iFly GPS Forum

FYI - I forgot to mention that I also tried setting the barometric altimeter to 29.92 to see the effect....there was still a difference of aprx 350ft.

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So then the altitude given by any GPS unit is basically irrelevant for flying unless you are very close to the ground? Seems odd but if thats the way it is then I guess thats it. It doesn't change my high opinion of the iFly but it does make me question why there is no way (with any GPS) to manually adjust the given altitude so it is relevant to the barometric altimeters readout.

Perhaps its too complicated but given that altitude is such an important factor I think it would be very helpful to be able to adjust the readout (to the barometric altimeter) once you have leveled off. I imagine there is a mathematical formula(s) that could help compensate for or minimize the discrepancy to a usable value as it seems relatively consistent that the higher you go, the greater the discrepancy.

Thanks for the info -

Dave

Thanks Steve -

This is all new to me and from my conversations with Scott and Walter it seemed that even they thought the difference was too much. I had no idea that GPS altitude worked as such and your explanation is the first I have heard that makes sense. This is my 1st aviation GPS - everything else the unit indicates has been dead on so it seemed pretty strange about the altitude.

I wonder how they do it with the G1000? Also, don't you think being able to adjust this value (or the way it is calculated) would be of benefit based on geographical differences as you explained. Seems the error is fairly consistent here in the North East - I would imagine that errors are consistent in other regions as well. Dialing them out would be a practical solution for a more accurate indicator (assuming its not too difficult to add to the programing).

Thanks again to all for the education.. and especially to Scott, Walter, and all at A.P. for being a great American co. Keep up the good work!

Dave

I'd agree but they really were not aware of this condition as being reported as a problem - that's why they were kind enough to take my unit in and test / replace. If this is something users would like to see addressed (if possible) we should put it on the wish list.

I wonder if there would be an easy way to enter the barometric pressure to compensate since adjusting it to std still didn't equal out. Perhaps developing an algorithm that would use the variation at a few key altitudes as entered by the user, the system could better guess the correct (pressure adjusted) altitude. This could be calibrated from time to time if the user chooses. I'm no mathematician or programmer so this may be far more difficult than it appears on the surface......but if its not.......I think this would be a very useful and significant update. What do you think?

Dave