Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    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

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site