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    Cook's surveys. was: Re: Circle charts
    From: George Huxtable
    Date: 2009 Feb 1, 20:50 -0000

    Nicolas de Hilster wrote-
    
    "For what I understood Cook charted New Zealand using a plane table from
    a ship and still did quite a proper job. He must have used similar
    techniques to get his positions right."
    
    I've no great knowledge of surveying, and I don't think Cook ever described
    in detail how he went about it. But I doubt if Cook ever tried to use a
    plane table from a ship, though he certainly would do so from on land.
    
    Cook learned about the techniques of military survey of land from Samuel
    Holland in Eastern Canada in 1758, during the war with the French. This
    acquainted him with the use of the plane-table, a simple analogue device for
    mapmaking. But that was a device that needed to be firmly planted on solid
    ground, so that the plotted azimuth of each landmark stayed constant, while
    bearings of other landmarks were added. He was to put it to good use in his
    surveys using land-stations, both in Canada and later in New Zealand. Cook's
    great advances in maritime survey mere made by combining such observations,
    from on-land viewpoints, with other observations taken from on board a ship
    by compass and horizontal sextant angle, both at anchor and under way.
    
    But a plane table couldn't be used at sea because its orientation was
    unstable, as the ship's heading would keep changing as the vessel yawed
    under way, or swung at anchor. Instead, accurately timed observations of
    horizontal angles between landmarks, compass bearings, steering compass
    readings, soundings, and Sun altitudes would be recorded. These would then
    be combined in a large chart, perhaps in real-time, down in the captain's
    cabin while the survey was progressing, but more likely in harbour later.
    The important thing was the careful noting of all the necessary
    observations, leaving nothing out.
    
    Cook's survey of Newfoundland was even able to align the whole series of
    charts to a precise Greenwich longitude, because he had been lucky enough to
    observe, and time, an eclipse of the Sun. Cook wasn't then confident enough
    to analyse the results on the spot, but he knew what he had to measure, and
    took his results back to Greenwich next winter for the gurus there to work
    out his longitude, a result that stood the test of time. You will find
    Eclipse Island, where the observation was made, on the Western coast of
    Newfoundland.
    
    As Nicolas says, Cook certainly did "quite a proper job" in surveying New
    Zealand. He wasn't provided with a chronometer (to test out) until his
    second circumnavigation, so the New Zealand survey was done without one. And
    the effects are noticeable, here and there, particularly around Coromandel
    peninsula in North Island, which ended up in Cook's chart quite a lot
    slimmer than later mapping showed.
    
    This was the problem. Endeavour had sailed up the East coast of that
    peninsula, including a stop, to observe a transit of Mercury, in Mercury
    Bay, which would help to fix longitude. That survey corresponds remarkably
    well with modern mapping. Then, after rounding the North end, he could make
    an equally accurate survey of the West Coast of the peninsula, down to the
    Firth of Thames. But the trouble was in rounding that North tip, where
    Endeavour met contrary winds that called for much beating about, enough to
    confuse any attempt to make a proper running-survey around that point. And
    so the only way those two coastlines could be connected was by comparing
    their longitudes, which relied on the accuracy of an ordinary pocket-watch
    (not a chronometer) over an interval of several days. As a result, Cook's
    chart shows the Coromandel a lot narrower than it really is. What was then
    called for was a shore-party to ascend a central peak from which both coasts
    could be seen, but that couldn't happen until much later.
    
    ============================
    
    Richard Pisko in [7213] mentioned a recent Canadian TV programme describing
    Cook's  surveys of Newfoundland, adding "I wonder if he had a
    way to keep the sextant level, so as to avoid taking the "slanted" angle,
    while still being able to align his chosen reference stations."
    
    My guess is that slanted angles would be avoided, where possible, by
    choosing landmarks at the shoreline, to keep everything in the same plane. I
    doubt if the height of the poop above water would have a significant effect
    on such horizontal angles. Where landmarks at an altitude were called for, I
    imagine that together with the slant angle, an angular height would be
    required. I expect that Richard understands such matters rather better than
    I do.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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