Navigational Math-North and The Magnetic Compass

For those new to cruising as well as those making their first off-shore passage there is a surprise waiting on their navigational charts, suddenly the only Compass Rose on the chart is a True Compass Rose!  Now the user must make the necessary calculations to convert the True Course obtained to Magnetic and quite possibly adjust the course further to account for any deviation in the Ships Compass.

The calculations required here are pretty simple, but first let’s take a moment to review a little bit about “North” and our “Ships Compass”

In Navigation, whether it be on land or water, there are three “Norths”, True North located at the North Pole, Magnetic North located in Northern Canada (the exact location changes naturally by up to 15 km per year so tough to say exactly where it is at any given moment) and Compass North as read on our Magnetic ships compass.

Let’s address Compass North first; Our ships compasses being magnetic are subject to magnetic influences scattered about our boat and in some cases by local magnetic anomalies.  A “Compass Adjuster” can in a half hour or so eliminate most compass errors caused by a boats magnetic field.  Those errors that are left the Adjuster will write on a “Compass Compensation Card” so the ship’s navigator can allow for them when plotting a course to steer.

When stowing gear or installing electronics in the vicinity of your magnetic compass you must give some thought to what influence the item might have on your compass.  Generally, items stowed lower than the compass have less magnetic influence than those stowed at or near the same level as the compass.  For those boats with a “Bulkhead Mounted” compass items stowed on the bulkhead behind the compass can affect the compasses accuracy.  I have seen SS galley utensils stowed on a magnetic rack on the bulkhead right behind where the compass was mounted, needless to say that compass had a massive error.  Be careful of what and where you stow items in the vicinity of your ships compass! Whenever I am in doubt as to whether or not something will affect the compass I will hold it close to the compass and see if there is any movement.  If there is I am particularly careful where I stow that item.

True North is the North Pole and Magnetic North is the “North of the earth’s magnetic field”.  With the exception of Merchant and Military Vessels most everyone navigates using a compass that is magnetic.

On our Navigational Charts direction is obtained from a “Compass Rose” which on Coastal and Harbor Charts consists of two roses superimposed on top of each other with the outside Rose being True and the inside Rose being magnetic.

In the center of the two Roses you will find the Magnetic Variation for the area covered by that Rose.  On Charts covering a small area (large-scale chart) like Long Island Sound, Western Half, the Magnetic Variation maybe the same over the whole chart.  On charts that cover a very large area (small-scale chart) like Cape Hatteras to Cape Canaveral, the Compass Roses found in several locations on the chart may have a different Variation listed. In general, the further North of the equator and the further east or west you are of the Magnetic Pole the greater the Variation will be.  Example: Boston Massachusetts has a Variation of 15° West, St. Thomas USVI 6° West, Western Gulf of Mexico 2° East.

If all your voyages are along the coast with a recently “Swung” magnetic compass then knowing how to convert from True to Magnetic to Ships Compass is a skill you are not likely to need. You simply plot your course on the chart using Parallel Rules and the Magnetic Rose and you are on our way.

But, if you elect to use a Course Protractor that uses the lines of latitude and longitude as a base line or when you venture off over the horizon on an ocean passage suddenly you find the chart only contains one Rose and it is True, now you need to know how to convert from True to Magnetic to Ships Compass!  If your passage is long enough, say from Newport RI to St. Thomas you will need to make several corrections in the True course as the Variation changes dramatically from Newport with a variation of 15° to St. Thomas with a variation of 6°

Thus, our subject for today is how to convert True to Magnetic to Ships Compass, and along those same lines how to convert a Magnetic Bearing to a True Bearing so it can be plotted.

We will begin by taking a look at the following diagram;

When converting a True Course to a Magnetic or Compass Course you are moving down on the diagram thus you would add any West variation or deviation and Subtract East variation or deviation

When converting a Compass or Magnetic Bearing to True you would be moving up on the diagram thus you would subtract any West variation or deviation and Add any East variation or deviation.

Example; You have plotted a course of 180° True, the variation in the center of the Compass Rose is 12° West, your Compass Card lists 0° deviation at 180°, what is your course to steer?

180°T

+  012°W variation

= 192° Magnetic

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=  192° Compass and your course to steer.

Another Example; You have measured a course of 137° True with a variation of 2° East, at 135° your Compass Correction Card says you have an error of 1° West, what is our course to steer

137° T

–   002° East variation

=  135° Magnetic

+   001° West Deviation

=  134° Compass and your course to Steer

Now let’s say you use your handheld compass to obtain a bearing on a light house and you need to convert its magnetic bearing to true so you can plot it.  Because a handheld compass can be moved around away from magnetic influences (watch out for sunglasses with metal frames) they are considered to be free of deviation so we will go direct to the Magnetic portion of our diagram and work our way up to True.

The bearing you obtained is 096° magnetic, your compass rose shows a variation in the area of 8° East.  What is the true bearing to the lighthouse?

096° Magnetic

+  008° East Variation

=  104° True bearing to plot

Another example of converting from Magnetic to True would occur when one uses a Course Protractor that relies on the lines of Longitude and Latitude as the base line for measuring angle.

You are steering a course of 223° on the ships compass and need to plot your course for Dead Reckoning purposes.  The variation in the area is 4° West and your compass correction card says you have a 2° East error on the compass at 225°.  You are very close to steering 225° so you must account for the deviation in your calculations.

223° on Ships Compass

+  002° East deviation

=  225° Magnetic

–  004° West variation

= 221° True, course to plot

Once you have made the calculations a few times they will become pretty easy.  Please feel free to copy the diagram into your Ships Log for reference, it isn’t something I invented, in fact I suspect this diagram has been around and taught to young Midshipman for several centuries or more.

As I have said in the past and will repeatedly say in the future, “Knowledge of Traditional Navigational Skills is imperative to vessel and crew safety”.  Electronics can and do fail, knowing how to navigate with traditional skills will make you a safer and more reliable skipper!