I agree with you. But I'm lost here :

zoom = 11;
mapSize = 640;

power = 2^zoom = 2^11 = 2048;
realWidth = 256 * power = 256 * 2048 = 524288;
oneDegree = realWidth / 360 = 524288 / 360 = 1456.355556;

=> onePixel = mapSize / oneDegree = 640 / 1456.355556 = 0.439453125°

That's what I found for longitude. It's always constant in all point
of the earth because it doesn't depend on the coordinates of the
center point so I can use it. It works perfectly. But for the latitude
span it's an other formula and I don"t manage to find it. I think it
depends on angle done with the center of the earth but I don't find.

On 1 juil, 14:00, Rossko <[email protected]> wrote:
> On Jul 1, 12:41 pm, ftessier <[email protected]> wrote:
>
> > Yes I read this. But I don't really find what I'm looking for...
>
> I thought everything you needed could be found at this reference 
> -http://www.articlesbase.com/ecommerce-articles/how-to-make-google-sta...
>
> > ... I don't find any formula to calculate it from center point
> > coordinates, zoom, image size...
>
> If you determine pixels-per-degree using those inputs, you can
> determine the spans; if you know the centre and the spans you can
> determine the corner coordinates very easily.
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