Projection

Raw OSM data (found in planet.osm and extracts) is in WGS-84.

Most slippy maps, including OSM's main slippy map layer, are projected in Web Mercator projection.

For clarification on projections, see:

• PROJ FAQ
• The Google Maps / Bing Maps Spherical Mercator Projection

For information on the projection and converting between lon/lat and tile formats, see slippy map tilenames.

Software

• PROJ — command-line tools and library

See also

• Wikipedia:Map projection
• Mercator
• Converting to WGS84