NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: DR vs EP vs MPP vs Fix vs Running fix
From: Chuck Griffiths
Date: 2002 Feb 8, 10:42 -0500
From: Chuck Griffiths
Date: 2002 Feb 8, 10:42 -0500
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 ShermanDate: 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. ********************************************************************** This e-mail and any files transmitted with it are confidential and may be legally privileged or otherwise exempt from disclosure under applicable law. This e-mail and its files are intended solely for the individual or entity to whom they are addressed and their content is the property of Smiths Aerospace. If you are not the intended recipient, please do not read, copy, use or disclose this communication. If you have received this e-mail in error please notify the e-mail administrator at postmaster@si.com and then delete this e-mail, its files and any copies. This footnote also confirms that this e-mail message has been scanned for the presence of known computer viruses. ***********************************************************************