[OSM-dev] Coordinate to Pixel at lowzoom

[Dan Homerick]

On Wed, Oct 7, 2009 at 6:36 PM, Lynn W. Deffenbaugh (Mr)
<ldeffenb at homeside.to> wrote:
> However, at zoom levels less than 4 (or so), I get a worsening offset to
> the north and south of the equator as I map objects onto the maps.  You
> can see this effect at the following URL:
>
> http://tinyurl.com/OSMLowZoom
> I'm using the floating-point X/Y value scaled by 256x256 to position the
> X/Y within a single tile.  This works fabulously for the higher zooms,
> but, as you can see, completely misses the continental land masses for
> the lower zooms.
>
> Is there a different/better set of equations to use at lower zooms?  Or
> is the project just that strange?

It sounds like you're not projecting from lat,lon coordinates to
Mercator correctly, which is introducing the offset. How are you
getting the "floating-point X/Y values"? Are you talking about lat,lng
coordinates in decimal form, or are you talking about the values you
got by projecting the coordinates into Mercator?

More detail about your code is probably necessary to provide any real help.

Cheers,
- Dan