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    Re: Coastal Plotting Sheets
    From: Nicol�s de Hilster
    Date: 2007 Apr 01, 11:28 +0200

    P F wrote:
    > Then drew it again with B well 'inland' of A and C. I ended up with a 
    > smaller circle wholly within the larger; there was no common point at 
    > which the two angles could have been observed. If this was the case 
    > then the possibility of an erroneous position being derived would not 
    > arise.
    Something must have gone wrong, as when you are able to sight all three 
    points from your vessel and you (your vessel) are not one one circle 
    with all three of them, then there should be two circles running through 
    your position and two of the three landmarks.
    Then he continued:
    > Presumably this technique would be useful if a compass was not 
    > available but horizontal angles between shore objects could be 
    > measured, with a sextant for example, although the distance inland of 
    > the middle object was unknown. Then the danger you warn of would be 
    > relevant.
    The distance to the middle point is irrelevant to a certain extend: it 
    simply places the locations with poor angle of cut at a different place. 
    Only when the angle ABC is getting well under 60 degrees it will start 
    to affect your accuracy. The technique has been used in hydrographic 
    surveying for many decades and even dedicated survey sextants have been 
    made to allow one observer to take both angles in one observation.
    When I first mentioned the station pointer I was merely saying that it 
    was a tool that did exactly what you were trying to do by creating a 
    plot, starting from a central point and radiating outwards with the 
    bearings. That method did not give any idea about accuracy nor does the 
    station pointer. I do realise that there are better methods to create a 
    fix from compass bearings, the station pointer however was simply made 
    for coastal navigation and hydrographic surveying using three beacons, 
    in which case it is impossible to show any form of accuracy apart from 
    the angle of cut (which is quite relevant of course). The station 
    pointer allows for speedy plotting of your position, very useful in 
    hydrographic surveying. When computers and plotters arrived the station 
    pointer was replaced by circle charts to make stuff even easier and 
    faster (I did work with those charts and a sextant in Nigeria back in 
    the early 90s).
    As said the station pointer was used in hydrographic surveying using 
    beacons. These beacons were placed on the shore or nearby high 
    landmarks, buildings etc. So a hydrographic surveyor would choose their 
    locations in a way that he would have a proper angle of cut in his 
    working area. Church towers and chimneys of coastal villages or even 
    high voltage masts serve a good alternative.
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