[OpenLayers-Users] maxResolution change

[Tom Carden]

On 20/06/06, Chris Holmes <cholmes at openplans.org> wrote:
> Schuyler Erle wrote:
> > * On 19-Jun-2006 at  3:14PM EDT, Chris Holmes said:
> >> Interesting.  Does it match it at the levels all the way zoomed out?  My
> >> understanding is that Gmaps uses Mercator for the first five or so zoom
> >> levels...
> >
> > Well, we haven't gotten that far yet. It's an interesting
> > consideration if true. I'd had the impression they used Mercator all
> > the way down.
>
> See:
> http://cfis.savagexi.com/articles/2006/05/03/google-maps-deconstructed
> for the real deal on what they're doing.

This doesn't seem to mention switching projections at all, it's just a
thorough (and good) overview of how the tiles are defined and how to
do mercator projection in javascript.

I recall that when Google introduced aerial and hybrid imagery for the
UK there were alignment problems with the hybrid map due to projection
issues. For example the outer circle of Regent's Park in London (so
long the first thing seen on OpenStreetMap) was not circular.  That's
corrected now of course.

Comparing with MS and Y! imagery (e.g.
http://www.mapstraction.com/ljn.html using +/- keys on the Y! map to
zoom) I don't see any significant differences in projection or
alignment... only a pixel here and there.  This leads me to believe
all three are using the same projection.

Given the look of the maps - Greenland as big as Africa when zoomed
out, circular things roughy circular when zoomed in - and class naming
giveaways such as GMercator, I'd side with Schuyler and assume that
it's turtles^H^H^H^H^H^H^H Mercator all the way down.  My own
experiments with lining up canvas and SVG elements with Y! and MS maps
would also suggest that it's Mercator all the way, since that's what I
used and things seemed to line up.  I confess to not testing
methodically or exhaustively though.

>
> http://docs.codehaus.org/display/GEOSDOC/Google+Maps is what we've been
> doing see:
> http://dist.codehaus.org/geoserver/gmaps-geoserver_scripts/wms-gs-1_0_0.js
> for the script that works with geoserver.  On close zoom levels it
> switches to 4326
>

Would be good to get this cleared up so people aren't making
unecessary mistakes, and it looks like you looked into it in some
detail.  Have you got an example of this up and running - perhaps away
from the equator where differences would be most pronounced?

Best,

Tom.