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Geoffrey wrote:
"Now you have three amalgamation position lines all crossing at the
same point�"
By bisecting roughly parallel position lines, the 6 have been reduced to 3.

Guy wrote:
"I highlighted my position lines with a orange highlighter and BINGO I
could see the
convergance of the lines."

With the exception of Rasalhague, this broad line convergence of 5
LOPs is only just to the west of the centre of the small triangle
indicated by Geoffrey's 3 LOPs (the difference is half of the effect
of Rasalhague; as one body out of six).

As to George's insistence that the ACTUAL position is not necessarily
located at the centre of a triangle, or other intersecting LOPs, this
does not seem especially helpful. As an example of how unhelpful it
is, Lu is now worrying about the precise statistical improbability
involved. Will this knowledge enable the calculation of a more
accurate position?

The value of the nominated position at the centre of position lines is
that it is the only possible CALCULABLE position. Saying that the
actual position could be somewhere else doesn't change this, or assist
the goal of position finding in any way that I can see.

Why do we want to define a position, anyway? Usually it is not to know
where we are, since after sight reduction and plotting in any sort of
moving craft we are no longer back there any more. Usually the
calculated position is then used to run forward our track since then,
a process largely beyond precision.

Moving this calculated position closer to identified danger seems like
simple common sense.

As Guy has found, 6 LOPs are potentially confusing when it comes to
deriving a position from them*. Geoffrey's solution has the appeal of
simplicity, although there seems to be little point in doubling the
sight reduction needed to produce 6 LOPs only to reduce them back to 3
in this rather arbitrary fashion.

What I would suggest as a better approach is to make sight reductions
from only three bodies with a wide spread of azimuths, but only after
comparing a number of observations from each body over a 5 minute
period with the slope of the bodies' rise or fall. This should
eliminate 'blunder' sights and lead to a more accurate position line,
which then leads to a more accurate position, although it will not
identify or eliminate systematic error.


[* Like so much else in nav, this is very much a case of 'horses for
courses'. If a land explorer or surveyor was seeking to establish as
precise a position as possible, perhaps using a theodolite, then 6 or
more position lines would not be redundant, and this and other data
could then undergo statistical and other analysis to enable the most
accurate position to eventually be established from them. Once again;
for position finding from a moving platform there is usually not much
point.]

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