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I used the cheapest Davis made thirty years ago. I think that it was good for 
something on the order of a dependable + - 5 to 7 miles under the best 
conditions. Of course I did not have GPS then but my landfalls were usually 
within 1 mile of the intended point.

I can achieve consistent sub mile accuracy under the best conditions and 2 to 
3 miles most of the time with my Cassens & Plath sextant when compared to 
GPS. I have not had the opportunity to compare sextant with GPS under 
stressful conditions yet. I have only made one trip to date with a GPS 
onboard and that was under beautiful conditions. Some of the practice sights 
near Bermuda were 1/2 mile from the GPS position.
Robert Gainer

>
> From: "Greg R." 
> Date: 2006/05/08 Mon PM 02:21:50 EDT
> To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> Subject: Re: Question about Davis Mk 25 sextant beam converger
>
> Fred:
>
> > One needs to check IE each time the sextant is used.  But one doesn't
> > need to ADJUST IE to zero each time.
>
> I would agree in principle (and the explanations for not doing so certainly
> do make sense), but am curious why Davis would say that index error should
> be adjusted each time it's used? Maybe what they're actually doing is
> encouraging people to wear out the adjustments earlier so they'll sell more
> sextants?...  ;-)
>
> BTW, does anyone know what kind of accuracy is possible with the plastic
> sextants? Is it possible to get a consistent zero (or very low) intercept to
> a known position, or with plastic construction are there just too many
> variables involved? I did notice that my index error varied over the weekend
> from around +5' to around -4' over just a couple of days.
>
> Temperatures ranged from mid-70s (my backyard) to mid-60s (beach), but I
> wouldn't think that would be enough of a gradient to cause that much change
> (or maybe that's typical for plastic?). The sextant wasn't dropped or
> handled roughly (and was always carried in its foam-surround case), so I
> don't think mechanical shock would be the cause.
>
> --
> GregR
>

   

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