Plans [was: Re: Gconf or GSettings? - leading to a wider question]

[Robert C. Helling]

On Jan 23, 2013, at 12:22 AM, Linus Torvalds wrote:

Linus,

> There are multiple ways to do cylindircal projections, and while osm
> *seems* to use the one that is just a pure linear version, there are
> other common formats like the "equal area" one (that compresses the
> poles in the north-south direction to make areas be comparable).

your impression is wrong. They use a Mercator projection

http://wiki.openstreetmap.org/wiki/Mercator

And it's not just the library that does that, as you can read here

http://en.wikipedia.org/wiki/Mercator_projection

they have good company. In particular, it's already the tiles that use that projection so you would have to use different tiles if they changed that in the library.

As this transformation for the latitude is non-linear, it is essential to do the computation in pixel coordinates rather than degrees (I had that wrong before). The projection then enters via osm_gps_map_convert_geographic_to_screen and its inverse. The upshot is, as long as you do it with pixels the computation is agnostic of the projection.

Hope that helps.

Robert


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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics  
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