With GPS, who needs a compass? You do. This column is all about that.
A Candle Held Where?
What if we told you that the signal from the GPS satellites reaches your boat with the intensity of a candle – held in Los Angeles while you are in New York…? Yes, that’s how it is designed. So, what happens if the weather really becomes foul? You can lose your GPS signal, that’s what! It takes a lot because of the redundancies built in but it can happen.
Where Is The Magnetic North Pole?
Most of us have seen diagrams or pictures of magnetic waves, just like those that come out of household magnets, coming out of the North and South Poles, encircling the Earth. The iron core of the Earth spins at high speed and creates this magnetic field. Of interest, the magnetic forces don’t emanate from the top of the world, i.e, the true North Pole. Right now, the “Magnetic North Pole” is just north of Hudson Bay. When George Washington was leading the United States, Magnetic North was near Norway. If you look on any paper chart for the “compass rose”, it shows in the very center what is called “Variation”, i.e., from the area that the chart covers, what is the angular difference, i.e., Variation, from True North to Magnetic North. Here, it is 14-degrees west, i.e., your compass points 14-degrees too far west at Magnetic North versus where True North lies. This means that when your compass is pointing to Magnetic North, you would turn the boat 14-degrees to the east (014-degrees) to be pointing to True North! BTW, this is interesting – but largely meaningless since all compasses sold above the Equator point to Magnetic North. But it is important to be aware of Variation.
What is Deviation?
Frankly, more important than Variation to the average boater is Deviation. Deviation is the sum of all the forces within your boat that keeps your compass from pointing to Magnetic North. Metallic objects (or magnetic objects like radio speakers) near your compass will “fool” your compass into thinking that that object is Magnetic North. How can you tell what the Deviation is of your compass? Well, if you have a GPS, it will be easy – all you need is mile or so of calm water and you can run down the rhumb lines of the four cardinal points, along with a number of intermediate headings in between, and record the differences between what the physical compass is reading from the GPS course you are running. Of interest, Deviation “deviates” differently at a given compass course so you need to check at least the four cardinal courses (when we develop our deviation tables for new boats, we measure at least 16 compass headings.) You need to know what your boat’s compass Deviation is so that, if you do have to use your compass in lieu of your GPS, you can compensate appropriately. Over enough distance, even a degree or two can add up to significant differences. If you don’t have a GPS, it is a bit more complicated but it can be done. Get your paper charts out, mark a rhumb line between two points that lie at a given magnetic course between each other. Run down that line and record what your compass is reading versus what your paper chart told you the compass should be registering. The difference is Deviation.
My GPS Has Failed and I Don’t Have a Compass!
Well, happily for this sorry skipper, there is a way to create a crude compass with a watch if you find yourself in such a state. Simply point the hour hand at the sun. Halfway between the hour hand (the sun) and 12 on your watch lies South. If you know where South is, you know where North, East and West are…
Don’t have an old fashioned watch..? Draw one and line it up as it were on your wrist. It works!