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Subject: Accuracy of Altitude sighting at Sea
From: Richard B. Emerson (navsys@XXX.XXX)
Date: Sun Oct 24 1999 - 22:24:45 EDT
My general rule of thumb is errors that can be sailed down in roughly
an hour (i.e., 5nm or less) are, practically speaking, immaterial.
Errors that take something like a few hours to sail down (say, under
20nm) are acceptable in ocean sailing. If the error would need a day
or more to sail down are cause to rethink the situation.
In the first instance, even when doing a landfall of Bermuda, for
example, there are a number of clues to the island's presence: land
birds or, in the case of Bermuda, longtails, clouds over the land
mass, ship and air traffic converging on the island. In cases where
the island has elevated terrain, it's going to show up above the
horizon at a greater distance than the island's shore. Of course,
when trying to locate yourself relative to a pass in a reef, for
example, precision is more important but there are often alternate
sources of positioning information: notably landmarks and buoys.
Errors on the order of 10 to 20 miles may come from poor DR work,
unanticipated currents, or observational errors but again it's
possible to get close enough to a landfall (or decide to postpone
landfall) with little difficulty. In mid-ocean, even a 30 mile error
on any one fix is really not a crime. There's probably enough time to
refine the ship's position with subsequent observations.
I just came back from a week of gunkholing on the Chesapeake and on
two different days worked sun lines and a noon line just to keep my
skills going. Even with a problem of a shoreline that was about 2.5
miles off instead of an open horizon, I was able to advance to line to
a noon line and get a fix that was 5 miles SSE of my true position (by
GPS and bearings). While hardly usable in the Chesapeake (I think the
fix put me in corn field), I'd be happy with this result offshore.
As was alluded to in an earlier note, high precision fixes based on
shots from a heaving deck are almost oxymoronic. From the bridge of
the QE2 it's certainly reasonable to expect these results but from the
deck of a 35 or 40' boat? Nah. [grin]
Rick Emerson
S/V One With The Wind, Baba 35
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