[SOLVED! [Proj] Re: Problem overlaying georeferenced images in Google, Maps(projection problem!)]

Jorge jorge.arevalo at gmail.com
Thu Jun 12 06:07:29 EDT 2008

Ok' I've solved the problem!!

One important thing that I hadn't considered was that Google Maps 
changes the meters-per-pixel ratio not only when you change the zoom 
level, but also with the latitude. If you move the map upper or lower 
(in vertical direction), this ratio changes.

I think that there is a good reason for this: Mercator projection 
"stretchs" the shapes near to poles (North and South), and maybe google 
wants to minimize the effect by  changing the meter-per-pixel ratio

I was considering that this ratio remained constant for each zoom level. 
Now, I calculate it with this formula:

m/px = cos(latitud) * ( 1 / 2^(zoom+8) ) * 40075017

That I got from http://slappy.cs.uiuc.edu/fall06/cs492/Group2/example.html

And now... it works!! I realize that when I was checking the coordinates...

Thanks a lot, anyway. You' ve helped me!

> Usually you do what ever to fit the shape, if you are sure that the reference shape
> is the correct one. Nobody cares about the map borders. It is more important
> to have the places where they are.
> Take some known points and make sure their coordinates are correct on
> any map and then start to deal with the rest of the problem...
> Regards: Janne.

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