Originally Posted by
Genghis the Engineer
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
I had a different experience. I was using a garmin gps60 from memory to log takeoff performance with a sample rate IIRC of either 1 sec or 1/2 sec. There appeared to be some interaction between the aircraft acceleration and the altitude. I started motionless at pretty much Airfield Elevation. As I accelerated, I dipped about 20 feet below the surface and then seemed to come up to the surface at about rotation, and then showed a normal climb out. It made it quite difficult to assess where the actual wheels off the ground event occurred. The deepest 'under ground' was at slightly over half my estimated ground run and there wasn't a really clear trajectory change anywhere. (As one would expect in a series of max gross takeoffs the initial climb was quite slow)