DEVIATION TABLE

Gene BjerkeGraph 1
Gene BjerkeGraph 2If there is any remaining deviation, you need to make a deviation table for your boat. A deviation table is simply a table of corrections to be applied to your compass to convert between magnetic courses and what you will actually steer using your compass. For example, if the magnetic rose on the chart indicates that your course should be 135º magnetic and your deviation table shows you have a deviation of 5º West at that course, you would add the two numbers together to get a course of 140º, and steer 140º by your compass.

There are two basic ways to generate a deviation table, by direct observation or by taking bearings. Since compasses on most small boats are not equipped to take bearings directly, we will not go into that technique (if your compass is so equipped, you can find the technique described in several references, including Chapman Piloting and Seamanship).

To create a deviation table by direct observation, first lay out your chart and find as many ranges as you can, a range being two fixed objects, on land or water, that you can line up visually from your boat and know you are running in a specific direction (magnetic). For example, you may find that if you line up with a beacon on the water and a steeple on land you will be running a magnetic course of 090 (and 270 in the opposite direction). That will give you two known magnetic directions to run.

Try to get as many different directions as possible. If you can find courses for every 15 degrees of the compass, great; if not, do the best you can. Then go out on your boat and run each of the ranges in both directions, taking careful note of what your compass says each time.

You can tabulate your results in three columns, either on paper or using a spreadsheet. The first column will list the correct magnetic course; the second column will contain what your compass read on that course; and the third column will list the difference between the two, which is the deviation for that course. For example, if the course you were running was 090 magnetic and the compass showed 092, then your deviation would be 2W, meaning two degrees west, because the proper course was west of what your compass showed (See Table 1).

If you were able to find a large number of ranges, the raw table might be good enough. But to make a neater table, and get a sense of the deviation for your boat, you can make a simple diagram. On a piece of graph paper, draw a vertical line and label it in appropriate increments, say every 10 degrees, to represent magnetic courses. Label the deviations every one degree, with easterly deviations to the right of the line and westerly deviations to the left (see Graph 1). Now plot the deviations from your raw data table, and draw a fair curve through all of them (see Graph 2). If any points are outside the curve, they should be treated with suspicion.

The result is that you can now create a more complete deviation table, because the graph gave you many more numbers than you could collect on your own. You can use any increment that you find convenient. In Table 2 we have used 15 degrees. To fill in the right column, you can pick the nearest whole degree from the fair line you have drawn.

USING THE COMPASS TO NAVIGATE

This may seem like a great deal of work, but it is necessary in order for your compass to be a useful and accurate guide. The day-to-day use of the compass involves two things: determining the course to steer and then steering that course.