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Mr. Fogg,the diagrams I sent were garbled in transmition,so what is on the
board is not what I drew.I don't know how to draw them on the computer so
they transmit properly.That was the simple exercise to teach the method out
of the books I have.I didn't want to take the time to go into an
explaination or the experiment of the more complex examples of the acute or
abtuse horizontal angle exercises.If you can find some of the older books on
pilotting they ,in my opinion,have more of these examples than do newer
books.The examples are explained in the books better than I am able to
explain them.CA and B are in a straight line with A in the middle.I did the
exercise for my own curiosity.I hope you do the exercise and let me know the
deltas of your experiment.Table 31 in my Bowditch (1984) are the trig
tables.It may be well that some people look at the title of Mr. Bowditch's
book and note the 2nd adjective in the title."practical".

-----Original Message-----
From: Peter Fogg [mailto:ffive@TPG.COM.AU]
Sent: Tuesday, March 25, 2003 14:44
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Horizontal distance off measurements


Thanks to Doug Royer for taking the trouble to explain in detail his
technique for horizontal sextant readings to measure distance off. I
intend to print it out and work through his example in the hopes of
better understanding it.

If I remember correctly the promise which led here was that horizontal
sextant readings would be more accurate than taking bearings with a hand
held compass, which is something I do a lot while cruising along the
coast. Typically the target, due to boat movement, swings from side to
side through the eyepiece and what I have to judge is the midpoint of
the average swings. I don't expect that it is more accurate than to the
nearest few degrees but 3 of them typically yield a tight enough
triangle, often enough confirmed by GPS reading.

Sure a sextant can measure to less than one minute of arc but I'm not
sure how practical this would be on a small boat in average coastal
conditions;  about 1-2 metres of swell. I suspect Doug is talking from
the perspective of a big stable ship which could be quite different.

Still, its all useful grist to the mill of knowledge.  Contributions
like this are much appreciated and I wouldn't, in his shoes, take too
seriously the occasional acerbic comments the list throws up.

   

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