NavList:

>>be suspicious with observations through double glazing units <<

I think as a general rule, this has got to be true.  Your opportunity is there to verify a particular observing location by comparing a number of shots with your GPS position, and making some inferences about the quality of glass you are looking through.

I have taken some hundreds of sights through the double-glazed front windows of my living room.  Whether it is a "personal error" related to how I hold the sextant, or a quirk of the windows I am taking my sight through, I have discovered that if I subtract 3 minutes from my Hs, I get a pretty accurate line of position.

Of course, while the existence of my error is unequivocal, the source of the error is a bit harder to identify.  I can say that I have several windows in the frame of the living room bow window, and that 3 minute error is fairly consistent no matter which one I am looking out.  I also apply that 3 minute correction to sights I take when I am bouncing around in a boat, and I have been pretty happy with the results.  So I am slightly inclined to think that I "see" the sun touch the horizon 3 minutes of angle before it actually does, and that my windows are pretty good.

Bob