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Subject: Re: Sextant design. [was: Another Davis...}
From: George Istok (george@XXX.XXX)
Date: Mon Mar 03 2003 - 21:21:04 EST
Rodney and all,
Thanks for forwarding George Huxtable's message.
I purchased the Davis Instruments Mark 15 used. It appears to have been
used very little. All of the adjustments on the sextant appear intact, the
index is positive with no play, and the micrometer is the same. There is an
error in the instrument in that the mark on the frame and the mark on the
arm do not align the same through the arc. The error is visible with the
micrometer at zero and the index set at zero as a slight misalignment. The
error disappears with the index set at 90 degrees. It was this error that
prompted my first question to Davis Instruments. It was their answer that
the "accuracy rate" was plus or minus five degrees that prompted my question
to this list.
I am interested in the application more than the theory of navigation and
piloting. I sail on Lake Michigan in a 24 foot Columbia Challenger and know
that it is nearly impossible to take accurate bearings and plot a fix along
the coast unless it is near calm. It is interesting to see that the same
condition applies to CN. I am new to CN, but it seems that, just like
coastal piloting, one is fortunate to determine a position from a series of
sights. I think the goal of taking bearings, sun sights, etc. on the small
boat is to allow one to compare the apparent actual position to the DR
position.
I think the final answer to my question about the accuracy of the Davis
Instruments Mark 15 is not a numeric specification. Instead, it is the
concept that even a plastic sextant that has been made with reasonable care
and is handled in a reasonable manner can produce data with errors much
smaller than those introduced by the conditions in which the measurement was
made.
With that said, I want to thank everyone who responded to my original
question.
George Istok
Senior Consultant
Signal Hill Consultants, Inc.
574 232 9888
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