AW: location providers

> Alright.  Let's go with altitude.  I take each source and 
> quantify its error.  The barometric altimeter is subject to 
> pressure variations so it is off less than 200m, 95% of the 
> time; I probably have to work this out ahead of time.  The 
> GPS directly provides altitude uncertainty, which I scale to 
> the same confidence.

Plus (warning - slightly OT) - barometric altitude is completely useless
in some situations, most prominently in the cabin of a commercial
airplane. 

Some GPS receivers insist on using the barometic altitude (and ignore
GPS altitude), which makes your GPS track stick at 2000 Meters,
essentially rendering the elevation measurement completely unusable, and
off by several kilometers.

Example here: http://timatio.com/traces/17225-hamburg-wien

Alex