Google Maps Projection and Coordinate system

Equal tiles: Google Maps, Virtual Earth, Yahoo Maps and others

Google Maps, Microsoft Virtual Earth, Yahoo Maps, and other commercial API providers are using the same projection and tiling profile and tiles are therefore compatible. The extent of available tiles and zoom (number of meters per pixel) is predefined for the whole Earth.
Difference is only in the URL they are using to access the same tile.

Documentation of the Tile System from Microsoft
Official Google Maps documentation of coordinate system

MapTiler/GDAL2Tiles is for tile names following OSGEO Tile Map Service_(TMS) Specification, because there is not any standard for tiling yet, OGC (Open Geospatial Consorcium) is working on the WMTS (Web Map Tiling Service) which is inspired by TMS.

Tile info for Latitude, Longitude and Zoom Level

Tiling the world (tile names on a picture are in quadtree format)

Click and info about the tile appears (several tile names - tms, google, quadtree).

WGS84 Datum and projection EPSG:900913 (EPSG:3785)

Any coordinate you use in the Google Maps API and is presented to the user is expected to be in Latitude/Longitude using WGS84 Datum (EPSG:4326).
For creating image overlay for Google Maps and compatible services you need to use "Spherical Mercator" which has coordinates measured in meters. It is defined as EPSG:900913 or EPSG:3785. Nice human description of this projection is provided by Microsoft. Exact parametric description for GIS systems (formats like WKT or Proj4), is in SpatialReference.org on-line database.

Source code for utility calculating tile bounds

I wrote a class GlobalMercator implementing all coordinate system conversion in Python. Code is documented and quite straightforward.

You can also download the source code globalmaptiles.py and run it on the command line. It prints for given WGS84 lat/lon coordinates (or bounding box) the list of tiles. For each tile several information is printed including bonding box in EPSG:900913 and WGS84:

	python globalmaptiles.py [zoom] [lat] [lon] 
	

Copyright © 2008 Klokan Petr Přidal