# [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)

_ _ __ ___ _____ ________ _____________ _____________________ ...
| Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC Canada
```

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