[PD] kml files

Mathieu Bouchard matju at artengine.ca
Fri Dec 14 00:24:19 CET 2007

On Thu, 13 Dec 2007, Mathieu Bouchard wrote:

> longitude has to be from -180 to 180. The epsilon is then the previous power 
> of two divided by 2^23. In metres this is 0.61 metre near equator. This is 
> the worst case. For latitude the precision is twice better than longitude at 
> equator. In northern europe and in alaska, the longitude precision is the 
> same as the latitude precision.
> It's twice more precise to use signed values than unsigned values, which is 
> why I wouldn't use longitudes from 0 to 360.

Oh duh, my computation was wrong.

A degree is about 111319 metres of longitude around the equator, or 111138 
metres of latitude. The max longitude error is 2^-16 degree or 1.69 metre 
and the max latitude error is 2^-17 degree or 0.85 metre. I don't remember 
how I computed it the first time.

You can get twice better worst case by using values ranging from -1 to +1 
(because 1 is a power of two, so it lies at the boundary of a new 
precision level)

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| Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC Canada

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