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    Re: Leap second today
    From: Geoffrey Kolbe
    Date: 2012 Jul 02, 18:12 +0100

    At 16:42 02/07/2012, you wrote:
    >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?
    
    I could respond by asking if professional sailors 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?
    
    Ah yes, but you need to know the azimuth angle of those traverse
    lines. And using a theodolite to do a timed observation of the sun to
    obtain a reference azimuth is a very quick and accurate way of doing
    it - even in these days of highly accurate GPS receivers. The NA is a
    very available and very suitable ephemeris for doing such work.
    
    Geoffrey
    
    
    
    
    

       
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