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    Sextant accuracies
    From: Doug Royer
    Date: 2003 Mar 17, 11:46 -0800

    I'll defend trying to get accuracies closer than 1 n.m. for one reason
    only.If one strives to use all the techniques of observation,correction and
    calculation to obtain an accuracy < or = 1 n.m. while at sea on a moving
    vessel then the results from a round of sights will most likely be as
    accurate as possible under the existing conditions.Remember that the LOP's
    obtained during a round of sights indicate where one was not where one is.I
    beleive that there are to many uncontrollable conditions especially on a
    smaller vessel to hope for greater accuracy even if all the above techniques
    are flawless.On a large vessel the sighting platform is more stable than a
    small vessel.The accuracy sought would also depend on where one is.Around
    landfall or congestion one would be more concerned with accuracy.Outside of
    congested areas of shipping or traffic the accuracy is not as important as
    in these areas.On larger vessels in the uncongested areas of the oceans one
    strives for the most accuracy one can acheive but should feel satisfied if
    one comes up < or = 5 n.m.Sea state,atmospheric conditions,ships
    heading,pressure to get the results all play a part in the final result.Take
    the following example: ships heading 325* psc. ships speed is 25 kt. sea
    state heavy  atmospheric conditions are stormy. Time for a round of sights
    we''ll say is 6 min. In the 6 min. of time for the sights the vessel has
    moved 2.5 nm.The vessels heading didn't remain at 325* but moved at so many
    * to either side of it for x amount of time.The horizon can't be accurately
    discurned because of the sea state,atmosphere and the observer moving.The
    observer may get the body on the true horizon but could be above or below it
    also.I think you get my drift.Remember that the sextant and sight reduction
    techniques are much older and open to more variables in interputation and
    manufactureing than is modern electronic nav. equipment.Even a bad round of
    sights is better than no round of sights.At least one will have an idea of
    the position held.On modern large vessels traditional nav. techniques are
    not the main position finding tools.What we officers will do,sometimes for a
    nice chunk of change,is use traditional navigation through a voyage and the
    position obtained that is closest to the actual position of the vessel when
    the master calls an end to the evolution wins the pot.I beleive it well to
    wring as much accuracy from ones equipment as possible,but think it
    foolhardy to compair the resulting position obtained with modern electronic
    navigation equipment positioning.
    
    
    

       
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