60CSx ...GPS Altitude or Barometric Altitude

Which provides more accurate information?

I fly airplanes and we are constantly adjusting the altimeter that is
barometric. In fact it would be 500 foot off if not adjusted some days.
(Low pressure somes in)

Depends on the circumstances. Barometric measurements are more stable
with less measurement fluctuation and also don't depend on having a
good skyview with line-of-sight reception of at least 4 satellites
with decent geometry. OTOH, these pressure measurements will drift
with time as the local weather conditions change which can lead to
apparent altitude variations of hundreds or even over a thousand
feet. So the barometric measurements need periodic recalibration to
remain accurate.

The Garmin units, like the 60CSx, let you use the GPS measurements to
do this recalibration automatically by having the unit keep track of
the barometric vs. GPS altitude averaged over about the last 30
minutes. In principle that should be fast enough to track most
weather-related changes while being long enough to smooth out
fluctuations due to GPS measurement error or periods of poor
reception. How well it works in practice is subject to some debate.

peter wrote:



I don't understand how or why GPS altitude would NOT be accurate.
Could somebody explain to me why it would not be accurate?

On Mon, 21 Jul 2008 16:19:24 -0700, Anthony Guzzi


The vertical axis of the GPS is usually has about 4x the variance of
the horizontal axis so if you have a +/- 3.6 metre accuracy (12') then
the vertical axis is approximately accurate to within +/- 14.4 metres
(48'). Depending on your requirements that may be as accurate as you
need. Surveyors off course need a much higher degree of precision.

3.6 m accuracy is pretty common but occasionally with heavy tree cover
or blockage by buildings/mountains the accuracy can be much worse.
Then the use of actual barometric pressure readings may prove to be
more useful provided you calibrated correctly in the first place.