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Bill,

> Answering my own question, I *finally* found the discussion of 3-arm
> protractors/station pointers in the archives under "Coastal Plotting
> Sheets."  The discussion ran from March 17, 2007 through April 1, 2007.

I looked at it. They are talking of a different problem.
Finding a position of the ship from BEARINGS.
You need TWO bearings to find your position if the bearings are exact.
Using three bearings is just for checking the error in bearings.
The size of the triangle you obtain more or less tells you what was
the accuracy of your bearings.

Bearing is taken with some kind of compass. The accuracy of a compass is
usually
something like 1/2 degree. Well, perhaps 1/4 degree with some very
advanced compass.

The 1' station pointer is not needed for this. Ordinary protractor
will suffice.

The 1' station ponter is used with a SEXTANT, not a compass.
And the position is found from two HORIZONTAL ANGLES, not bearings.
You need THREE objects on the shore, 2 is not enough as with bearings.

You measure two angles with your sextant, then set these angles on
the station pointer, and apply it to the map. The center
of the pointer shows you
your position. Perhaps they intentionally made it with 1' scale,
so that you transfer the sextant reading directly to the pointer,
without rounding or whatever. It has a drum (two drums) which look
exactly like sextant drums.

As we already discussed, the method is super-accurate for most ordinary
purposes, you cannot even see the difference between two points on the map
which correspond to 1' difference in the angles.

When I said "ridiculously accurate", I meant that this is not needed
for ordinary navigation purposes, that compass bearings would be enough.

One evident situation when this can be needed is when you try to hit a
target on the shore with the first gun shot...
Another is map-making, hydrographical surveys etc.
Of course, the method has an advantage tht you do not need to know your
compass correction. But you need a very good map to use the full
power of the sextant and 1' station pointer.

Alex.

   

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