[OSM-dev] OSM and OpenLayers Projections

[Robert (Jamie) Munro]

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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? 

Because the world is round, and (most?) computer screens are flat.

> Is this a unique projection for OSM?

No, it is Mercator.

> What are the reasons for choosing it? 

It is used by Google, MS Virtual Earth, and Yahoo maps, for a start.

It means that countries away from the equator don't get squashed
vertically compared to how they look from directly above, but they do
get subtly distorted. Bearings are preserved. Horizontal scale and
vertical scale are the same at every point, although they both get
larger as you move away from the equator.

The main disadvantage is that it makes northern countries look too big
compared to Africa. Africa is a lot bigger than Greenland in real life!

> Is there a wiki page for this?

Not sure.

Last time I looked, the way that Mercator was handled in OpenLayers, it
seemed very overcomplicated. It used a version of Mercator that took
into account that the earth wasn't spherical and translated everything
into "meters in the mercator projected plane". The tiles use a simpler
spherical-earth mercator projection. I like to think of them in degrees
of longitude and mercator degrees latitude.
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