Layer URL parameter
The layer URL parameter on the OpenStreetMap homepage is one of several parameters forming a 'permalink' URL. It takes a special code representing your layer selection, so that others following your permalink will see the map browsing interface initialised with the same layer selection.
The parameter is optional and if you leave it out, we will default to showing the best map layer we have to offer (currently "mapnik").
If you do link to a layer code to select a different layer, note that we may not be able to guarantee that this code will remain the same forever. Your permalink URLs are unlikely to ever break completely (we'll always try to show the map in the right place) but a change in layer definitions may mean your link ends up pointing at a different layer selection. That said, we will endeavour to keep things the same to avoid changing the behaviour of incoming links.
Layer code structure
The layer code currently works as follows:
- M = Mapnik
- C = Cyclemap
- T = Transport Map
- Q = MapQuest Open
- H = Humanitarian
- N = Notes - Enables the Notes overlay (this is as an alternative to adding notes=yes in the URL)
- D = Data - Enables the Data layer overlay
Because these are optional overlays they can be combined e.g. layers=QND means show the MapQuest basemap with Notes and Data overlays added.
Previously we supported an 'O' value for 'Osmarender'. This was a layer generated and served by Tiles@home. This project was discontinued and the tile layer is no longer available. The rendering system Osmarender is still available.
Previously we supported an 'N' value for 'Noname'. The NoName map was particularly useful at a particular time in our history. Nowadays we still have (a new incarnation of) the NoName map, but now this is only one of several QA tools available on various separate websites.
Pre 2010 the parameters were formatted in an old layer URL parameter format (default OpenLayers behaviour) but this was simplified to the above single letters, a scheme which is specific to OSM.