July 9, 2020 at 10:22 AM #9503David WebbGeneral Member
Saw an interesting discussion among the San Diego group. An issue was reported to Ayvri regarding altitudes occasionally showing incorrect values in their app. Here’s Ayvri’s response:
Ayvri is showing you your barometric altitudes, while your other track visualization tools are likely showing your GPS altitudes. Your instrument (and pretty much all paragliding instruments) record both into the IGC. Here’s the first sample point from your track:
The last 10 characters there are the barometric altitude followed by the GPS altitude for that first point, which translates to:
Barometric: 1304 meters = 4278 ft
GPS: 1397 meters = 4583 ft
The reason that the barometric altitude does not match the GPS is due to pressure variation day to day. In order to get accurate baro readings, the instrument needs to be calibrated (either automatically, which most new instruments do, or modifying the QNH) so that the altitudes match. Traditional aircraft do this by syncing with their local airport. Here’s a pretty good article about all this from a free flight perspective: http://www.free.aero/en/media/altitudes-E.pdf
The reason we show barometric altitude instead of GPS is that the height differences between samples along the track are much more accurate for barometric than GPS, even though the absolute heights may be “wrong”. The higher you go, the more inaccurate GPS altitudes become, so we felt like this was the right choice for paragliders.
But, we’ve received a lot of bug reports about this in the past, and I think it’s time we changed it. What I’d like to do is use the GPS altitudes instead if we detect that the barometer is not calibrated correctly, as in this case.
Keep an eye out for the fix in a coming update.
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