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    Re: Leap second today
    From: Lu Abel
    Date: 2012 Jul 2, 08:42 -0700
    Geoffrey:

    I have only the vaguest knowledge of land surveying techniques, but I am forced to ask two questions:

    1.  Do land surveyors do astronomical observations to establish position any more?   Every surveying crew I observe these days seems to have one of these high-precision GPS devices.   In fact, did they ever do it except, for example, to establish the position of some island in the Pacific?    Every illustration of surveying techniques I've seen seem to be based on distances and angles from known starting points (called, I believe, traverses).  The Great Survey of India, a 60-year effort to map the boundaries of the subcontinent, was strictly trignometric; if celestial observations were part of the surveyor's repitorie, would they not have been used here?

    2.  If land surveyors were to attempt to do high-precision location via celestial sights, would they use the NA for celestial data or a publication giving celestial data to even higher precision?

    We're talking about the Nautical Almanac, which is intended for celestial navigation of ships.  I suspect an almanac with data that, with perfect observations, gives location to within 0.1 nautical mile is quite sufficient for the task.


    From: Geoffrey Kolbe <geoffreykolbe@compuserve.com>
    To: NavList@fer3.com
    Sent: Sunday, July 1, 2012 10:04 PM
    Subject: [NavList] Re: Leap second today



    Gary wrote:

    > Before I would start worrying about the difference between UT1 and UTC, I would first have to figure out how to time the sight to a precision of better one second.
    >
    > gl

    It is an argument that the precision of marine sextant measurements is such that we need not bother ourselves about the difference between UT1 and UTC, but for other users of the NA such as surveyors or land based navigators who would use a theodolite, the limiting factor becomes the accuracy of the NA tables and their accuracy is better than one second.

    Geoffrey Kolbe





       
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