A line of position is a navigational line drawn on a chart to a known object using a bearing from a compass. Plotting a line of position from your boat while you are travelling along the coast allows you to fix your position, or in other words determine where you are. A single line of position lets you know that your boat is located somewhere along this line. By plotting multiple lines of position, you can pinpoint your exact location. While GPS have made offshore and coastal navigation much easier, it is still important to learn basic plotting skills. Learning how to plot a line of position will allow you to navigate if your vessel loses power, or if you simply want to navigate without the use of a GPS.
To determine your location, you can plot a line of position using several tools: you can use a compass to take a bearing off landmarks that appear on your chart, you can use radar to determine a line of position or a range to a landmark, or you can use your depth finder and plot a fathom curve. The most basic way to create a line of position is to use a compass.
Compass - preferably a handbearing compass, but the ship's compass will work as long as you can see your landmarks from the location of the compass.
Chart - For plotting a line of position, choose the largest scale chart that has all your landmarks on it.
Parallel Ruler or two Triangles - You can use either of these plotting tools based on your own personal preference.
To plot a line of position, locate a landmark that you can identify and that also appears on your chart. The best objects to use are narrow (i.e. radio tower) and aren't too far away. If the object is too far away, you may not get an accurate bearing and if it is too close, the bearing may change too rapidly to get a bearing. It is much better to use completely stationary objects that are on land, but if you must use a buoy, be aware that the buoy may not be located exactly where it is drawn on the chart and you should use caution when relying solely on this bearing for navigation.
Once you've located your object and identified it on the chart, it is time to take a sight. If you are using a handbearing compass, hold the compass at arm's length while pointing it directly at the object. Use the sighting line on the compass to line up the object and take a sight. When using a handbearing compass, take the sight off the numbers closest to you. If you are using the ships compass, bring your eyes level with the compass and look across the compass to the object. Take your sight by marking down the numbers on the far side of the compass. On a piece of paper record the compass bearing and the time.
Now you must convert your compass bearing to a true bearing. To do this you must account for deviation (differences in your personal compass) and variation (the difference between magnetic and true north). Unless you know the deviation of your ship's compass, you can ignore deviation since it is usually minimal. To account for variation look at the variation in compass rose located on your chart (if you chart is very old, you will need to do extra calculations since in some locations variation changes significantly over time). If the variation is East you are going to add the variation to your bearing, and if the variation in your location is West you are going to subtract the variation from your bearing. For example: if your original compass bearing was 355 Â° and the variation is 10 Â° E, you would add 10 Â° to 355 Â°. Since there are only 360 Â° in a compass, your true bearing is 5 Â°. Record your true bearing on the piece of paper. You can also skip this entire step by simply using the smaller compass in the compass rose that is based on magnetic north when you are plotting your bearing, but it is good practice to be able to convert a compass bearing to a true bearing.
The final step is to plot the true bearing on the chart. Assuming you are using the true bearing, line your parallel ruler or triangle up in the middle of the compass rose with one edge connecting the center of the compass rose with the true bearing.
Since you are using a true bearing, use the outside circle of the compass rose. If you are using a parallel ruler walk the ruler across the chart until it lines up with the object you used for your sight. If you are using triangles, you can line up the triangles and slide them until you have the edge lined up with the original object. Take your pencil and draw a line along the edge of your plotting tool.
It is very important to label your line with the time and the bearing. This is your line of position and your boat was located somewhere along this line at the time you took your compass reading. In order to determine your exact position on this line you will need to plot an additional line of position to create a fix. Ideally a fix has 3 lines of position, but this is not always possible. If you only have one object to use for your sight, you can create a running fix by taking multiple sights off the same object at different times and advancing your earlier lines of position based on your ships speed and direction.
When taking a sight be aware that metal onboard your boat can negatively affect your compass reading. If possible, take the sight away from the engine or other large metal object aboard your boat.
Be certain that the object you identify is the object you see on your chart. It is not helpful to put a line of position on a different object and it can be dangerous to assume that you have determined you location. For this reason it is helpful to have multiple lines of position.