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    Index error vs. index correction
    From: Paul Hirose
    Date: 2020 Mar 13, 14:38 -0700

    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 +
    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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