iFly GPS Forum

All,

This is normal. Just like airspeed and ground speed are different - pressure altitude and GPS altitude are different. The GPS measures your actual altitude over the ground, "pressure altitude" is basically an estimate based on the air pressure. When you enter the barometric pressure in your altimeter this is based on a ground level reading, and there is a presumed standard decrease in pressure with altitude. But actual pressure changes with altitude vary up to 10% based on weather conditions.

I found a good explanation here: http://bhgc.wikidot.com/tutorials:dif...

Bottom line: ATC and other aircraft are all using pressure altitude, and this is what you should use when communicating. Airspace ceilings/floors are also based on pressure altitude, so keep this in mind as well. This is why our airspace alerts have an altitude buffer which we recommend setting to 1000' or more.

Hope this helps,
Walter

Just my two cents worth! Of all GPS functions and capabilities, altitude is the least accurate. There are many reasons for this, but basically it resolves to parallax or geometric functions. The only time a GPS unit will give semi-usable altitude readings is when multiple satellites are locked and the signals are excellent. I own a total of 7 GPS receivers, and ALL of them will tell me that I'm climbing or descending when I haven't moved an inch! Even my IFR certified GPS will wander around, changing altitude readings when the airplane is parked at the gas pump!

As Walter said, GPS uses radio signals and a multitude of calculations to determine altitude. Our aviation system uses atmospheric pressure. I recently installed a blind altitude encoder in one of my airplanes and there was ONE calibration - the reference is set to 29.92" and that is the end of it. From that point forward it will report altitude according to the standards adopted by the FAA.

Depending upon atmospherics, GPS signal quality and local barometric pressure, you might "get lucky" and observe the GPS and your altimeter agreeing with each other, but it would be rare indeed!