A nautical compass is simply a compass designed for use on the water. The top of the compass is usually bowed out, like a contact lens or the top of a ball. The compass operates the same as a regular land compass, but works off degrees gathered from the hatch marks on the compass that correspond to different locations on a nautical map. Using the nautical compass is largely a math problem, subtracting or adding the magnetic north variations to the true north map readings.
Look at the outer ring of the nautical map rose to determine which direction is true north. True north is slightly different from magnetic north, and when you follow directions on a map you must adjust for the difference between true and magnetic north.
Look at the second ring from the outside to determine where magnetic north is located.
Inspect the inner ring of the nautical map rose to determine the amount of variation from true north to magnetic north. This will vary by map because the variation is different based on location. The map will have a number and direction variation, such as 21 degrees east. The inside of the ring also tells the number of degrees that magnetic north shifts. Multiply the shift number listed on the chart by the number of years from the map year to the present to determine the shift for the current year.
Determine the course to travel by finding the magnetic north listed on the nautical map. Follow the degree variations based on the inner magnetic ring to find the easiest and most accurate path to follow.
Chart the degree path onto the map. Locate the traveling point by degrees, which are the hatches on the compass. A location of 3 degrees north on the magnetic compass map reading will mean that you can directly follow the path outlined on the nautical compass. This is the easiest way to determine the correct way to go in a boat.
Alternatively, you can follow the TVMDC formula for finding magnetic north on a regular map with a nautical compass. Take the true north reading and divide it or subtract it from the magnetic north variation. For east variations, subtract the number. For west variations, add the number. Adjust the deviation of the compass. This will change for each compass. The resulting number is the degree path you should follow.