Tiles à la Google Maps: Coordinates, Tile Bounds and Projection...

Published Nov 27, 2008

Tiles à la Google Maps: Coordinates, Tile Bounds and Projection... image

I prepared an online tool I was missing when I started to study how the tiling using Spherical Mercator runs in Google Maps, Virtual Earth, Yahoo Maps, OpenStreeMap and others… This tool can help you to understand how are the tiles referenced and how they are stitched together in the browser.

I think this mashup could be an excellent starting point for people who are interested in this subject, because it contains all the math what you need for overlaying your own geodata from external sources: it can be tiles pre-generated by MapTiler/GDAL2Tiles following the OSGeo TMS recommendations, it can be raster data from WMS servers probably cached by TileCache, or tiles from MSR MapCruncher etc.

The tool is excellent for fast debugging of tiling for particular area, because you can use the search for display the tiles of a given place.

A transparent tile with correct coordinates is displayed for every original map tile by calling Google Chart API service. Whereever you click the longitude/latitude boundaries of the underlaying tile are displayed in WGS84 datum. Boundaries are also displayed in projected Mercator coordinates (EPSG:900913, EPSG:3785) and in pixels for active zoom level. This information is all you need for generating custom map tiles.

The site contains also a Python script doing the coordinates and bounds calculation offline.

So enjoy this mashup:

Tiles à la Google Maps: Coordinates, Tile Bounds and Projection

BTW: This demo is part of the documentation of MapTiler application.

Petr Pridal

Published on Nov 27, 2008

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