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Taking Magnetic Bearings

This article was originally published on SailNet in September 2000.

Step one—finding your target and and getting a bearing. The handheld compass is a great instrument for this step.
Since the primary source of lines of position is visual, you need to know the correct procedures for taking and plotting them. In previous articles on LOPs, I mentioned taking LOPs and how to change them from magnetic to true prior to plotting, but I really didn't get into the best ways and methods of taking magnetic bearings.

Unless you are using a gyrocompass, all visual bearings will be made with some sort of magnetic compass. It may be the ship's compass, a handheld compass, a pair of binoculars with a built-in compass, or something like the KVH Datascope® that gives a readout of magnetic bearing, range, and time. A pelorus will give relative bearings that must be converted to true using the ship's compass. Whatever the source of your bearings, there are several methods and techniques that will give you the best results.

Basic Considerations    The most important aspect of taking observations is the proper identification of the aid. It will do you little good to make accurate observations on the wrong aid. Of course this always brings up the argument, "If you can positively identify the aid, then you know where you are and there is no need to take bearings and plot them." Well, not exactly. You may have a good idea of your approximate location, but for accurate navigation, you need to have an exact location plotted. A good DR position will alert you to the relative position and locations of the various aids to navigation in your area and a pair of binoculars will help to identify them. At night, use the light's characteristics to help identify the aid.

Once you have selected two or more aids to navigation to use, you can begin taking some bearings. However, there are several caveats worth considering beforehand. The position on the chart for a floating aid to navigation is only approximate and you should not take all your observations on them. It is best to mix them up with bearings on fixed aids to navigation and prominent landmarks. It is also well to remember that observations taken on nearby objects will be subject to smaller errors. Any angular errors will be the same no matter what the distance, but the farther the object the greater the linear inaccuracy will be as the LOP is extended a greater distance.

For example, a one-degree error will be off by one nautical mile for a 60-mile plot, but only one fourth of a nm for a 15-mile plot. You can easily see that a four-degree error will result in an LOP that is a mile off, if the object is 15 miles away. A handy rule of thumb is that an angular error of one degree extended to one mile will give about 100 feet of error, and at two miles the error will double. For this reason it is vital to take accurate bearings and not to make any math errors in the conversion from magnetic to true. As a matter of course you should take three bearings on each object and average them. Choose three objects that are about 120 degrees apart. This will minimize any consistent errors. Another good rule of thumb is to take bearings on objects that are either ahead, slightly ahead, aft, and slightly aft of the boat first and take the beam bearings last.

By using a binnacle-mounted ship's compass, the person on the helm can easily call out each bearing.

Using the Ship's Compass    OK, now we are ready to start taking some bearings. To use the ship's compass, you can either steer the boat until it is pointing at the object and take the compass reading, or you can sight across the compass by lining up the object with the lubber line and the pivot post of the compass. When the object is slightly ahead, any steering maneuvers to line up the object will not affect your DR plot. However, you must take into account your location and any nearby traffic to safely make the off-course swing. After taking the bearings, steer back to the desired course as quickly as possible. Obviously, beam bearings and bearings abaft the beam should not be taken by pointing the boat at the object.

If you point the boat at an object, record the deviation for that compass heading to use when converting the bearing from magnetic to true. If you sight across the compass, use the deviation for the compass course steered. In many cases the difference in deviation can be several degrees or more, and that much error on a distant light at night could be serious.

Using a Hand-Bearing Compass    A handheld compass, binocular compass, or range finder can be used at any time to take bearings without affecting the course steered. Select a place on the vessel that is free of magnetic influences, clear of obstructions, and fairly secure. Remember that any metal, such as in eyeglass frames, buttons or grommets in your hat, flashlights, and batteries, can affect the compass. Once you are free of any magnetic disturbances, bearing compasses need not be corrected for deviation, but only for variation. Once again take three or more bearings on each object and average them. It is best to use an assistant to record the bearings and the time of each observation.

