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While watching the movie Master And Commander the other night, it made me think of how far navigation has come through the ages. First it was line of sight, then came dead reckoning, then celestial, then Loran, and now GPS (Global Positioning System).

We have gone from navigational systems that told us where we “sort of” are to a system that allows a captain to zero up on a wreck and hit it when he drops his hook.

That is all fine and good. I love my GPS, but why does it always give me a slightly different heading than my compass? My GPS says 45 degrees and my compass reads 52 degrees. One of them must be right and the other wrong. Right?

Wrong. It is just a matter of understanding how each one works.

GPS works by receiving signals from satellites in space. It produces a fix by calculating its position in relation to the known position of the satellites. By continuously updating this information, GPS can give you position, heading, speed, and other variables to help you navigate.

Most GPS sets are set to true north because that is how all nautical charts are produced.

True north represents the geographic north pole. If you laid out a flat map of the world, true meridians would constantly be straight up and down.

Most vessels use a magnetic compass to help navigate. A compass works by always showing Magnetic North as zero degrees.

With charts, you can then determine heading, and in most cases, speed and position.

Magnetic North represents the magnetic north pole which is determined by earth’s magnetic field.

Magnetic north is hundreds of miles from true north, which causes differences between true and magnetic bearings. So now we know why our magnetic compass does not read the same as our GPS.

The difference between our compass heading and true north is called compass error. If we can compensate for compass error, then we should be able to have the two be in line with each other. Compass error is then broken down to variation, the difference between magnetic and true north, and deviation, which is the difference between magnetic north and where our compass points.

Variation can be determined by looking in the compass rose on any nautical chart. It will tell you in degrees what the variation for that area is and how much it changes over time. Variation changes by the location of a vessel, not its heading.

Deviation is a measure of how the ship’s compass is affected by the unique magnetic field of that particular vessel. There are many variables to determine deviation, such as hull construction, other electronics, compass position and any metal objects.

To see an example of this, while reading your compass, pass a metal object near it (like a hammer) and watch what happens. It is like entering the Twilight Zone. Large vessels have their compasses compensated with other magnets and deviation tables to reflect no deviation.

You could do this for your vessel, but it must be done by a professional because of the many variables present. Deviation will change depending on the vessel’s heading, not its position.

So what does all this mean?

In theory, we should be able to make our compass heading and GPS reading work together. If we take our compass heading, add or subtract the variation off our chart, and then add or subtract the figure from our deviation tables, we should come up with what the GPS reads. Because deviation is so complicated and there are so many factors involved, it is hard to come up with the exact bearing, but you should be able to come very close.

So before you get frustrated with your compass or want to throw your GPS overboard, just remember that they operate on different systems and they still have a place at the helm or on the navigation station.

Most GPS units these days can calculate deviation and can be switched from true to magnetic in the systems page. By enabling this and using a hand held compass to eliminate most of the deviation, you can get close to making them match without any calculations.

My heading is 45 degrees true and 52 degrees as per ship’s compass, eight bells and all is well.

Remember that with all the electronic and other sophisticated navigational equipment available today, you still have to know how to use them.

Even with mapping, Global Positioning Systems and everything else, it is still essential to have good charts and be able to read them.

Have a great week.

Billy Harris welcomes ideas, comments and information about sailing. If you would like to inquire about sailing on Marco Island, e-mail him at defcon1bh@earthlink.net.