On 25 Dec, Andrew Nikitin explained why he though it would be unproductive
for a navigator using graphical techiques to reiterate a fix of LOPs, as
follows-
"A navigator using plotting techniques introduces greater errors
which stem from these techniques (placing
a ruler, drawing line along its
edge are not exact operations, especially on a small cluttered badly lit
table which rocks back and forth). Assuming your AP is within a degree,
the error introduced by Hilaire method would be smaller then the errors
introduced by the act of plotting lines. Doing second iteration would be
a waste of time."
===================
That is a fair comment, but it needs a bit of qualifying. It depends. It
depends on the altitude, the geometrical errors getting worse at high
altitides. It depend on angle-of-cut between the azimuths of observed
bodies. It depends on the accuracy of the Assumed Position, on which Andrew
sets an upper limit of 1 degree, but that in turn depends on how good is
the dead-reckoning. In the heyday of celestial navigation, how good was the
DR of a sailing vessel which had spent a week battling adverse weather,
without a sight of the Sun or anything
else? Such conditions were not
unusual, especially in the Atlantic, in Winter.
I attach Table 4 from vol.2 of my 1981 Bowditch. This is a table of
offsets, showing how to bend a straight-line approximation to an LOP, to
make it sufficiently curvaceous to be an accurate position line. It also
shows the error that remains if, as usually happens, no such adjustment is
made.
The maximum such discrepancy in AP that the table contemplates is 45', and
because the resulting errors go as the square of that , then Andrew's
assumption, that the AP was within a degree, could result in errors that
are nearly double those in the 45' column. Those errors can then be
considerably greater than a competent plotter would accept on a
plotting-sheet.
The careful navigator can disregard the need for reiterating if his
altitudes are all low ones (less than 60º, say), at a good angle of cut,
and his
Assumed Positions are known to be close to the truth. Or if he uses
table 4 to bend his position lines suitably. Or if, being in mid-ocean, he
doesn't really care about his position being a few miles out. But
otherwise, reiteration may need to be considered seriously.
George.
contact George Huxtable, at
george@hux.me.ukor at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
