In these days of GPS navigation, people tend to ignore changes in deviation caused when the boat heels and lumps of structural iron alter their relationship with the compass. Some boats, especially those with iron keels, can exhibit enough ‘heeling error’ to make a significant effect, but it is almost impossible to tabulate. The best way to deal with it is to find a spot on board where the hand-bearing compass is free of deviation (check by steering in a circle and sighting it on a distant object. If it doesn’t change its bearing, you’re OK). Next time you’re steering a long leg well heeled over, verify the heading against the hand-bearing compass. Any discrepancy is deviation. If you’ve none of this ‘on the flat’, it’s heeling error.