NavList:

Thank you David. I understand what you are saying and did some further computations for various values of HP and Ha. Indeed, as you mention, correcting SD before P-in-A (serialising addtions) or using uncorrected SD after P-in-A (so purely additive) lead to almost the same result. For instance for HP = 61 and Ha = 80 (maximising parallax and augmentation corrections, and minimising refraction) give me those correction results:

• Ha - R + P(Ha) + SDgeo = 27.0403' <--- uncorrected SD
• Ha - R + P(Ha) + SD + SDaug = 27.3307' <---- purely additive P + SD + augmentation
• (Ha - R + SD + SDaug) + P(Ha-R+SD+SDaug) = 27.0401' <--- serialised: P-in-A calculated at the end from the corrected Ha, which is as you mentioned close to the result obtained with the uncorrected SD.

What troubles me, is that Norie's tables give a total correction of 27.3'. Capt Khan's guide or navsoft website also describe a purely additive method (would lead to 27.3'). The Nautical Almanac also adds all terms calculated from Ha and table xxxv gives for HP=61 and H=80 a correction of 20.5+6.4=26.9', which is closer to either an uncorrected or a serialised calculation. I am wondering if there is not some inconsistency in various sources on how to use or properly add both the augmentation and parallax?