Originally Posted by
peterh337
GPS altitude is normally accurate to within 10-20ft - so long as you get good reception.
However some older products, notably those using the popular SIRF-2 chip, had a systematic altitude error of about 160ft. I can't remember which way it went. Basically they did not correct for the deviation from the ellipsoid, IIRC, which in the UK is a certain fairly constant figure.
Pretty irrelevant - between start and end of the manoeuvre, the total altitude change will be below 1000ft in probably any light aeroplane. In analysing something like this, it's just differences, not absolute values - and the GPS is in that case, giving a more exact reading than anything a pressure altimeter will give you anyhow since it's unaffected by PEC.
OAT and pressure altitude will give density altitude near enough, and those can just be recorded at stable conditions before manouevring.
Even if flying take-off to landing for a manoeuvre like this, the same applies for data analysis because GPS will flatline on the runway (and given that PEC varies with airspeed and may be affected by ground effect, a pressure altimeter may not, although it'll be close enough to allow analysis if you had airspeed as well).
G