[OSM-dev] Sphere / Web-Mercator

Dominik Spies kontakt at dspies.de
Thu Oct 9 16:38:31 BST 2008


Hi,

I'm trying to implement some projection-code for a project of mine..
But I have some trouble with the Mercator projection.

I understood that the projection we use in the slippy map is called
Sphere / Web-Mercator, or EPSG:900913 or EPSG:3785 and is also used by
Google, MS, Yahoo and so on..

So there is some description on MSDN
(http://msdn.microsoft.com/en-us/library/bb259689.aspx) which says:

y = 0.5 – log((1 + sin(Latitude)) / (1 – sin(Latitude))) / (4 * pi)

on some other pages and wikipedia I found that one:

y = 0.5 * ln( (1 + sin(Latitude)) / (1 – sin(Latitude)) )

where this is called the projection for GoogleMaps.


So - this is not the same. I understand that the first one is somehow
clipped to Lat +-85.05... and then scaled to 0 to 1.

Huh, the second one. what scale is this? I can't figure out how to use it..

Can someone point me into the right direction? Something to read?

Regards,

Dominik




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