Using a Pelorus    The pelorus can be adjusted to the boat's compass and sights can be made directly as compass bearings. However, it is best to set the pelorus to zero degrees dead ahead and use it to make relative bearings. Relative bearings are converted to true by adding the relative bearing from the pelorus to the boat's true heading. The boat's true heading is calculated by adjusting the compass heading for deviation and applying variation. Always use the deviation for the boat's compass heading and not the pelorus bearing. Use an assistant to record all the information. When you yell "Mark!" the helmsman will note the compass heading and pass it to the assistant. The assistant will also record the time and the pelorus bearing you took at the mark.

Plotting your bearings is always a critical step in the navigating process.

Plotting Bearings    The direction measured for all bearings is from the boat toward the object, but the plot of the LOP is always from the object toward the location of the boat. In other words, it will always be plotted in the reciprocal direction of the true bearing. You can align the plotter by inserting one point of the dividers into the object on the chart and aligning the plotter with the true direction of the bearing by using the nearest line of longitude. On a Mercator chart you can also use a latitude line and the plotter's inner scale. Draw the LOP on the chart in the reciprocal direction of the bearing, which will be toward the DR position. If this is confusing to you, then add or subtract 180 degrees from the true bearing and plot this direction from the object's location on the chart. Label the LOP with the time on top of the LOP and the magnetic bearing on the bottom.

Use True and Avoid Magnetic Plotting    It is possible to convert compass bearings to magnetic by applying deviation and to plot LOPs directly from the compass rose. This is not recommended because it would require that all directions be plotted as magnetic, i.e., courses, ranges, currents, celestial LOPs, etc. Plotting using true is the traditional method and continues to be taught and used by all marine institutions and professional mariners. Plotting magnetic is not wrong and if it works for you, just remember to label your LOP directions with the uppercase M after the direction to show that you used magnetic versus true.

Using Ranges for LOPs    If the conversions from magnetic to true are too much of a hassle for you, then consider using ranges for LOPs. They are free of all magnetic effects and are easier to take and plot than all the other bearings previously discussed. You can use the aids to navigation specifically designed to be used as a range when in a narrow channel, or you can use any two identifiable objects that can be aligned. Simply note the time when the two objects are in alignment. Then plot the LOP by lining up the symbols with a plotter and drawing the LOP in the direction of the boat's location. Repeat this procedure one more time on a different range and advance the older LOP to the time of the last LOP for a quick and easy two LOP fix.

Remember that it is your responsibility as captain to know your vessel's location at all times. When coastal cruising, taking and plotting visual bearings to verify your DR position will fulfill that obligation. To ignore your navigation responsibilities can put the safety of your boat and crew at risk. Once you know the theory of navigation, putting it to work for you only requires some diligent practice.

Light Lists, Coast Pilots, and Sailing Directions

By their nature, nautical charts only have room to present a limited amount of information. To ensure correct identification of an aid to navigation, prudent mariners often equip themselves with Light Lists (published by the US Coast Guard), Coast Pilots (published by the National Ocean Service), or Sailing Directions to Non-US Waters (from the Defense Mapping Agency) as supplemental resources.

As their name implies, Light Lists contain information on lights in a given area (seven volumes are published for seven US regions), but they also provide information on sound signals, buoys, day beacons, RAYCONS, and radio beacons.

Coast Pilots and Sailing Directions contain a vast array of information on the routes between ports for their respective areas. You’ll find descriptions of channels with information on controlling depths along with notes regarding particular hazards or obstructions. Remember, neither of these publications are meant to replace good dead reckoning, but they can assist you in confirming your navigational findings, particularly in waters that are unfamiliar.

Suggested Reading:

Understanding and Using the Magnetic Compass by Jim Sexton

Lines of Position, Bearings, and Fixes by Jim Sexton

Compass Rose Navigation by Jim Sexton

SailNet Store Section: Binnacle Mount Compasses

Last edited by administrator; 02-05-2008 at 01:49 PM.
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