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Subject: Re: DR vs EP vs MPP vs Fix vs Running fix
From: Chuck Griffiths (griffiths_chuck@XXX.XXX)
Date: Fri Feb 08 2002 - 10:42:42 EST
In answer to the poll, I've checked with my sailing friends and after rereading
a couple reference texts in addition to Bowditch we've all come to the following
conclusion - we badly misuse the term DR position. Yes, I've always taken leeway
and estimated set and drift into account and called that my DR. I can't however
find a reference that suggests that one should call such a position anything
other than EP. My 1975 Bowditch does concedes that in the "time of sailing
ships" leeway was included by some in the DR, but it is quite clear that the DR
needs to be corrected for estimated set and drift to arrive at EP. The resulting
line from departure to such EP is labeled with "course made good". After a fix
(a true fix not DR or running fix) the line drawn from departure to the fix is
labeled "course over ground" actual set and drift can now be calculated.
In lieu of "course made good" Richard Hobbs'"Marine Navigation" (Which is a text
book used at Annapolis.) says the DR line is labeled with course(C) and
speed(S). The line from departure to EP, after correcting for estimated set and
drift, is labeled as "track"(TR) and "speed of advance"(SOA) and the line from
departure to a fix is labeled with "course over ground"(COG) and "speed over
ground"(SOG). I wish I plotted half that precisely.
Chuck
____________________Reply Separator____________________
Subject: Re: DR vs EP vs MPP vs Fix vs Running fix
Author: Jared Sherman <jared.sherman@XXX.XXX>
Date: 2/7/02 6:36 PM
Chuck - and all-
<"The DR position is only an approximate position because it does not allow
for the effect of leeway, current, >
Is there anyone here who would "reckon" their position without attemping to
factor in ALL known effects? If one is plotting a DR position while crossing the
Gulf Stream, even without knowing the current, is there anyone who would not
indicate *some* offset as a best guess of the current, since they approximate
current direction and speed are almost certain to exist to some extent?
I ask this as a poll, not as an arguement. My understanding of DR, the good Mr.
Bowditch aside, was that a navigator placed EVERYTHING he could into the DR
position and that it was supposed to include all "guestimates" of where you
actually were, not just ignoring the ones you couldn't specify to five decimal
places.
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