Information on generalization
Most OpenStreetMap-based maps are rendered from exactly the same data on all scales. This means that at low zoom levels there is usually way too much geometric detail which produces a lot of noise and blurriness in the results by rendering details that cannot be properly represented at the map scale and resolution used.
The process of adapting the detailed map data as it has been surveyed to the target map scale and resolution is called cartographic generalization. This is neglected in most digital zoomable maps. Key to good quality generalization and ultimately clear and well readable maps is the elimination of unimportant details that cannot be properly shown and at the same time selection and emphasis of key elements. This is not to be confused with mere geometric simplification which is just one element of generalization and which is widely used in digital map production.
OpenStreetMapData.com provides a generalized version of the OpenStreetMap coastlines for use in web maps in Mercator projection. The coastline is usually the most important element of maps at the lowest zoom levels so generalization in most important here.
The technique used for that is a raster-based process implemented in the coastline_gen tool. It ensures that straits and land bridges are properly represented at the low resolutions. Different levels of generalization are produced for the different zoom levels up to z=8.
In contrast to the original coastline data two further preprocessing steps exist:
- The lakes currently tagged natural=coastline have been removed.
- Some marine coastal water areas, primarily at the mouths of rivers, that are not included in the coastline in OpenStreetMap are added based on the maritime=yes tag.
Here an example of use of these coastlines - together with generalized waterbodies and relief shading - from maps.imagico.de.