Section 5

Plotting Basics



5.3

Dead Reckoning and Fixes

Lesson 5.1 covered how to plot a course and 5.2 explained the use of the distance, speed and time formulas. This lesson builds on those two skills and demonstrates how to plot a dead reckoning or DR course.

It's uncertain where the term "dead reckoning" came from, but it's probably from "deduced" reckoning. "Dead" doesn't refer to the fate of sailors who don't practice it, but it could. Boats and mariners have been lost from failure to maintain an accurate reckoning of course and position.

Plotting a DR Course

Navigating by dead reckoning involves calculating a new position or positions from a known departure point using estimates of speed, course and elapsed time. The DR course is plotted on the chart with a series of DR positions and may indicate the intended path of travel. Sometimes it is a planning tool only, as the known path of travel will be elsewhere. This occurs in waters with currents. This subject will be covered in Section 8.

A DR course should be plotted, in advance, or from the moment a boat leaves harbour or weighs anchor, until safe arrival at its destination. The plotted course allows the navigator to avoid hazards and plan and monitor an efficient route. The DR positions along the route at regular intervals provide approximations of the boat's position relative to a known position. In an emergency a DR position may serve as the best guess of the boat's real position.

Starting With a Fix

A navigator's prime concern is to know the boat's position at all times. Sometimes there's no doubt where it is: secure in a known anchorage, for example. Close aboard a charted buoy the navigator can be almost absolutely certain of the boat's position. However, in open water the navigator should be wary. Is the chart up to date and is the buoy in the right place and correctly identified?

Highly reliable positions can be obtained by the Global Positioning System (GPS) or other satellite or electronic positioning systems.

A fix is a position of which the navigator is highly confident. The act of establishing a fix is called "getting a fix" or "fixing a position". Sections 6 and 7 look at methods of fixing a position.

Important: A dead reckoning (DR) course always begins at a fix.

Continuous Plotting

A DR course can be plotted in advance, say the night before, but chart work on the water should go on while under way. As navigator, you should constantly correct and update the plotted course by extending it, drawing and labeling changes in direction or speed, taking and plotting fixes to determine position, and comparing DR positions with fixes to discover if the boat is off course.

Errors and Accuracy

If a skipper steers a course due south at a speed of five knots, after one hour the boat's position by dead reckoning should be five nautical miles south of where it started. Actually, it's unlikely that it will be there. It'll probably be close to where it should be, but steering errors, an out-of-date deviation table, leeway, drifting due to current, and other factors can put the boat off course.

So why bother plotting a DR course at all? There are two reasons; first, it's better to have an inaccurate idea of where you are than to have no idea at all; and second, it's only by starting with a DR course that you can determine how much the boat has been nudged off course, at what rate and in which direction. You can then figure out what could be causing these errors and take action to correct them.

Dead Reckoning Today

Navigating by DR has been used since the time of Columbus and it's still the fundamental basis of course planning today. Even high-tech navigational aids such as electronic plotters, radar and GPS can't replace dead reckoning. DR plotting isn't just a backup in case electronic instruments fail, although that would be reason enough to use it. With a plotted representation of a boat's course on the water, a navigator can see what lies ahead on the chart. The unwary boater who has neglected to plot his intended path and position would have no warning of impending dangers.

DR Principles

Here are five golden rules for plotting a DR course:

 
1.   Start from a fix, that is, a known departure point, and note the time.
2.   Label the DR course in TRUE degrees*.
 3.   Use speed through the water (from a knotmeter) and the
60D=ST formula to calculate distance.
 4.   Plot DR positions on the hour every hour**, and whenever there's a change in course or speed.
 5.   Follow labeling conventions.

Sail Canada requires that courses and bearings be labeled in true degrees. Note that the course plotter is designed to plot in true degrees.
** In inland or coastal waters it's common practice to plot a sailboat's DR position at least every hour and a powerboat's every half hour or more often for higher speeds.

Labeling Conventions

Sailing and power boating schools world wide, and coast guard, military and merchant marine institutions teach similar plotting and labeling conventions. These conventions are tried and proven. With the same systems widely taught, all navigators speak the same navigational language. They can understand each other's log books and chart notations, and readily take over from one another at the helm.

A fix is plotted as a small circle with a dot in its centre. It's labeled with the time, using the 24-hour notation, printed parallel to the bottom of the chart (to show that it's known with certainty).

The course is placed above the plotted line and labeled with a C, followed by a space and the three-digit heading. If a magnetic or compass course is plotted instead of true — not recommended, as you know by now — it should be indicated with an M or a C following the course.

Speed is placed below the line under the course, labeled with an S, and noted to tenths of a knot.

DR positions are marked on the plotted course line. The symbol is a small semi-circle above the line with a small dot or short stroke at its centre. The DR position is labeled with the time or estimated time of arrival using the 24-hour notation. The time is entered at an angle to the bottom of the chart (to show that it's an estimate).

Classic Versus Sail Canada Conventions

Looking back on the history of plotting, it's clear that a fundamental principle has been to keep plotting and labeling simple and uncluttered. There are good reasons for adhering to these principles and a system of clean, clear plotting and labeling:

conventions are easy to learn and memorize;
it's quick to draw and label;
it's easy to read and understand; and
it's logical and intuitive.

In recent years, and in some circles, there has been a move away from clear and simple plotting to more complex methods and conventions. The reasons for this are vague and weakly reasoned.

Those who have learned and used classic plotting — a system that has served leisure and professional mariners for centuries — might find modern plotting cumbersome, illogical, time consuming, and of no benefit to their tried-and-true practice.

Sail Canada has been caught up in the move away from classic plotting. It's new Navigation Standards now includes a nine-page document illustrating the association's "Uniform Navigation Symbols and Terms".

Students who intend to write the Sail Canada Basic Coastal Navigation exam, will need to use the Sail Canada plotting symbols.

Download the Sail Canada Uniform Navigation Symbols and Terms .pdf document.

This course teaches both the classic and Sail Canada's methods of plotting and labeling. This enables the student to learn and eventually put into use the method of their choice. Throughout the material you'll find references to descriptions, illustrations, and comparisons between the two systems.

See Sail Canada plotting and labeling of a Course, and Dead Reckoning Position

 

Plotting a DR Course using the QuickPlotter and Dividers

View an interactive movie illustrating how to use the QuickPlotter and dividers to plot a DR course. Play Movie
Flash Player 6.0 or newer is required. If you can't see the movie, you'll need to install the latest version of Flash Player.
Download it for free and follow the installation instructions.
 

 

 


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