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    Re: Horizontal distance off measurements
    From: Trevor Kenchington
    Date: 2003 Mar 25, 19:56 -0400

    Peter Fogg wrote:
    > If I remember correctly the promise which led here was that horizontal
    > sextant readings would be more accurate than taking bearings with a hand
    > held compass, which is something I do a lot while cruising along the
    > coast. Typically the target, due to boat movement, swings from side to
    > side through the eyepiece and what I have to judge is the midpoint of
    > the average swings. I don't expect that it is more accurate than to the
    > nearest few degrees but 3 of them typically yield a tight enough
    > triangle, often enough confirmed by GPS reading.
    > Sure a sextant can measure to less than one minute of arc but I'm not
    > sure how practical this would be on a small boat in average coastal
    > conditions;  about 1-2 metres of swell. I suspect Doug is talking from
    > the perspective of a big stable ship which could be quite different.
    Back when I was an undergraduate student in the Department of
    Oceanography at U.C.Swansea, as it then was, we operated our research
    trawler out of Swansea Docks. One time, locking out into the river, we
    were alongside the Port Talbot hydrographic survey launch -- a vessel
    that would have made a respectable motor yacht but nothing out of the
    ordinary. (She was much smaller than our 100ft trawler.) This was long
    enough ago that Port Talbot still had one of the world's largest (and
    most uneconomic!) steel mills, the ore being hauled in by some of the
    largest dry-cargo ships of the time -- up to 120,000 tons deadweight and
    more. They needed a dredged channel, which (the Bristol channel being
    what it is) was continually silting up and needed constant maintenance
    dredging. Indeed, it sometimes seemed that the major traffic in and out
    of Swansea was the fleet of dredgers continuously working the Port
    Talbot channel. The work of the survey launch and its surveyors was to
    make sure that the dredgers had achieved the charted depth throughout
    the width of the charted channel, so that we didn't get some giant ore
    carrier running aground and breaking her back when the tide fell.
    Since this was all long before GPS, the surveyors had set up their own
    Decca HighFix chain, which gave them comparable precision to modern
    military GPS receivers, though only over the local area off Port Talbot.
    (We had HighFix receivers installed on our ship and tuned to their chain
    for a while, in connection with a project someone was doing.)
    Nevertheless (and here I come at last to my point), when we were
    alongside the survey launch, her huge chart table had sextants scattered
    across it. They were there as the only non-electronic way of fixing
    position with sufficient precision to meet the surveyors' need and they
    thus allowed checking of the sometimes-troublesome HighFix.
    So ... if you really need to know where you are in coastal waters, with
    high precision, and if you are averse to relying on electronics,
    horizontal sextant angles are the way to go. Even in a quite moderate
    boat, they will give you far better precision than compass bearings can.
    [The one time I have done this for real was to fix the position of an
    archaeological site we had finally found after 20 years of trying.
    Finding it had required GPS with the Selective Availability turned off
    but I wasn't about to trust GPS alone to tell us where we were and
    should go back to, especially since the war had just started in
    Afghanistan. The dive platform in that case was a converted lobster
    boat, the day was calm and the observations were easy.] On the other
    hand, if all you need to do is to keep far enough offshore that you can
    be confident of not hitting anything more solid than another boat, then
    you don't need the precision of sextant angles and the greater
    convenience of a hand-bearing compass may well make that the instrument
    of choice -- not least for the ease of plotting the results.
    But a final point: It is not necessary to read your sextant to the
    nearest 0.1' of arc for horizontal angles. That would be nice to do but
    how would you use the data on a yacht's chart table (as distinct from a
    hydrographer's computer)? No station pointer or protractor will allow
    you to plot with that sort of precision. What you can do is to measure
    the angles to the nearest 10' (still far higher precision than you can
    achieve with a compass) and then to plot that using even a cheap station
    pointer. (Mine has verniers marked to the nearest minute, though how
    anyone could keep its legs fixed in position to such fine precision,
    while searching for the fit to the three charted objects, is beyond me.)
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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