[tiling] worldKit meets OnEarth with tiling

[Mikel Maron]

Yes, the OnEarth Landsat cache does use a tiling scheme. Though it's slightly different from the usual, so I'll endeavor to explain this use case. Hopefully any generic tiling scheme description format can encompass this scheme, and it won't be too hard for other tools to implement.



As normal, there is a power-of-two pyramid of tiles. Each tile is 512 x 512 pixels. Requests are made through the normal OnEarth WMS, and BBOXs corresponding to the tiling scheme are served from the cache.

The images start from the northwest corner (90,-180) and are placed in non-overlapping tiles covering the world. At the lowest zoom level, each tile covers 256 degrees of latitude and longitude. So the global image is not divided in equal portions, and the tiles contain areas "off the map. The bounding boxes at the lowest zoom are -180,166,76,90 and 76,166,332,90

http://wms.jpl.nasa.gov/wms.cgi?request=GetMap&layers=global_mosaic&srs=EPSG:4326&format=image/jpeg&styles=&width=512&height=512&bbox=-180,166,76,90
http://wms.jpl.nasa.gov/wms.cgi?request=GetMap&layers=global_mosaic&srs=EPSG:4326&format=image/jpeg&styles=&width=512&height=512&bbox=76,166,332,90

At the next zoom level, each tile covers 128 degrees of latitude and longitude. The bounding boxes are [-180,38,-52,90] [-180,166,-52,38] [-52,38,76,90] [-52,166,76,38] [76,38,204,90] [76,166,204,38]

The next zoom level is 64 degrees per tile, and so on. At 2 degrees per tile, the tiles no longer overlap the world boundary. The lowest resolution tiles are 2**-5 degrees per tile.


Hopefully this makes sense -- it's pretty simple, probably easier to communicate with a couple pictures!

How can we describe this and other use cases in a generic form? Is there any way to include this information in a GetCapabilities document?

Mikel



----- Original Message ----
From: Allan Doyle <adoyle at eogeo.org>
To: tiling at lists.eogeo.org
Sent: Saturday, March 25, 2006 2:59:48 PM
Subject: [tiling] worldKit meets OnEarth with tiling

Just when I was thinking it's time to give this list a poke, along  
comes Mikel with a great example of why tiling is a good thing.

http://radio.weblogs.com/0100875/2006/03/25.html#a1233

At least I'm assuming that this uses a tiling scheme...

   Allan

-- 
Allan Doyle
+1.781.433.2695
adoyle at eogeo.org



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