Coastline

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logo Feature: Coastline
One example for Feature: Coastline
Description
Where the land meets the sea
Tags
natural=coastline

The coastline is a rich and complex environment at the edge of the marine environment including cliffs, beaches, harbours, headlands, bays, wetlands and islands.

Tagging coastlines in OSM

The natural=coastline tag is used to mark the Wikipedia Mean high water spring which is the point of the highest tide. See natural=coastline for all the details. See also de:Coastline for much more details.

Using OSM coastline data

The presentation of the coastline is often more complex that for other features and there can be delays between changes being made to the high water line and it appearance on the map.

At low zoom levels (used for z0-9), up to and including zoom level 9, Mapnik renders all the sea as a solid fill of blue, generated from the shapefile simplified-land-polygons-complete-3857.zip, which has a relatively low resolution. At higher zoom levels the coast polygons used are generated from a larger shapefile (land-polygons-split-3857.zip) which are automatically generated almost daily from planet dumps + diffs (note: if you edit the coastline then you have to be patient for the result of the changes to render at the higher zoom levels).

The main problem of showing coastlines from OSM data is that Mapnik really needs closed polygons. The polygon can (and normally should) be made of several ways, but the ways should join end-to-end and eventually loop back round without interruption, to close the polygon. An imperfect coastline with many small gaps, reversed ways and other defects will cause the polygon to break, and the coastline to be rendered incorrectly . One of the features of the Coastline error checker is to try and join coastlines in such a way as to make high-quality coastlines even with these imperfections. As the coastlines are uploaded and completed the quality can be checked.

See also