[Mapbuilder-proj] RE: Pixel Coords <--> Lat Long coords

[Adair, M. <Mike.Adair@CCRS.NRCan.gc.ca>]

>>In mapbuilder/lib/tool/Extent.js we have functions getXY() and getPL()

Actually getXY() returns map projected XY value, which is lat/lon only if
the maps are displayed with EPSG:4326 (or equivalent).  To get lat/lon to or
from map projected XY you need the Proj/cscs forward()/inverse() methods,
which happen to be the identity for lat/lon.  

Google's Mercator projection is a map projection like any other (unless I'm
missing something) so we should continue to use the same design where
Proj/cscs implements the math to convert between Mercator and lat/lon.  It's
in that code that non-linear axes is taken care of.  The Extent object will
only ever go between pixel-line on the screen and map projected XY.

Mike



> -----Original Message-----
> From: Cameron Shorter [mailto:cam...@gm...] 
> Sent: Friday, April 14, 2006 5:06 PM
> To: Richard Greenwood
> Cc: map...@li...; Adair, Mike; 
> Patrice Cappelaere
> Subject: Re: Pixel Coords <--> Lat Long coords
> 
> I can see I'm showing up my lack of training in geography.
> 
> I agree that we should be using cscs as soon as you are ready 
> to share 
> it with the world.  I only suggest using Proj.js as an 
> interum measure 
> until your code is ready.
> 
> In cscs do you have a function which converts pixels to latlong?
> 
> Richard Greenwood wrote:
> > On 4/14/06, Cameron Shorter <cam...@gm...> wrote:
> > 
> >>Richard, Mike,
> >>
> >>In mapbuilder/lib/tool/Extent.js we have functions getXY() 
> and getPL()
> >>which translate between Pixel coords and Lat/Long coords.
> >>
> >>The functions make the bold assumption that both the X and 
> Y axes are
> >>linear (which they are for EPSG:4326) but is not the case 
> for Mercator
> >>which is used by Google Maps and many other projections.
> > 
> > 
> > In my mind, I do not see a longitude/latitude coordinate system as
> > "linear" but I don't know what you're trying to do either. At the
> > equator, a degree of longitude==latitude, but 45 degrees north or
> > south a degree of longitude/2==latitude.
> > 
> > 
> > 
> >>What needs to be done to correct this?  Can we extend the existing
> >>Proj.js code to handle pexel->latlong translation?
> > 
> > 
> > I was kind of hoping to see cscs employed over Proj.
> > 
> > Rich
> > 
> > --
> > Richard Greenwood
> > ric...@gm...
> > www.greenwoodmap.com
> > 
> 
> 
> -- 
> Cameron Shorter
> http://cameron.shorter.net
>