This is your intended route
, from A to B
COG - Course Over Ground
Your actual course over the ground affective at that moment in time (or at least based on a calcuation by the GPS
CMG - Course Made Good
This is your actual course between A and B, of if you didn't do accurately counter the set and drift, A to C...
So for instance, if you travelled along a courseline of 090 out of a bay and were steering
of a heading of say 090 Compass
, with your GPS
showing COG 092. This 2 degrees is probably due to you not applying the Variation and Deviation to your compass
heading, and also could be due to the enivronmental conditions (Wind, Set, Drift).
So you then exit the bay into the open ocean, still steering
090, but your COG now shows 099. Your now exposed to more environmental conditions since you have left the lee of the land.
Eventually because you haven't applied any corrections to your heading, you have a CMG of lets say 95 degrees, which is a combination of the COG from before you left the lee of the land, and then after you left the lee of the land.
COG 1 092
COG 2 099
In general, when you are in the deep blue, you'll find that your CMG and COG over the period of an hour will be fairly similar. As you pass over large waves or swell, your COG and SOG may rapidly change for a moment, but then it will stabilise again afterwards.