[Tilecache] World Wind + TileCache

Christopher Schmidt crschmidt at metacarta.com
Fri Aug 1 09:34:24 EDT 2008

On Fri, Aug 01, 2008 at 03:25:44PM +0200, Martijn van Exel wrote:
> Hi all,

I'll start by saying that my first reaction to your post is that you're
misunderstanding something pretty seriously, but I'm going to try to
understand anyway.

> TileCache supports World Wind type tile requests, which are of the
> form [LEVEL]/[ROW]/[ROW]_[COLUMN].[FILETYPE_EXT] as described in more
> detail here: http://issues.worldwind.arc.nasa.gov/confluence/download/attachments/394/world+wind+tile+systemt.gif

TileCache does not support this type of requests. This is a description
of the on-disk worldwind 'Cache packs': these are not accessible over
the web.  It supports requests of the type '?t=layer&l=zoom&x=x&y=y'.

> But World Wind allows you to define the base zoomlevel as
> LevelZeroTileSizeDegrees parameter, which defines the width of a level
> zero tile in degrees. By default this is 36 degrees, so the Earth is
> 50 tiles on Z0, 200 tiles on Z1, etc - but if you set
> LevelZeroTileSizeDegrees to a lower number, I guess TileCache will be
> confused.

No, LZTD maps to 'maxresolution': The WorldWind '36 degree' LZTD is
equivilant to a maxResolution of .140625, for example. TileCache does
not try to do any magic here, but it does generate WorldWind metadata:


You can look at any of the "EPSG:4326" layers and see that they have
appropriate WorldWind Metadata XML so far as I understand it.  

> Even when I set the L0 tiles to be 36 degrees wide, the resulting WMS
> requests are not valid. I'm supposed to get something in the range of
> bbox=577252.433917 6819405.91068 587036.373536 6829189.8503
> (just the BBOX part)

This doesn't make sense to me. WorldWind caches are geographic: your
bbox should be in degrees. I don't understand why you're getting
something other than that. Is this after some kind of transformation?

TileCache can't/doesn't do any reprojection. You have to set up a
worldwide EPSG:4326 cache in order to use the TileCache caches with
WorldWind. Also, WorldWind clients tend to break with tilesets that have
a depth of more than 10 levels, in my experience: with default
'worldwide' caches with maxresolution of .706125, this breaks down
before things really get interesting. Also, the WorldWind .Net client
didn't like 256x256 pixel images last I tried it: it wanted 512x512 to
do things right.

Hopefully this helps you somewhat.

Christopher Schmidt

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