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    Re: Dip from Shore
    From: Bob Goethe
    Date: 2016 Feb 9, 12:12 -0800

    >>I hadn't even considered wave height.  <<

    When taking sights from a yacht a couple of years ago, my first few shots yielded a location that was out by scores of miles.  My accuracy improved as I sorted out how to take my sextant altitude at the moment when my vessel was on the crest of a wave.  That way, I am on a wave top, looking across to a horizon mostly defined by wave tops.  I was able to do this when the waves were coming from the same side of the vessel as the celestial object was visible, or from ahead or astern. Hold my sextant up to my eye, and dial in the approximate height.  Then glance out (or fore or aft) to watch the next big wave rolling towards us.  Once we moved onto the crest, we seemed to more or less hang there for 3 or 4 seconds...which was enough time to finish my sight. 

         Failing in this, I was essentially looking "up" at a horizon defined by nearby wave crests, instead of "across" at a horizon.

    I found it impossible to acquire a knack for looking over my shoulder at a wave approaching from the opposite beam.  Just too much head movement.  The trick there was to have an assistant, who would watch the wave approaching from the other side of the boat, and do a countdown:  "3...2...1...1.............MARK!"  And I record my height at the "MARK!"

    I was regularly able to get 2-3 nm accuracy (compared to our GPS position) in 10-12 foot waves, with occasional sights within 1 nm of true.

    >>One of my future (ie, ain't gonna ever happen) projects would be to run multiple iterations of a sight reduction program, a round of say 4 or 5 well placed objects, and just increment the dip by 1 foot each round to see if the fix closes in on the real height of eye.<<

    You might not need to do multiple iterations with the SR program.  Do a set of sights.  Figure out what the dip would need to be to make those sights translate into LOPs that, on average, run through your position. 

    I identified a "personal error" in much this same way.  I found that my sextant sights, both good shots and bad, always tended to end up with an LOP that was "towards" the celestial object.  If I consistently subtract 1.5' from any sight I take, then my LOPs hover around my actual GPS-defined location, with no apparent preference between "towards" and "away".  Best of all would be to correct my technique, of course.  But if my "PE" is consistent enough, I can leave my sight-taking technique where it is, and just add one more correction-line to my sight-reduction worksheet.

    Bob

       
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