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    Re: Coastal Plotting Sheets
    From: W F Jones
    Date: 2007 Mar 17, 18:37 -0500

    I also purchased a unit similar to the Warren-Knight version in your first 
    image.  It isn't near me, so I can not  check to see who manufactured it 
    but I know it was made for a the US Navy (BuShips).  A very impressive 
    I have often wondered why the instrument needed to be so accurate, My 
    old unit has verniers that permit measurements to 1'.  Better sextants will 
    do about 10".  What does this phenomenal instrument accuracy translate 
    to when working with a real chart?  Even using the finest width pencils 
    available today (they weren't around in WWII) how can you really plot 
    angular data to 1'.
    Of course today this is done mathematically and such wondrous devices 
    are no longer really needed, right?  Any clue what these cost today, new 
    from factory?
    Frank J.
    Rochester, NY   
    Date sent:          Sat, 17 Mar 2007 09:49:14 +0100
    From:               Nicol�s de Hilster 
    Subject:            [NavList 2391] Re: Coastal Plotting Sheets
    To:                 NavList@fer3.com
    Send reply to:      NavList@fer3.com
    [ Double-click this line for list subscription options ] 
    P F wrote:
    > What this coastal nav course proposed was taking the same three
    > corrected bearings but plotting from the fix position onto clear
    > material from a point on the plastic sheet representing the side
    > opposite to the land � the seaward side. So three position lines
    > radiating outwards.
    > Then placing this clear sheet upon the chart and adjusting it until
    > all three position lines are placed over the features they point
    > towards � and beyond. You can immediately see the advantage � there
    > is no ambiguity about the fix as the triangle proposes; the fix is a
    > point.
    For this purpose a nice instrument was constructed: the protractor or
    "station pointer". The station pointer consists of a divided circle
    with three legs and was used mainly for sextant observations using
    beacons on shore. The legs could be adjusted by either a vernier or
    drum micrometer. The two measured angles (or reduced bearings as in
    this case) could be set on this instrument, after which the whole
    instrument could be placed on top of a chart of the area. After moving
    the instrument around until the three legs crossed the beacons on the
    map the middle the station pointer would then be your position.
    Examples of the instrument can be found on my web-site:
    The first one comes with a whole range of plotting aids like a 
    push-button pin and a cross-hair with pencil hole. The second one
    simply has a ruler in the centre with a small notch in it.
    To post to this group, send email to NavList@fer3.com
    To , send email to NavList-@fer3.com

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