74 seamanship secrets
- Use standard international symbols. Use one of three symbols for any
plotted position (see illustration).
Dead reckoning (DR)—dot, half circle, and time.
Estimated position (EP)—dot, square, and time. Estimated positions
upgrade the reliability of a DR plot. Simply take a bearing and plot a DR
position for the same time. Th e EP lies at the point along the bearing that
is closest to the DR position.
Known/fi xed position—dot, circle, and time. Your GPS provides a fi x, or
known position. Alternatively, if you are sitting next to a buoy or light, you
know beyond a shadow of a doubt where you are, and that is your fi x. More
broadly speaking, a fi x results from plotting two or more bearing lines
(see the sidebar on page 73) and is thus more accurate and reliable than an
estimated position. (See “Secrets of the Most Accurate LOP on Earth” later
in this chapter to learn why ranges yield the most accurate fi xes of all.)
THREE GOLDEN RULES FOR SUCCESSFUL
DR PLOTTING
- Plot a DR at your chosen time interval. If you maintain your DR plot
faithfully, you’ll have a rough idea of your location in the event your
navigation electronics go off -line. - Plot a DR at the same time you take a bearing or range. If you take
a bearing at 0912, plot a DR for 0912. You must do this in order to
determine the impact of the current on your boat. In Chapter 5 we’ll
Use the standard international symbols for plotting on your charts. Th is includes a solid
line for your TR (as well as course steered); a dot, half circle, and time (in 24-hour time)
for a DR position; a dot, square, and time for an estimated position (EP); and a dot, circle,
and time for a fi x, or known position.