[OSM-dev] OSM and OpenLayers Projections

[Christopher Schmidt]

On Fri, Aug 10, 2007 at 06:31:42PM +0100, mike wrote:
> > OpenStreetMap tile data is projected.
> 
> Sorry if this is another really dumb question (or set of questions) but why
> is osm tile data projected?  

Maps are, almost universally, drawn in some projection. (Maps on the web
are, many times, drawn with the 'projection' actually being
'unprojected'. This is because people are lazy ;))

If you compare an unprojected version of Sweden to a mercator projected
version of Sweden, you will see an obvious difference. The former is
'squished' looking, and right turns are not at right angles (for
example). The Mercator projection fixes this. 

> Is this a unique projection for OSM?  

The projection for OSM is the same projection used for Google Maps,
Virtual Earth, Yahoo Maps, etc. It is a standard mercator projection,
using a sphere as the earth model. The tiles are therefore possible to
'drop in place' in Google/Yahoo/MultiMap, and important quality
(according to all the people I've talked to who care at all).   

> What are
> the reasons for choosing it?

Primarily the above, as far as I understand it. There are many Mercator
projections which would offer benefits -- where angles are not distorted
-- but the specific one that OSM uses by default is designed to make it
possible to use with other commercial APIs easily.

> Is there a wiki page for this?

No clue.

Regards,
-- 
Christopher Schmidt
MetaCarta