OpenStreetMap coastline

From a bike ride round Regent's Park to a world coastline

OpenStreetMap was founded in 2004 by Steve Coast, then a student at University College London, with the idea of a free map of the world that anyone could edit. The first street went in on 10 December 2004, after he had cycled round Regent's Park in London with a GPS receiver. The OpenStreetMap Foundation, the non-profit that holds the database and runs its servers, was set up on 22 August 2006.

Mapping took off when imagery became available: from December 2006 volunteers could trace over Yahoo's aerial photographs, and from November 2010 over Bing's, so that a GPS was no longer needed to contribute. The data moved to the Open Database License (ODbL) on 12 September 2012, and the project passed 10 million registered users on 19 January 2023.

The coastline came early, and in bulk. From August 2006 a script imported the Prototype Global Shoreline (PGS), a shoreline extracted automatically from public-domain Landsat satellite images, first piece by piece, then over nearly the whole planet. It gave a rough outline of every continent and island, with gaps traced by hand and a sawtooth look that volunteers have been replacing ever since, coast by coast, with finer traces.

Turning those lines into land a computer can use is the work of OSMCoastline, a program written by Jochen Topf. Its output was published for years on openstreetmapdata.com, which closed in June 2019; it is now served by osmdata.openstreetmap.de, run by the German association FOSSGIS.

What the line means and how it is drawn

In OpenStreetMap the coastline is a set of lines ("ways") tagged natural=coastline that mark the mean high water springs line, the highest line the sea reaches in normal conditions. It is not used for lakes or rivers, which are mapped as water areas; the Caspian is the only inland sea that carries it. Bays and tidal salt-water areas connected to the sea count as sea, and a large river is closed by a coastline drawn across its mouth, the exact place of that junction being left to local mappers.

The direction of each line carries information: walking along it, land is on the left and water on the right, so an island is drawn counterclockwise. The lines must join end to end, without crossing themselves, into one unbroken ring around each continent and island. What dries between high and low water is tagged separately, as sand, mud, bare rock or tidal flat, mangroves as wetland=mangrove, and areas always under water but close to the surface as natural=reef: none of them is part of the coastline.

No single instrument stands behind the line. Most shores began as the PGS import or as imports of official data, then were refined by volunteers tracing aerial and satellite imagery, walking or sailing with a GPS, or using national data whose licence allows it, such as Ordnance Survey's open high and low water lines in the United Kingdom. The project's wiki warns that imagery can be offset by a few hundred metres, which is why traces are checked against GPS tracks.

From lines to land polygons, every day

OSMCoastline reads the whole OpenStreetMap database, gathers every coastline line and joins them into closed rings, closing small gaps on the way. It then builds polygons from the rings, with holes where land encloses water, and splits the largest into pieces of at most a set number of points (1,000 by default) that overlap very slightly so that nothing shows at the seams. Errors it cannot fix, such as rings left open or lines that cross, are reported on the coastline view of Geofabrik's OSM Inspector, where any mapper can correct them.

The processing runs once a day on the latest data. Because a single broken or reversed line can turn a continent into sea, each new result is first compared with the previous day's, originally by drawing the land into a 2048 by 1024 image and comparing the two images bit by bit. If the difference is too large, the new files are not published, the previous ones stay online, and a person has to look at the change and decide whether it is real.

osmdata.openstreetmap.de offers the result as land polygons, as their inverse, water polygons, and as the coastline itself, in geographic coordinates (WGS84) or Web Mercator, either whole or split into small overlapping pieces. Derived from OpenStreetMap, these files are under the ODbL like the database itself: anyone may use them, provided the source is credited as "© OpenStreetMap contributors" and works built on the data itself stay under the same licence.

At sea: a fine line, not a chart

Where mappers are numerous, around harbours, marinas and popular coasts, the line often follows quays, breakwaters and beaches to within a few metres. On remote coasts it may still be the old PGS outline, a sawtooth that can be off by much more, and a new breakwater or dredged channel appears only once someone maps it. Its precision is therefore not one figure: it varies from one bay to the next.

By definition the line is high water springs: at low tide a strip of sand, mud or rock dries out on its seaward side, wide on flat coasts with a large tidal range. Reefs, shoals, rocks awash and isolated dangers are not land in the polygons, unless someone has drawn them as an islet, and the edge of a mangrove is hard to trace from imagery. A route that keeps clear of the polygons can therefore still cross a reef or a drying bank.

The coastline says nothing about depth, buoyage, traffic separation schemes or restricted areas. It is a good guide for planning a passage, never a substitute for the official nautical charts and notices to mariners of the hydrographic offices, which remain the only reference for navigation.

In Saily WR

Saily WR uses the processed OpenStreetMap coastline, the land polygons of osmdata.openstreetmap.de, in their edition of 19 October 2026, simplified to a precision of about 10 m. It is the line every route keeps clear of, whichever wind model the route is sailed in, so the routes compared side by side all respect the same land.

Around that line the route keeps a safety margin that you set. Choose it wider than the uncertainty of the coast where you sail and of the foreshore that dries beyond high water: more on a remote or reef-fringed coast than along a well-mapped harbour approach.

The coastline only keeps the boat off land. If you also ask for a minimum depth, the depth comes from EMODnet where it covers and from GEBCO elsewhere, each described on its own page.