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    Re: A noon fix inland with an peri-sextant
    From: Antoine Couëtte
    Date: 2018 Jan 8, 02:52 -0800

    Dear David,

    One question pertaining to my numerical solution to your Noon Fix published yesterday.

    In this last post I ignored or performed the following corrections:

    • No instrument corrections (maybe an uncorrect procedure : see here-under),
    • No dip corrections,
    • Refraction Corrections have been done for each specific observation (all close to -4.0' throughout observations),
    • No Sun Semi-Diameter correction as per your information.

    Regarding the ignored "Instrument Correction", would there be any "dome" corrections to be performed from your data published on Dec 18th, 2017? I am not familiar at all with your "ex-RAF Smith’s Mk2C pendulous reference periscopic sextant".

    • If so, such dome corrections would likely be more or less constant throughout the observations given their narrow range. Hence their effect would esentially modifiy the computed Latitude result only.
      • This it could be done easily on the spot without reprocessing the entire data set. A simple first order correction would involve modifying in the right direction the computed Latitude by the amount of such dome correction.
        • As I can remember a dome correction is always negative. Such correction would put the Observer's computed position a further bit to the North.

    The reason I am raising this point comes from the fact that maybe there is some sytematic error in my height reductions.

    • The computed 53°07.6' N Latitude is 2.6 NM south of your published GPS N53°10.2' Latitude (Hint ! One typo yesterday which incorrectly quoted such GPS Latitude as N53°17.2').
      • On such a sizeable set of 31 Observations, the SDEV (σ = 0.7') must be certainly quite reliable and such a 2.6' difference between both Latitudes is equal to almost 4 times their computed Standard Deviation..
        • I have never until now observed such an actual dispersion / difference of almost 4σ between one value assumed to be "perfect" (i.e. your GPS N53°10.2' Latitude) and an independent determination through observations (i.e. the 53°07.6' N Latitude computed from your sextant Observations). Such differences could, can and will happen but such a "3.7 σ" situation is supposed to happen extremely rarely.

    Hence my query about any possible systematic correction I might I have forgotten in my height reductions?

    Any comment from your part or from anybody else? Thanks in advance.

    Antoine Couëtte

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