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    Re: London symposium (on Cook's mapping)
    From: Nicol�s de Hilster
    Date: 2006 Nov 25, 14:11 +0100

    At a certain moment in time George wrote [NavList 1745]:
    
    >Well, it isn't so simple to make a map of an unknown coastline as you 
    >"meander" along it. Land surveyors can measure a baseline and take 
    >bearings from its ends and triangulate from that. From sea, any 
    >baseline is somewhat fluid. For a coast that tends North-South, it can 
    >be done by creating a baseline between two sea-positions with spacing 
    >determined by precise celestial latitude measurement. Where it tends 
    >East-West, it's much harder, when the surveyor lacks a chronometer, as 
    >Cook did on that first circumnavigation, and had only lunars, and dead 
    >reckoning, to rely on for longitude differences.
    >
    >  
    >
    I discussed this bit the second evening in Bar du Mus�e with Jeremy. As 
    you all know I am a hydrographic surveyor [NavList 1734] and with my 
    background I do see an option of using a plane table on board of a 
    vessel. As the plane table needs a fixed orientation this could be 
    obtained by using a far direction (mountain, cliff). Any movement of the 
    ship would be small in comparison to this far direction. If they added 
    two visors to the plane table and operated it with two men (one for 
    orienting the plane table using the visors on this far direction and one 
    for drawing the lines using an alidade) it would have been possible to 
    do a plane table survey.  The survey itself would only take a minute or 
    so as the main goal of the plane table would be getting the reference 
    lines on paper. Filling in the details could be done on sight.
    
    The 'double protractor' with compass Jeremy showed may have been very 
    usefull as well as they might have been able to do some resection with 
    that. I say 'might' as I would have to test the possiblilities of this 
    instrument. The resection could be done using specific landmarks, that 
    were well visible during the voyage.
    
    It is known that longitude posed a problem as Cook cut some of his field 
    sketches in two (in north-south direction) to correct for longitude errors.
    
    As someone after Jeremy's paper suggested it would be a good idea to 
    check the lunar distance tables Cook used for errors, in order to find 
    out the influence of them (if there are any) on the survey.
    
    Nicol�s
    
    
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