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    Re: Taking four stars for checking accuracy of fix
    From: Hewitt Schlereth
    Date: 2008 Aug 1, 23:38 -0400

    I'm new here and your reference to NavList archives caught my eye. How
    does one access them?  HewS
    
    On 8/1/08, Peter Fogg  wrote:
    >
    >  Greg wrote:
    >  >     A quick google search of TITANIC web sights showed the TITANIC's
    >  > ocean floor position to be in good agreement with the distress call
    >  > latitude but not the longitude where there is a 20 minute discrepancy.
    >  > What would explain this discrepancy best chronometer error or DR speed
    >  > error? The water was very cold at the time so I doubt that the gulf
    >  > stream was a factor.
    >
    >
    > 1. Sights taken "before 8 o'clock", collision "at 11.43, I think"
    >  during which interval the Titanic was steaming full bore to the west.
    >  Sinking quite some time after the collision, during which time the
    >  ship was drifting ... back to the east (it was a calm night), although
    >  at a considerably reduced rate.
    >
    >  2. The sea bottom was deep below the water surface, and that long way
    >  down taken by the ship may not have been vertical ...
    >
    >  As to the elimination of constant error, this was discussed in some
    >  detail, complete with diagrams, some time ago (thus findable in our
    >  archives).
    >
    >  Yes, sights using opposing azimuths will lead (with constant error
    >  only present, ie; assuming no erratic error which could complicate
    >  things) to a 'box' shape.  Since with each pair of intersecting LOPs
    >  the true fix will lie along a line bisecting the angle formed by their
    >  intersection, with such a box shape the fix will be found ... at the
    >  centre of that shape!
    >
    >  How about that.  Who would have thought.  Incidentally, the same also
    >  holds true for a triangle formed by 3 LOPs.  Assuming only a constant
    >  error, and a spread of azimuths of more than 180d, the fix (free of
    >  that constant error) must be found at the centre of that triangle.
    >
    >  It is only when the assumption is made that the sights are somehow
    >  free of any constant error, and thus subject only to erratic error,
    >  that the  fix becomes 3 times more likely to lie outside than inside
    >  the shape.
    >
    >  How can either presumption be made?  It seems to me that, a priori,
    >  any round of sights may contain some extent of both types of error.
    >
    >  Therefore the most useful approach is to eliminate erratic error at
    >  source.  This can be done via the comparison of a number of sights of
    >  the same body taken over a few minutes with the slope caused by the
    >  apparent rise/fall of the body observed over that period.
    >
    >  Then, erratic error eliminated (to a usefully practical extent), any
    >  remaining error, now assumed to be the constant type, can be dealt
    >  with by bisecting those intersecting LOPs and finding the fix ... at
    >  the centre of the shape.
    >
    >  Et voila tout.  That's it.
    >
    >
    >  >
    >
    
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