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Author
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Topic: COG vs CMG
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David Burch
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posted March 10, 2004 10:44 PM
What is the difference between Course over ground (COG) and Course made good (CMG). The answer is in our coastal nav glossary, but that it not online yet till the full course is running.
In the meantime:
in a sense these are the same thing. in the days before loran or gps was popular they were use pretty much the same way,and nearly always early on called just CMG.
these days, we can make a useful distinction between the two and the electronics companies have pretty much come to agreement on the meanings.
the key point is COG is thought of as an instantenous value, whereas CMG is the combined net course after some period of time and maybe several course changes.
if i sail with a cog of due E for 2 hours and then sail with a cog due S for 2 hours, my cmg after 4 hours will be due SE.
needless to say, one could use the terms interchangably and not cause confusion in most cases, but there is a value in the distinction.
"my heading is 050 right now and i have a course over ground of 060, but my course made good over the past hour has been 055" etc.
note that the traverse tables in bowditch is how you would take a series of course legs and convert them numerically to a final course and distance made good.... the other option, of course, is to plot them out then draw a line from start to finish.
From: Starpath, Seattle, WA
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