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    Re: Historical charts and mapping
    From: Brad Morris
    Date: 2016 Aug 18, 23:14 -0400

    David

    0.07 seconds is 70 milliseconds.  7 milliseconds would be 0.007 seconds

    Just as today, when people  will quote results to 15 decimal places, the 1800's witnessed a similar phenomena.  In certain navigation texts, results are quoted to minutes, seconds and "thirds".  Thirds being 1/60th of a second.  It's absurd and is merely a calculation artifact.  There is little or no value in calculating to thirds, yet there it is in navigation tomes.

    Example:  My GPS offers positions to many decimal places.  But those decimal places are just calculation artifact.  0.000277 is 1 arc second, or just around 2 feet of displacement.  I seriously doubt that the GPS is affording 2 foot accuracy.  But when folks offer decimal degrees, many decimal places are offered!!

    Brad


    On Aug 18, 2016 7:05 PM, "David C" <NoReply_DavidC@fer3.com> wrote:

    As I read this thread I realised that for many years a relevant map has been hanging on my study wall. The map (published in 1885) shows the results of a geodesical survey of the North Island of New Zealand. At five sites latitude was determined by astronomical means. At one of these sites - Mt cook Initial Station - the longitude was determined astronomically.

    All other lat/long pairs were determined by spherical trigonometry.

    NZ was connected to London by telegraph cable in 1876 but I have seen a comment that the  telegraph cable was not used for time until about 1914. The comment lumped  wireless and cable in one phrase  so maybe 1914 was when wireless was first used?  It is not a subject I have studied but maybe I should.

    Note that the observed long is given to  0.1 of a second of arc. Norie's arc to time table tells me that 1 sec of arc is  .07 sec of time so the surveyors were claiming that they could measure time with a resolution of 7ms. Could time be measured with this resolution in 1885? I have my doubts.

    Note that Mt Cook Initial Station is not the 3764m peak in the South Island but rather an insignificant hill in Wellington whose only other claim to fame is that it was once the site of a jail.

    I have also attached three photos I took about ten years ago at the site of the Mt Cook initial station.



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