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Herbert wrote:

> While I concede that plastic sextants adequate for coastal piloting are
> available, Starpath's claim that the Mark 3 be one of them, indeed be
> the "sextant of choice" for that purpose seems absurd.

I feel you are being a little too harsh on the Mark III in this regard.  I
fully agree with your assessment regarding distance by height (vertical
measurement).  In my last post I stated, "I would personally prefer my metal
unit for height-of-object distance-off sights,..."

Using plane trig in one table, dubious heights above water (many charts
state height above the base) and possible parallax make it iffy enough.  A
highly accurate instrument is required for the potentially small angles.

I went on to state,  "...but believe a plastic unit more than adequate for
horizontal measurements unless navigating a large military or commercial
vessel.

Consider the alternative used by most small craft navigators for horizontal
angle measurements--a compass.  Be it sighting over a built-in unit or a
hand-held sighting type one would be talented and fortunate to have two
combined readings come within a couple of degrees of reality--before
deviation and variation are figured in.  Here a Mark III could be far
superior to a compass when determining angles between shore object or the
craft and shore object.  Here is where it could be a remarkable value,
especially for someone that does nor practice cel nav or own a metal
sextant.

> While making their claim, Starpath admits to not expecting
> better accuracy than 10' from a Mark 3.

Actually they state, "One should consider a consistent under 10 miles as
good for this device."  By using miles I read this as a fix/EP from two or
more observations on the water with all the fuzzy variables thrown in.
Several sources have done much better than this from land, especially with
one intercept, but 10 nm does establish a practical safety limit for
navigation.  It does not say, IMHO, that a good reading is 10' off.  It
would appear the instrument can be accurate to almost 2-3' in careful hands.
That is far better than one can do with a compass.

As to distance by height, I have done many trials with Chicago buildings,
and the Michigan City (Indiana) light house, cooling tower and smokestack
with an Astra IIIB.  I was amazed at my very poor results, using Bowditch
tables or trig.  I won't discount my technique, but results were often 30%
off our DR/GPS position.  I have much better results dipping lighthouses ;-)

Respectfully,

Bill

   

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