NavList:

   

Reply

Rick:

First of all, I know you're an active and knowledgable member of this list;
if some of what follows is a bit elementary it's because I also want to
educate those who may be less expert.

Anyone on this list knows that navigation is both art and science.  For a
navigator the science is knowing how to obtain various sorts of position
data for his/her boat.  The art is judging how good this data is and how
far to trust it.  And so it is with advancing LOPs.

LOPs are advanced along a vessel's DR based on the assumption that outside
effects such as current do not significantly move the vessel from her DR
position during the period between obtaining an LOP and advancing it.

For how long can one assume the vessel has not deviated from its DR??  That
all depends, and here's where art is required.  If I know I'm in an area of
significant current (for example, in the Gulf Stream or anywhere along the
southern coast of New England), I know that my boat is being likely being
moved at a knot or more off its DR.  In those circumstances I'd know that
positional information derived from an advanced LOP would degrade very
rapidly.  (I'd also use my knowledge of the currents to decide how rapidly
the quality of the LOP was deteriorating -- for example, if direction of
the LOP and the current were the same (or opposite) I could trust the LOP
longer than if the two were perpendicular).  On the other hand, if I know
I'm in an area where there's little current I can advance an LOP much
further with confidence that it's still reasonably accurate.  I need also
remember that a sailboat can deviate significantly from course due to
leeway, especially if its hard on the wind.

What all the words above say is "it depends."  So there's no hard and fast
rule about an advanced LOP being good for 1, 2, 6, 12, or 24 hours.  But
navigation texts like to give rules of thumb.  Dutton's suggests that "a
terrestrial LOP should not be advanced more than 30 minutes" (and we need
to remember that commercial and naval ships move a lot faster than ours).
Dutton's doesn't give a similar rule of thumb for celestial observations.

The Power Squadron texts allow one to advance or retard an LOP up to 20
minutes and still claim a fix, more than that and the position is labeled
as a running fix (presumably as a reminder of more questionable positional
accuracy); Dutton's requires all fixes using advanced or retarded
(terrestrial) LOPs to be labeled as RFIXes.

We also have to remember that the required quality of a fix varies vastly
with my location.  If I'm well offshore, knowing my position within 10 or
20 miles is fine, but I'd better know it a lot more precisely when I'm on
soundings.  So offshore I'd feel fine taking Sun shots several times during
the day and advancing them to get running fixes (in fact, I've heard of
experienced offshore sailors who navigate solely using morning, afternoon,
and meridian transit Sun shots, never bothering with star shots at all!).

If I had to boil all this "it depends" down into hard rules, I'd say:
1.  Offshore, it's fine to advance celestial LOPs by up to 12 hours
2.  Onshore, never advance either a celestial or terrestrial LOP by more
than 30 minutes (one hour at the outside).
3.  The more you advance an LOP, the more uncertain your position.

Lu Abel

At 09:03 AM 8/3/00 -0400, you wrote:
>I recently ecountered a set of circumstances that left me confused and
>somewhat shaken about my nav skills.  Specifically, this past June on
>a trip from Cape May to Block Island, with Montauk Point in sight, I
>came up with a running fix that was about 30-35 miles off from my
>predicted DR position.  The running fix was based on the previous
>day's noon line at about 1700 (all times GMT), a round of sights at
>2000 for a second sun line, and, the following morning, at about 1200.
>The day 2 1200 sights clustered reasonably well (about a 2 to 3 mile
>spread in distance from the AP) but consistantly produced a result
>that was blatantly wrong.  I have since diagnosed the problem and I'm
>very comfortable with the corrected results.
>
>I want to ask a question that was posed by this incident: while
>acknowledging that there is dilution of accuracy in advancing (or
>retiring) LOP's as they age (that is, the older an LOP is, the less
>confidence a navigator should have in advancing that LOP down a DR
>track), it's been my impression that any LOP within the current
>navigation day (i.e., noon to noon) is fair game for advancing or
>retiring.  This implies that, with the understanding of loss of
>accuracy with time, LOP's as much as 23 hours 59 minutes old can still
>be used and that LOP's over 12 hours old are certainly still usable.
>
>Finally, for very specific reasons, I do *not* want to discuss where
>the initial error came from or how the sight reductions were done.
>The only question I'm asking is regarding the age of LOP's and their
>suitability for advancement or retirement.
>
>Rick
>S/V One With The Wind, Baba 35
>

   

Reply