Terms Associated with the DR Plot
Use the following table to identify and learn the meanings of terms
associated with DR:
The ships heading is always expressed in degrees
measured clockwise from 000° through 360°. Commonly
referred to as the ships head, the heading can be
referenced from true north, magnetic north, or compass.
The ships head is always changing due to the constant
yawing motion caused by the effects of the sea and
The course is the direction on which the ship is to be
steered. As an example, the helmsman is ordered to come
left steer new course 090° T . The helmsman would
respond by putting the rudder left and steadying the ship
on new course 090°T.
The course line is the graphical representation of the
course that is being steered laid on to the chart. Looking
back at our example, lets assume the original course was
094°T. The chart would have had a 094°T course line
laid on it. When the helm was ordered to steer 090°T, a
new course line of 090°T would be laid on the chart.
This is the ships ordered speed. For example, lets
assume that ordered speed is 12 knots. For purposes of
DR, we assume that the ship will travel 12 nautical miles
in 1 hour
This position is determined by laying out the ships course
(course line) and speed on the chart. A DR position does
not take into account any current that may speed or slow
This is a best guess position using available information.
In practical usage, it starts with the DR position and adds
other data such as the estimated speed and set of the
This position is established at a specific time that is
believed of high accuracy. With the recent addition of
Global Positioning System (GPS) WRN-6 satellite fix
data, it is now possible to obtain a highly accurate fix 24
hours a day.