Re: [Xcsoar-user] Barometric Alt vs. GPS Alt

[Ian <ian...@zs...>]

On 30/09/2010 11:36, Henrik Bieler wrote:

> The GPS-Altitude is pretty precise in my opinion. The pressure altitude
> (corrected by QNH) is only precise at the ground of the airport, where
> the QNH was measured.

I think it would be nice if Xcsoar could automatically adjust the QNH 
setting to make the measured altitude on the ground the same as the 
altitude for the take off location taken from the way point database, or 
the terrain model.

Maybe then it should offer the user an opportunity to correct this (or 
to set it if no suitable waypoint or terrain data is available).

> Remember the old saying: "In Winter the mountains are higher" ;-)
> With GPS-Altitude, you don't get this error.

For terrain clearance, GPS altitude is probably best.

For final glide it probably makes no difference if you calibrated QNH 
before launch (unless your landing airfield is at a very different 
altitude to the take off one).

> However for vertical airspace distances, I think QNH-Alt should be the
> Master, since Airspaces are defined that way. The "true" vertical
> boundarys just "fluctuate" a little bit with temperature.

Airspace defined in height ASL, should be measured with pressure 
altitude with corrected QNH.

Airspace defined in flight levels should be measured with pressure 
altitude measured with standard (1013mb) subscale setting.

This might become an issue in competitions if there are penalties for 
airspace violation.


Ian