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Fascinating background Nicol�s.  I remember glancing at compass 
markings over the years (including military, commercial and 
recreational vessels) and don't recall one that was readable to better 
than 0.5 degree.  However to be fair, my interests never caused me 
to look closely or consider the matter carefully.   Plotting compass 
derived bearings with a precision three-arm protractor seems a little 
absurd but if the accuracy issue is put aside, the method easily 
yields a position solution.

Thanks for providing comments and wonderful insights Nicol�s.  I 
created the "�" with ALT 133 here.

Frank J
Rochester, NY


Date sent:          Sun, 18 Mar 2007 11:36:53 +0100
From:               Nicol�s de Hilster 
Subject:            [NavList 2401] Re: Coastal Plotting Sheets
To:                 NavList@fer3.com
Send reply to:      NavList@fer3.com

[ Double-click this line for list subscription options ] 


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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