[ale] WAY [OT] but geeky - how do I calibrate GPS barometric altimeter

[Jim Lynch]

On 02/25/2013 11:59 PM, Sean Kilpatrick wrote:
>
> The civilian rade GPS units by and large only claim to be accurate 
> within 50 feet or so -- in any direction. That certainly was proven 
> true during a recent trip to Amelia Island, Florida -- just north of 
> Jacksonville.
>
> the unit reported elevation on different days between -50 and +75 
> feet, when the car was parked ~maybe~ eight feet above mean high tide.
>
I agree, my sailboat varies from being a  submarine to a low flying 
aircraft when I'm on the water according to the gps.  Also the 
mathematical model of the earth GPSes us doesn't not accurately describe 
the actual surface of the earth.

There are a number of surface models, including sphere oblate spheroid, 
ellipsoid, geoid and others.  Each has it's accuracies and 
inaccuracies.  To compound matters, the actual surface of the earth 
constantly changes its shape.


It's best for hiking to use the altimeter as a relative measuring 
device.  "I am ascending" or "I am descending" or "This hill is 50 feet 
higher than my house".

Jim.
Jim.