Hi, The EPSG:900913 an EPSG:3785 are coordinates measured in meters, you will see the ranges for bbox when you click on a tile in:
http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/. You need this codes only for creating raster tiles, especially those covering the whole Earth or continents (when zoomed out), because when you use just bbox in Lat/Lon coordinates (EPSG:4326 with WGS84 Datum) your data will not be correctly aligned to the original map layers which use Mercator projection. You will realize there is a north-south shift in the middle of every tile. For zoomed-in tiles (large scale maps) the shift is minimal. First code (900913) is supported in most open-source software tools like MapServer, GDAL, OGR, Grass, QGIS, etc.. but it is not official. The second (3785) is official but unfortunately not yet implemented in most GIS software tools because it is not in the EPSG database for a long time. The raster tile projection of Google Maps and similar interactive maps is a bit tricky, because the shape of the Earth is simplified to sphere, but Lat/Lon coordinates are on ellipsoid. That is something very unusual for GIS software tools. It is a bit advanced subject.. I tried to summarized it on the mentioned website. To simplify this coordinate conversion for existing raster maps I am programming a user friendly tiling application GDAL2Tiles/MapTiler (http://www.maptiler.org/). I will write more about it later, now it is under heavy development and testing. It generates mashups from a supplied raster files with georeference (GeoTIFF, MrSID, ECW, BSB, HFA, ...) similar to: http://www.maptiler.org/example-usgs-drg-grand-canyon-gtiff/ It is going to be available for free under an open-source license... somewhen in the beginning of the next year. Right now there is only command line version for that application. Best regards Klokan On Dec 2, 11:00 pm, bratliff <[EMAIL PROTECTED]> wrote: > On Nov 27, 7:52 am, Klokan Petr Pridal <[EMAIL PROTECTED]> wrote: > > > > > 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 thetilesreferenced 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 betilespre-generated by MapTiler/GDAL2Tiles following the > > OSGeo TMS recommendations, it can be raster data from WMS servers > > probably cached by TileCache, ortilesfrom MSR MapCruncher etc. > > > The tool is excellent for fast debugging of tiling for particular > > area, because you can use the search for display thetilesof a given > > place. > > > A transparenttilewith correct coordinates is displayed for every > > original maptileby calling Google Chart API service. > > > Whereever you click the longitude/latitudeboundariesof the > > underlayingtileare displayed in WGS84 datum.Boundariesare 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 maptiles. > > > The site contains also a Python script doing the coordinates and > > bounds calculation offline. > > > So enjoy this > > mashup:http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/ > > > Best regards > > > Klokan Petr Pridalhttp://blog.klokan.cz/ > > An excellent article. I have tried EPSG:3785 with the same "bbox" > parameters I use for EPSG:4326. It is not working: > > www.polyarc.us/epsg > > Apparently, it does not expect Lat/Lon coordinates. What is the > correct way to specify the "bbox" ? > > Thanks very much. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Google Maps API" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/Google-Maps-API?hl=en -~----------~----~----~----~------~----~------~--~---
