[OSGeo-Standards] Handling of resolution (or scale?) in WMSC

[Christopher Schmidt]

On Wed, Sep 03, 2008 at 02:51:01PM -0700, Jody Garnett wrote:
> I hope this one will be a bit quicker; and not push quite so many buttons.
> 
> I would like confirmation on how TileCache and/or GeoWebCache is 
> calculating the resolutions they return as part of their 
> Capabilities/VendorSpecificParameters/TileSet information.
> 
> In particular ...
> - WMS calculates scale information based on an assumption of 90 DPI; and 
> the size of the center pixel in a GetMap request
> - My customer has found that they need to make an assumption of 103 DPI 
> (when converting between GWC resolution and scale) for central Europe

"Scale" requires DPI assumption. "Resolution" does not.  


> My guess is the resolutions are calculated at the center of the tile 
> set; 

Huh?

A dataset is a specific bounds: say, for example, -180,-90,180,90.

The resolution is the description of the number of geographic units per
pixel for a given tileset.

180 - (-180) = 360, / 256 -> 1.40625 (Default resolution for some
things)

I think the "global profile" for lon/lat says half of that: world in two
tiles is .703125.

But there is no need to consier 'at the center of a tileset' or anything
else: the nummber of geographic units in a particular tile can't change,
all the tiles are the same.

Regards,
-- 
Christopher Schmidt
MetaCarta