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    Re: Measure of All Things
    From: Kieran Kelly
    Date: 2003 Sep 30, 18:07 +1000

    I recently read Ken Alder's excellent work The Measure of All Things and was
    struck by Delambre's struggle to improve the accuracy of celestial sights
    and transit observations for the French metric survey. In the book there is
    a discussion of the difference between Precision i.e. random errors and
    Accuracy i.e. constant errors when making a celestial observation either
    with a theodolite, sextant or transit circle.
    I am still a bit confused. Is a random error-precision-a human error either
    in the sighting technique or in the calculation such as an arithmetic error?
    These errors would presumably be expressed as a bell shaped curve. If so
    then the accuracy must relate to the instrument and would always be
    constant. Thus errors of accuracy would be expressed as a straight line when
    plotted. Those who use sextants are familiar with errors such as index
    error, collimation error or the errors along the arc. But I thought these
    errors could be eliminated. Apparently not completely eliminated. Is this
    interpretation correct?
    Does it mean that because of random errors no observation is ever completely
    precise as minute human variables come into play? Similarly if there was to
    be no errors of accuracy then the measuring instrument - sextant, theodolite
    etc would need to be perfect. On the basis that no man made machine is
    perfect it would appear that all navigational fixes and surveying
    triangulations are ultimately only approximations.
    This was certainly the case with the French metric survey which was found to
    be inaccurate with the advent of satellite navigation. What I don't
    understand is how do scientists know it is inaccurate when there appears to
    be no absolute standard if Delamere is to be believed. Is it possible that
    in future a further technological development will prove the satellite
    survey to be inaccurate?
    Any assistance would be appreciated.
    Kieran Kelly

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