|Where the land meets the sea|
Tagging coastlines in OSM
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 shoreline_300, which has a relatively low resolution. At higher zoom levels the coast polygons used are generated from a larger shapefile (processed_p) which is generated every few weeks from planet dumps (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). ( Note: the '300' in shoreline_300 refers to the value in meters used in the simplification threshold used when generating the file.)
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.
- OSM Inspector view that helps fixing the coastline
- OSMCoastline A tool to create shapefiles for the coastline.
- Coastline error checker (currently offline) A longstanding tool to convert OSM coastline data in the database to shapefiles for use in Mapnik