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who uses a compass?

So who does use a compass? Serious question. A compass is not the vital piece of kit it used to be. I am not suggesting that you should put to sea without one but it is quite possible to sail round all day without actually using it. I often use the compass just to hold a course. Without using any calculations or taking a bearing off the chart, I find it useful to keep me heading in the same direction. I mentioned this to my colleague Andy and he promptly pointed out that I was not holding a course at all but just a heading. Depending on stuff like wind and currents my actual course could be wandering all over the place. The compass just keeps the boat pointing in the right direction but not necessarily going in the right direction. None the less it still feels quite satisfying to hold the compass steady.


Another thing I find I use the compass for is picking up landmarks. No great accuracy required. Just measure the bearing to the tower or whatever and then mark it off on the chart to identify what you are actually looking at. Of course it is possible to be much more accurate with a bit of effort. We have just been updating the magnetic variation tables in Nuno™. The raw data comes from NOAA as a series of tabulated values which refer to the World Magnetic Model (WMM).

The World Magnetic Model is the standard model used by the U.S. Department of Defense, the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field.

Pretty good credentials eh? In Nuno™ we have code which can interpolate the model and so calculate the magnetic and secular variation for any point on the planet at a given time. Secular variation is the rate of change of magnetic variation. Variation is also sometimes called declination. The current model is valid until the end of 2014 by which time another set of data will be published. At school I was taught that the earth’s magnetic field is as if there were a bar magnet on the spin axis. The reality is a little more complicated, the Earth's main magnetic field is generated in the conducting, fluid, outer core, but it is still not a bad analogy. There is some more interesting detail here on the NOAA website. The field is not completely aligned with the earth’s spin axis. Currently the magnetic north pole is at 80.02° N, 72.21° W. The field is not completely even as you can see from this picture.


Do we need magnetic navigation when Global Position System (GPS) is available? GPS provides precise point location but only measures travel direction when in constant motion. A GPS receiver must collect several sets of latitude and longitude pairs to obtain direction. In addition, GPS signals may become blocked due to obstructions, adverse terrestrial and space weather, ionospheric conditions or being underwater. Hence, compasses complement GPS receivers to attain precise and immediate navigational headings for air, ground, and water-based systems. Electronic compasses and the WMM commonly co-exist in GPS receivers.

If you have fixed ships compass, say in a binnacle, then to be as accurate as possible you need to account for compass deviation. This is the effect of the boat itself on the compass. Anything close to the compass which can affect the earth’s magnetic field will cause deviation. Lord Kelvin patented a system around 1880, Kelvin’s Balls, which involved two spheres mounted on the binnacle. These were common place for a long while and can still be seen on older vessels. The effect of deviation is directional so if you point the vessel in different directions and measure the deviation it is possible to construct a correction table. You need to get organized a little bit to swing the ship like this but it is possible. If you find the right place you can obtain accurate bearings by sighting on known objects and so eventually you construct a deviation table. In our Henry product you can enter the deviation table – it looks like this:

deviation table

So now it is possible for the system to quite accurately use true bearings (T), magnetic bearings (M) and compass bearings (C). A true bearing is the angle from true North, a magnetic bearing is a true bearing corrected for magnetic variation and a compass bearing is a magnetic bearing corrected for the compass on a particular vessel. The question is :- does anybody, would anybody, use this? We could quite easily add this feature into Nuno™ but, realistically, is trying to use a magnetic compass accurately a bit out-dated these days?

One little lurker that I should mention in closing is electrical equipment. When Lord Kelvin was adjusting his balls this was not much of a concern. Once set the deviation would stay fairly predictable. However anything electrical can generate a magnetic field and these can play havoc with a magnetic compass. A good experiment to try is to see what happens when you switch things on and off. Get your boat still and steady and then try messing with any bits of electronics near the compass. While you are at it have a look at what your mobile phone can do to the indicated direction. If you are lucky then very little will happen, but this is not always the case.

Who is for a deviation table in Nuno™?

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