NavList:

I took six sights and what I had done is just base the calculation on the one I thought looked best. When I plotted a regression line, and removed one as an outlier, I get a position line 106m from my position given by Google Maps staelite image  (and confirmed by my highly accurate Samsung S4 phone with GPS and GLONASS).  So the moral in that is that the errors are due mainly to the variability of my technique (and small ripples on my tray of water), and linear regression is a good way of reducing these errors.

I was wondering though when close to local noon (but either before or after) is linear regression still a good idea? The curve altitude against time is less straight the closer you get to noon. As a rule of thumb how far away from local noon should you use linear regression?