[OSM-dev] Good explanation of latitude-to-pixel conversion?

Robert (Jamie) Munro rjmunro at arjam.net
Fri May 23 17:30:10 BST 2008


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David MENTRE wrote:
|
| The code you give is to translate a (lat, zoom) to x coordinate, you
| have a more direct formula applying the Mercator projection to the lat
| value:
|   http://wiki.openstreetmap.org/index.php/Mercator
|
| In the above page, it helps to know that r_minor and r_major are Earth
| Equatorial and Polar radii[1].
|
| The original post helps to understand why taking into account the
| variation of radius was necessary:

|   http://osdir.com/ml/gis.openstreetmap.devel/2006-12/msg00016.html

I don't understand. Why was it necessary?

AFAICS, taking into account the variation of radius is *not* necessary,
but can be done if you want to make the maths much harder for yourself.

Last time I looked, the Google, Microsoft, Yahoo and OSM default slippy
maps do not take the variation of radius into account, they just assume
the earth is a sphere.

Robert (Jamie) Munro
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