Many boaters today prefer to use electronic navigation tools such as GPS rather than traditional position fixing techniques. While I use and appreciate GPS chartplotters, I do not rely on them exclusively. Being able to plot your position on a paper chart is a rudimentary skill all boaters should be able to perform. And having a paper chart as backup to electronics is prudent seamanship. Power outages, missing chart chips, improper settings and range scales as well as emergency situations do happen. So today, I’ll show you how to use cross-bearings from visible landmarks to determine your position.
In my earlier navigation blogs, How to Judge the Tide and Not Run Aground, and How to Identify Hazards on a Chart, I stated that a competent boater should also be able to fix their position on a chart. The easiest way to be able to fix your position on a chart is by using your compass: In addition to the paper chart, you’ll need some chart plotting tools: a hand bearing magnetic compass or, alternatively, a pair of binoculars with an internal compass, a pencil, parallel rules, and dividers, to chart your position.
With your paper chart in hand, look around. Once you are able to visibly identify fixed landmarks such as lighthouses, water towers, smoke stacks or even a prominent hill or point of land and then find their corresponding symbol and location on the chart, it is an easy process to use such landmarks to fix your position. Using a magnetic compass, take a bearing of the landmark. You are positioned somewhere along that bearing line (Line of position). If you take another bearing on a second landmark (preferably at nearly right angles), you will establish your position at the point where the two bearing lines intersect.
In order to use your compass accurately, there are a few things you should know about it. Saying that a compass points north isn’t exactly true. The magnet in a compass aligns itself with the magnetic field of the planet. This field flows between the earth’s two magnetic poles, the northern of which is the magnetic north pole. The north geographic pole is the axial pole centered at the top of the planet where the meridians of longitude converge. This is true north.
The north magnetic pole moves slowly over time due to magnetic changes in the earth’s core. In 2001, it was determined by the Geological Survey of Canada to lie in the Canadian arctic near Ellesmere Island about 700 miles from the true north pole. From most places on earth, true north and magnetic north aren’t in the same direction. This angular difference between the directions of the two poles is variation and is expressed in degrees.
On every paper chart you will find a compass rose. Notice how the compass rose on your chart reads from 0 (north) through 360 degrees. Always express a bearing or course in three digits, 005 degrees for instance. The outer circle of the rose points to true north and the inner circle points to magnetic north—again, the difference is variation. To transfer our readings of known landmarks from our magnetic hand bearing compass to the chart, we’ll use the corresponding magnetic inner circle on the compass rose which is adjusted for variation in our locale.
Say we have taken two bearings with our compass: one on a water tower that bears 055 degrees from our position, and another from a lighthouse that bears 175 degrees from our position. Lay your parallel rule on the compass rose, with one edge on 055 of the inner circle and the very center of the compass rose. Maintaining this exact angle, slide one side of the parallel rule to move this angle over the symbol for the water tower on the chart, draw a line through the symbol, and extend it for some distance. This creates your line of position; just how far from the water tower you are on this line will be determined by plotting the second bearing of the lighthouse in a similar manner.
Again, for the lighthouse bearing of 175 degrees, lay the rule through the center of the compass rose and align it with 175 degrees on the inner circle, slide the rule over the lighthouse, and draw your second Line of Position (LOP). Where the two LOP’s intersect is where you are. You should be able to identify the exact depth of the water and relative position of hazards from the previous blog exercises.
Two or more cross bearings is an easy and precise way to plot your position. There are other more advanced ways to fix your position while underway using multiple timed bearings of a single landmark. It has taken me many years of practical experience and training as a professional mariner to understand and be able to use all the information presented on our modern paper charts. So for those of you who would like to know more, I recommend, How to Read a Nautical Chart by Nigel Calder. Online, a good resource is Starpath’s chart trainer. Safe boating!