[mapserver-dev] RFC: GMaps API for mapserv

[Christopher Schmidt]

On Thu, Apr 10, 2008 at 02:40:08PM -0700, Paul Ramsey wrote:
> >  Yes, I agree with the fact the metatiling is out of scope. I think the
> > point that I did not make so clear is that assuming someone wants to do
> > metatiling, say using some script or tilecache or whatever. Is there
> > anything in the implementation or design that would prevent one from
> > generating metatiles and chopping them up via a script and storing them
> > somewhere outside of mapserver? If yes, is removing that within the scope of
> > things or not?
> 
> The main thing preventing using this as a metatile server is the
> assumption of 256x256 tiles.  Basically (and this relates to Paul
> Spencer's request to name it mode=tile) saying "gmap" implicitly sets
> a number of conditions:
> 
> - The CRS will be "spherical mercator"
> - The bounds be global in extent. The top level will have 1 tile
> (http://mt.google.com/mt?x=0&y=0&zoom=17)
> - The "levels" will run from 17 downwards

Are you sure this is what Google actually passes into the GTileOverlay
stuff? This is kinda silly, if so: I thought they made their
GTileOverlay code smarter than that. (All of my work has been in tiles
with minimal code, but zooms all start at 0.)    

> - The y axis of the tile coordinates will run from top to bottom
> - The tiles will be 256x256 in size
> - Each zoom level will be related to parent and children by powers of two
> 
> (Digressionary question to anyone who knows or can guess: why would
> google choose 17 for their top level and count down, rather than 0 and
> count up?)

Originally, Google only had 17 zoom levels. "Why would you ever need
more than 512kb of memory?" In time, they realized that that was
ridiculous: their API now works entirely in 0-as-most-zoomed-out, with
different layers supporting different 'max' zooms (15 for terrain, 17
for roads, 19 for satellite ... though roads recently changed to also be
19, I think).

I would confirm that the GTileOverlay API still works from 17 down: they
definitely don't do that in the API these days, I'd be surprised if they
do in the GTileOverlay.

Regards,
-- 
Christopher Schmidt
MetaCarta