed features/Tidal Rivers
The Great lakes, and other very large bodies of freshwater, are treated as being sea even though they are actually freshwater, and tagged as coastline.
Errors
Some renderers/tools also require the ways to form a complete polygon (eventually looping back on itself). For Europe you can use OSM Inspector coastline debug view, to check that the coast line is correct.
Do not cross a coastline over itself - this has no meaning and may cause renderers to behave badly. Don't use the same point twice (pinch point) other than the start/end point (if appropriate). If you want to make an island, start a new way.
Renderer usage
More detailed technicalities of the way the data is used by the main renderer Mapnik can be read on the Coastline page. Note, that due to the way the coastline is generated for the main OSM map, updates to coastline ways may take some weeks before they appear on the map.
See Also
- Tag:natural=wetland
- Tag:waterway=riverbank
- Key:coastline:survey_quality
- natural=water is for inland lakes, and should not be used for tagging ocean coastline. Equally natural=coastline should not be applied to the edges of a lake. FIXME: even if it's a really huge lake? (see discussion)
- Key:waterway may list some useful related features.
