By Alberto de Capitani

Of all the navigation tools you have on board the magnetic compass is the most basic and , in many respects, is still the most useful.
Mechanically simple, self–contained, comparatively rugged, the compass can be a highly reliable instrument. After all, with nothing more than a compass and a primitive version of the sextant, Columbus managed to discover America and Magellan found his way around the globe.
But compasses can be tricky. For one thing, the very simplicity of their design implies that they cannot distinguish between the various sources of the magnetic field that surrounds your boat. Put a flashlight (or a walkman, or a cellular phone, or a screwdriver, the list can go on and on) too close to your compass and the compass will merrily tell you that North is West or vice versa. Even if you are super careful and avoid placing magnetic objects in the vicinity of the compass, a boat –even a fiberglass boat is full of things that can deflect the compass needle in the wrong direction: for instance, your engine, various components of your rigging, your electrical or electronic gear, even the relatively weak currents that flow through your boats electrical wiring. And then you have the variable difference, due to irregularities in the earth magnetic field, between the magnetic North and the true North ( known as the variation or, as they prefer to call it in Italy, the magnetic declination). The results can be quite unattractive: a five degree of error in your course produces a cross track error of five miles for every 60 miles of distance. You may miss your next island or worse.
For all these reasons, it is prudent (as well as legally compulsory, in some countries) to have your compass regularly checked and compensated by a professional adjuster. The adjuster will move the tiny magnets that are built in your compass (or add new ones) in such a way as to offset the magnetic influence of the main structural elements of you boat and will leave with you a table showing the residual deviations for every 15 degrees or so of heading. Armed with the deviation table and knowing the local variation (which is shown in the nautical chart), the process of translating true headings to magnetic headings and then to compass headings is pretty straightforward: add West variation (or subtract East variation) to the true course and you obtain the magnetic heading; add West deviation (or subtract East deviation) to the magnetic heading and you obtain compass heading. If you want to move from compass headings back to true headings, you simply subtract (instead of adding) West variation/deviation and add East variation/deviation. To remember all this, there is a jingle : True Virtue Makes Dull Company– Add Whiskey.
The problem with the above is that deviation tables are inherently perishable commodities. Doing structural work on the boat,adding new electronic gear, replacing electrical wiring, even keeping your boat in the same position for a long time – all this can significantly change the boat magnetic field and thus render your deviation table obsolete, with obvious risks for your navigation. It is therefore good practice to check your deviation table from time to time – I would say, at least once a year. The traditional approach is to choose a range or a distant object whose magnetic bearing can be derived from the chart, run your boat for at least one minute on magnetic bearings 15 degrees apart 000, 015, 030, etc. through 360, and then write down for each heading the difference between the compass bearing of the range or object and its magnetic bearing as obtained from the chart. Alternatively, one can use the calculated azimuths of the sun for the time when compass bearings of the sun are taken on various courses, and then compare the two sets of values. The difficulty here is that you need to have a pelorus (plus an azimuth table, if you use the second procedure).
Slightly less accurate but quicker approach is to use your GPS. You need to do the exercise on a calm day, with little wind or current, so that leeway is minimal. Choose any waypoint and then steady your boat on course–over–ground (COG) of 000, 015, 030, etc. through 360.
For each heading, add West variation (or subtract East variation) to the course over–ground and write down the magnetic heading thus obtained. Calculate the difference between the magnetic heading and the heading as shown by your compass. If the compass reading is less than the magnetic heading, the deviation is East; if the compass reading is greater, the deviation is West (Compass least, deviation East; compass best, deviation West). At the end of the procedure - which may take half an hour or less– you will have a table with a first approximation of the degrees of deviation (East or West) for each magnetic heading. Transfer these data to a simple diagram, showing courses on the horizontal axis and deviations on the vertical axis (say, West deviations above the axis and East deviations below it). The points on the diagram should roughly identify a snake–like curve crossing the horizontal axis at least once. If there are points significantly off the curve, check them out: chances are that they are the result of observation errors. If the curve does not cross the horizontal axis (i.e. is entirely on the upper or lower quadrant), the likelihood is that the compass lubber's line is not perfectly aligned with the boat's fore and aft line: adjusting the position of the compass on the binnacle would then be necessary. Once you have drawn (interpolated) a smooth curve through the points on the diagram, you can estimate the deviation for any heading. Compare the data thus obtained with the deviation table prepared by your adjuster. If the differences between your table and his table are significant, it may be time to give him a call.
The moral of the story is this: even if your GPS breaks down, by knowing your local variation and having an up–to–date deviation table, you can safely go a long way, guided by your old faithful, the magnetic compass. Just remember to keep your cellular phone out of the way....