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    Accuracy, to half a metre?
    From: Peter Fogg
    Date: 2003 Sep 8, 16:48 +1000

    Some contributers to this list, who are land based, have detailed their
    endeavours, using sextants and reflective devices, to obtain fixes as close
    as possible to their known position. In the recent discussions which have
    centred around appropriate methods for amateur navigators in small boats
    using nav. as a practical tool, I hope I haven't appeared to criticize or
    disparage their efforts. To them I dedicate the following anecdote.
    
    Recently I needed to have some survey work done here at home, local council
    nonsense, and found that the surveyor was old enough to have been taught and
    to have used field astronomy, meaning taking sights with a theodolite
    mounted on a tripod. He told me that surveying outback properties, which
    here in Australia may be measured in thousands of square miles, used to be
    good business for surveyors. The team would go out in 4-wheel-drive vehicles
    and set up camp then take multiple sun and then star sights to establish,
    for example, a corner of a property. Then break camp, pack away the tents
    and drive, sometimes for hundreds of miles across trackless semi-desert, to
    another site. These days they arrive by helicopter, set up the GPS, and lift
    off shortly afterwards to arrive in no time at all at the next location,
    already fairly accurately determined in advance.
    
    On accuracy, he said that the ultimate accuracy using field astronomy was
    constrained by a number of factors: the accuracy of the time was important
    (he used a stop watch) but also the precision of the astronomical data
    available. We use Declinations and Hour Angles stated to the nearest minute
    of arc or tenth of a minute of arc but they required far more detailed
    precision. They would take multiple sights and then work on them; plotting
    the pattern of sights against the rise/fall of the body; also using
    reiteration, meaning reworking sights using the best position as a new DR,
    and also using the techniques discussed on the Nav. List in May this year
    (Bisectors and MPP, etc) to identify and overcome systematic error. He said
    that these methods combined to increase the final accuracy by about a factor
    of 10.
    
    He said it was possible to get within about half a metre of the real
    position, which I found fairly astounding, given that a second of arc
    (measured on the earth's surface) is nominally 30.87 metres. Half a metre is
    about a sixtieth part of that - is there a term for this in the hexigesmal
    system?
    
    The brightest young men (and they were all men then) of his generation (I'm
    guessing 30 odd years ago) went to NASA in the United States to work on the
    space program, but at that time he was more interested in Ironman
    competitions and surfing than studying. He said he had a bad habit of
    turning up for classes with wet hair and excuses instead of completed
    assignments. One of the perks of his surveying course was that during the
    holidays the students found it easy to obtain berths on yachts as
    navigators, and so went off sailing in summer to Hobart or even to Tahiti.
    Then as now there were plenty of sailors who were reluctant navigators. This
    is an important fact to bear in mind, appropriate systems for them are not
    necessarily those of surveyors. Its a matter of horses for courses.
    
    Now don't get me wrong, I'm not claiming this half metre accuracy as fact.
    Far from it. I'm reliably informed that accuracy stops, even with a
    theodolite mounted on a tripod used by an experienced surveyor, at a second
    of arc, about 30 metres. I guess the point I'm making is that for those
    interested in squeezing as much accuracy as possible from their
    circumstances and equipment, the practical tools that can be used are
    readily available and have been discussed here on the Nav. List from time to
    time.
    
    
    

       
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