| natural = coastline
|Used to mark a coastline.|
|Used on these elements|
|Tools for this tag|
Where is the coastline?
By common agreement the coastline is tagged at the mean high water spring line. Basically that is the highest line the water reaches in normal circumstances. (See the images to the right for some ideas how to decide where that line is.)
The Mean low water spring is the position of the lowest tide. There is currently no agreed way of tagging this line in OSM.
How to enter the data
Tagging the coastline is a bit different than with all other features in OSM and you should be careful to get this right.
For smaller islands a single closed way can be drawn around the perimeter of an island, looping back on itself. For larger landmasses the coastline is defined as a series of linear ways connected end to end. These ways must create a continuous interlinked coastline, the end node of one way must be the start node of the next way. A break in the coastline or other error can cause serious difficulties when using this data. If you are not confident about editing, it is best to not make major changes beyond adjusting nodes to align better with aerial imagery.
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 counterclockwise.
Coastlines often coincide with other features, such as river mouths, landuse and boundaries. You can split up the coastline in as many pieces as you need to, just make sure there is always a continuous line.
Do not tag nodes or relations with natural=coastline.
What about lakes?
Before multipolygon relations were added to OSM, some large lakes were also tagged with natural=coastline, but most of them have been converted. Currently there are a few large lakes left (like the Great lakes), that still use this tag. Because of the complexity and size of these lakes, this tagging is difficult to change, so they might stay this way for the time being. But you should not introduce any new inland water areas that are tagged this way.
River ways tagged as waterway=river or waterway=stream end at the coastline on a common node for the river way and the coastline way. Larger rivers tagged with waterway=riverbank or natural=water and water=river have a way across their mouth that is tagged natural=coastline. In those cases the river line can also extend on the other side of the coastline, for example if there is a dug deepwater channel or a marked shipping lane.
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. There is a proposal outlining upper and lower limits of the placement of this junction and there is another proposal describing tagging of tidal rivers.
The shoreline data for most parts of the world was originally generated from a low-resolution data using the PGS import and other imports. Inaccurate shorelines can be seen mostly in a sawtooth shape, whereas in reality the coastlines are mostly gentle curves. In many parts of the world the coastline ways have subsequently been improved either with tracing from aerial imagery, or replaced by better vector data.
Be careful when tracing coastlines from aerial images. Those images are often not quite in the right position, so your coastline might look like it fits perfectly to the images, but in reality it is a few hundred meters off. Make sure that the data fits to GPS data.
- Ordnance Survey Opendata (Which includes official high water and low water data for the UK).
Errors and consistency checking
It is very important that the coastline is formed and tagged correctly. A single error in the coastline somewhere in Africa might mean the whole Asia-Africa-Europe continent can't be drawn correctly! Always make sure the coastline has the right orientation (land to the left, water to the right). 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.
Use the daily updated OSM Inspector coastline view to check for errors in the coastline.
Other coastal features
Along the coast it is often useful to add natural=beach and natural=cliff for beaches and cliffs; also natural=wetland for various forms of wetland, including wetland=tidalflat, wetland=mangrove etc. man_made=groyne can be used for sea defenses.
Using OSM coastline data
Please see Coastline for information how to use OSM coastline data in your own maps.