NavList:

David Pike - agreed!  Averaging works great so long as the slope of the altitude vs time stays about the same at the end of the series as it was at the start.  At meridian passage, the slope goes so far as to change sign, of course, but the rate of change in altitude stays near 0 so an averaged altitude over short time periods should be very close. And, as you say, I think the 2 minute interval fits that short time period bill.

I am still learning how to best manage the centering of the object in the bubble.  As a result, I tend to have better results with objects that are changing altitude quickly.  When I shoot something with the bubble that is at or near meridian passage I tend to miss by 2-3' - faster changing objects I tend to get very accurately.  I suspect this is me adjusting and correcting more often for faster moving objects whereas with slower moving objects I set the object and then let it sit there with whatever initial bias I had on it.

-- Arthur