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Thanks to Jeremy for removing my worries about the operation of his
SkyMatePro navigation program, in the circumstances in which it was being
used.

He has kindly re-analysed the same data, taking a different DR as the
starting position, and found that the program homes in on exactly the same
deduced position as before, just as it should. So he has clearly shown that
my suspicions about the way it was analysing that data were quite
groundless. It was doing its job properly, in rather difficult
circumstances.

And given a single observation, with a DR, and nothing else, it refuses to
deduce a new position. Again, just as it should. Can't be faulted.

So that disposes of the doubts I put forward, in posting [8085], under
comment 2. Which leaves comment 1, as follows-

1. As we've seen before, when looking at scatter, a single result tells us
almost nothing. It may "hit the spot" just by chance. Consistency is what's
needed.

==================================
Jeremy helpfully offered this-

"To the first point, I will make this a priority experiment on my next trip
which should
offer excellent oppurtunities to shoot rapid fire fix data on a variety of
bodies and at
a variety of latitude in both the northern and southern hemispheres.  I
should also have
a variety of courses both north/south and east/west. I suspect that the moon
will be the
best body as the azimuth changes so rapidly, but we will see what the data
shows once I
shoot them."

Good. Yes please, Jeremy; the more, the better. Nothing special about the
Moon, though, to make its azimuth change much faster than other bodies that
get as high in the sky; feel free to choose anything. Frank's proposed
procedure hasn't (yet) had limits attached, specifying when it applies, and
when it doesn't.

To be fair to Jeremy, he has claimed nothing more for this observation (data
listed in [5416]), than his comment, in [6066], that- "As we can see, it is
fairly accurate, certainly accurate enough for a deep sea position." It is
Frank's use of this result, in [8049], as the (only) numerical example
quoted in support of his proposed procedure, that endows it with extra
significance. It isn't Jeremy's claims that need testing, but Frank's, once
they have been stated quantitatively.

=================================

Jeremy ended-

"I think that George may be barking up the wrong tree in a way.  Far more
important than the DR position used in this multi-shot sight of a single
body over time, is the accuracy of the course and speed used to
advance/retard the lines.  This will grow far more important as the time
spread between the first and last shots increasing. "

Well, I was indeed barking up the wrong tree in suspecting his analysis
software. The DR position should have no effect on the result, and it was
pleasing to learn that was indeed the case.

It's certainly true that allowing for course and speed is always important,
but that doesn't become a bigger (relative) problem when observations are
made that are extended in time. As you increase the time interval, the shift
of the vessel, over that time, increases in proportion, and so also does the
shift in the position of the body that's being observed. As both change in a
similar way, the speed allowance gets no more important over long intervals,
and isn't minimised by choosing short ones. No barking, in that case.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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