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post #1 of Old 01-04-2002 Thread Starter
Jim Sexton
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Navigation Basics

This article was originally published on SailNet in November, 1999.

Along with the Rules of the Road, every mariner should be very familiar with the principles of navigation.

For the safety of your boat and crew, there are two areas that every mariner needs to know cold. One is the Rules of the Road and the other is navigation.

The word navigator comes from two Latin words, "navis," meaning ship and "agere" meaning to direct or to move. Navigation is defined as the process of directing the movement of a craft from one place to another. The craft may be, in its broadest sense, any object requiring direction or capable of being directed. Navigation of a watercraft is called marine navigation. It is also important to understand that marine navigation methods and techniques will vary with the type of vessel, the prevailing conditions, available equipment, and the navigator's experience. There is a world of difference between navigating a pleasure craft and an oil tanker, even though you may use the same type of equipment and navigation techniques. An experienced professional navigator will navigate a pleasure craft in an easy and offhand manner that belies the complexity of the techniques involved.

Basic to the study of navigation is an understanding of certain terms that could be called the dimensions of navigation. These so called dimensions of position, direction, distance, and time are basic references used by all navigators. A clear understanding of these dimensions, as they relate to navigation, is necessary to provide a means of expressing and accomplishing the practical aspects of navigation.

Hand-bearing compasses like the one shown here are a useful tool for finding a relative position.

Position    The exact location of a boat or object is a point defined by stated or implied coordinates. Though frequently qualified by such adjectives as "dead reckoning," "estimated," "GPS," and "Radar," the word position always refers to some place that can be identified. It is obvious that the navigator must know the ship's position before being able to direct the vessel to another position or in another direction. The ship's position can be expressed in coordinates of latitude and longitude or Loran TD's. The position may also be expressed as being "close abeam" a particular aid to navigation or so many miles in a particular direction from any geographic location. For example, "two miles due south of Point Lookout."

Direction    The position of one point relative to another, without reference to the distance between them, for example, "due south," is termed direction. Direction is not in itself an angle, but it is often measured in terms of its angular distance from a referenced direction, such as true north or magnetic north. For example, a course of 135 degrees true.

Distance    The spatial separation between two points is what we call distance, and this is measured by the length of a line joining them. On a plane surface this is a fairly straightforward and simple measurement. On a sphere the separation between two points may be expressed as a variety of curved lines, i.e. great circle and usually requires a segmented measurement. The length of the line can be expressed in various units, such as, nautical miles, statue miles, yards, or feet.

Time is an important concept for almost any form of navigation, but particularly so for celestial navigation.

Time    This concept is defined in many ways, but those definitions used in navigation consist primarily of either the hour of the day in reference to a particular line of longitude (i.e. GMT, LMT, and EST) or as an elapsed interval between events.

The principal divisions of navigation are dead reckoning, pilotage, electronic, and celestial. Within these divisions navigation can often be further broken down into sub-divisions such as radar navigation within electronics. There are also certain areas of knowledge required in order to fully understand a navigation division, such as having a basic understanding of tides and currents when determining an estimated position. Each of these types of navigation will be covered thoroughly in future articles.

Dead Reckoning (DR)    The determination of position by advancing a known position along the vessel's steered course (C) for a calculated distance (D) is what sailors call DR. This distance is based upon the speed of the vessel through the water and the elapsed time since departing a known position. An Estimated Position (EP) is determined by applying the set and drift of the current to the DR position or by using the vessel's Speed and Course over the Ground (SOG & COG) from the GPS to calculate the EP directly from the last known position. Dead reckoning is the basis of all navigation and should be carried on your paper chart at all times as a safety backup to pilotage and electronic navigation. A good DR is absolutely essential for celestial navigation.

Piloting    Also known as pilotage, piloting is navigation involving frequent or continuous accurate determination of the vessel's position, or a Line of Position (LOP) relative to geographic points, using visual or electronic means. Pilotage is practiced near land and aids navigation. It requires good judgement, constant attention, and alertness by the navigator, especially when near hazardous areas.

Handheld GPS units like the one in use above are one of the most popular forms of electronic navigation, but savvy mariner repeatedly warn against using them as your sole means of navigation.

Electronic Navigation    
Sailors who use electronic equipment in any way to navigate(including radar, Loran, depth sounders, and GPS), are engaging in electronic navigation. The latest means of electronic navigation is the use of a computer connected to the GPS, or a stand-alone plotter, to display the boat's position on an electronic chart. This type of equipment has been covered in detail in earlier articles and in future articles I'll cover using it for navigation as it overlaps pilotage to a considerable degree.

Celestial Navigation    Using information obtained from celestial bodies to determine one's position at sea (a process that relies considerably on DR techniques) is known as celestial navigation. Today, many mariners consider celestial navigation to be archaic now that GPS is available. However, celestial is the only way to evaluate the reliability of the GPS position and is a necessary backup to the GPS if your power or electronics should fail at sea.

These divisions are considered separately here for the purpose of instruction; however, they are so intertwined in actual practice that it's often difficult to separate them. For example, you'd use DR to get an ETA for a sharp change of depth contour; then use the depth sounder to get an LOP as you cross that depth; take a visual bearing and a radar range to confirm your DR position; and note the time and mark your position on the chart. Finally, you'd compare this position with your computerized charting program or GPS plotter to confirm its reliability. Now that's what I call navigation. Practicing these techniques on the sunny days will pay dividends on the foggy ones.

Remember that the science of navigation can be taught, but the art of navigation must be developed from experience.

Suggested Reading:

General Navigation Techniques by Jim Sexton

Understanding and Using the Magnetic Compass by Jim Sexton

Nautical Publications by Jim Sexton

Buying Guide: Binoculars

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