To travel accurately and safely from point to point on Earths surface,
charts have been constructed to show the locations of most all prominent
places. Using these charts, a navigator can plan the voyages. By
drawing a line on the chart from one place to another, a navigator
establishes a line known as a course line, the purpose of which is simply
to provide a graphic representation of a vessels course. Careful
attention must be paid to ensure that there are no dangers to navigation,
such as rocks, reefs, islands, and so forth, along the route of intended
travel. From this line, the navigator determines the direction in which
the ship must sail to arrive at the desired location. By measuring the
distance between the two places and knowing the speed of the ship, the
navigator computes how long the voyage will take.
As defined in the terms table, course (C) is horizontal direction of travel,
expressed as angular distance from a reference direction, usually from
000° clockwise through 360°. For marine navigation, the term course
applies to the direction to be steered, which sometimes differs from the
direction you intend to make good over the ground. Course is most
often designated as true, but may also be designated as magnetic,
compass, or gyro.
Often while the ship is following the intended track, it will be necessary
to change course to avoid other ships or make adjustments for current
that sets the ship off the intended track.
Maintaining the DR plot is a matter of closely following the six rules of
DR. Lets look at an example of what is required to maintain a sample
plot. The example shown in figure 8-9 illustrates a typical DR plot. At
0900 your ship departs point A en route to point B on course 065°T,
speed 10 knots. In this particular example, DRs are laid out every 30
minutes; you expect to arrive at point B at 1200. At 0941 you change
course to avoid shipping traffic. At 1000 you obtain a fix which places
your ship right of your track line. Based on the 1000 fix, you
recommend course 075°T to arrive at point B on time.