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On 2020-03-10 12:32, Tony Oz wrote:
 > I think there should be only one term: either IE or IC, it depends on
how you define it. That value - whatever you call it - is just the
difference between the true value and the one displayed by an instrument.
 > Hence the formula: TRUTH - APPARENT = INSTRUMENT ERROR; from which
clearly follows if we want to know the TRUTH we must do APPARENT +
INSTRUMENT ERROR.


On the other hand...

On 2020-03-11 14:46, Antoine Couëtte wrote:
 >  From a recording you either :
 > - Remove (i.e. "subtract") an error, or
 > - Perform (i.e. "add") a correction.


I agree with Antoine. In my view, the error of an observation is the
observed value, minus the true value. The correction has the same
magnitude but opposite sign. To recover the true value, you either add
the correction or subtract the error.

Bowditch (1984) is surprisingly elusive on the "sense" of error and
correction. However, the master table of sextant corrections says
refraction and dip corrections are always negative, which implies
corrections should be added.

But the Nautical Almanac agrees with Tony. To calculate apparent altitude:

H = Hs + I - D

where I is index error and D is dip. (See page 280 in the 2020 edition.)

Index error, index correction... is there any consensus? Are the
definitions whatever you want them to be?

   

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