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Comparing the lines from points A and B and from points A
1
and B
1
you see
that while the boat travelled 34 NM it only moved 30 NM along the courseline.
If he moves 30 NM on the courseline in 1 hour, his SOA is 30 knots, whereas
SOG would be 34 knots.
SOG. Speed over ground is the speed at which the vehicle/vessel is moving in
respect to the earth. SOG is sometimes referred to as ground speed and is
measured in knots, miles per hour, and kilometers per hour. (This is not the
same as speed through water.)
STR. The difference between COG and BRG. If BRG is180° and COG is 183°,
then STR is 3° Left.
TTG. Time to go to the active waypoint.
VMG. Velocity made good is the portion of the velocity that is in the direction
of the destination. In the above example, the boat is travelling directly to-
wards the buoy even though not on the courseline originally set. In this case,
all of this velocity is being applied towards arriving at the destination, (i.e.,
VMG = SOG). Should the boat veer away from this course and travel in a line
parallel to the original courseline (A
1
;B
1
) he would be travelling on a course
that would not intercept the buoy waypoint. As the boat moved along, less
and less of his velocity would be applied toward arriving at the buoy. By the
time point D is reached, none of the velocity is applied to getting closer to the
buoy and the VMG would be 0. As he moves to point E, he is moving away
from the buoy and his VMG is a negative value.
A
Courseline
B
C
D
E
XTE. XTE (cross track error) is the distance to the left or right of the courseline
that you are travelling. As you will see in the discussion of other data items,
XTE is important in computing them accurately.
Keeping XTE at a minimum will help
maintain the most direct route to your
destination. This illustration shows a
boat and the courseline. The arrows
between the boat and the courseline
is the distance of XTE that this boat is
to the left of the courseline. This boater
would need to steer right to close the
XTE.