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    Re: Coastal Plotting Sheets
    From: Nicol�s de Hilster
    Date: 2007 Mar 18, 11:36 +0100

    The Warren-Knight (http://www.warrenind.com/WKNavigation.html#Three Arm 
    Protractors) has a vernier that reads down to 1 arc minute and can be 
    estimated to about 30 arc seconds. The Kelvin-Hughes has drum 
    micrometers that read down to 1 arc minute as well, but can be estimated 
    down to at least 15 arc seconds. So the accuracy is about equal to 
    period sextants.
    I never worked with the instrument myself in the field, but I know it 
    was still in use in dredging some 30 years ago (which was about 10 years 
    before I stated as a hydrographic surveyor). It was mainly a very cheap 
    and easy way for positioning. Later on when maps could be easily made 
    using computers and plotters these instruments were replaced by circle 
    charts. Those showed the measured angles as circles on the map of the 
    area at for instance 5 degree intervals (similar to Decca charts, but 
    those are hyperbolic). So I cannot tell you the achievable accuracy from 
    first hand.
    3-4 years ago I was asked to deliver 20 of these instruments to a 
    company in Bangladesh. So I contacted Warren-Knight and asked for an 
    invoice. The regular price was around US$6000 each, but 20 instruments 
    would give a US$1000 discount per instrument, making them roughly 
    US$5000 each. The Bangladesh company asked me to send brochures and 
    certificates ahead of ordering the instruments (which I did not do of 
    course) and in the end they only seemed to be interested in the 
    paperwork. I suspect they were after creating cheap replica's and 
    selling them using the documentation they wanted me to supply.
    FJones wrote:
    > 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 
    > instrument.
    > 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:
    > http://www.dehilster.info/instrumenten/stationpointer1/index.html
    > http://www.dehilster.info/instrumenten/stationpointer2/index.html
    > 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.
    > Nicol�s
    > >
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