chart plotting and preparation 33
to nothing at the poles. Th us, use only the latitude scale (or the chart’s distance
scale) to measure distances.
Always express distances in nautical miles and tenths of a nautical mile. On
charts that show degrees, minutes, and tenths, you can read distances directly
from the latitude scale. On charts showing degrees, minutes, and seconds, you’ll
have to divide the seconds by six to convert them to tenths of minutes.
CHARTS WITH DEGREES, MINUTES, AND TENTHS
- Open your compass or dividers until the two points fall exactly on
the two chart locations whose distance apart you want to measure.
Preserving that span, move to the adjoining right or left chart
scale. - Stick the point of your compass or dividers into an even minute and touch
the other leg above. You’ll notice each minute is divided into ten small
segments, or tick marks. Each tick mark represents a tenth of a minute
(which is a tenth of a mile). - If you measure 4 minutes and 3 additional tick marks, the distance
is 4.3 nautical miles. If you measure 12 minutes and 9 additional tick
marks, the distance is 12.9 nautical miles.
CHARTS WITH DEGREES, MINUTES, AND SECONDS
- Open your compass or dividers until the two points fall exactly on the
two chart locations whose distance apart you want to measure. Preserving
that span, move to the adjoining right or left chart scale. - Stick the point of your compass or dividers into an even minute and
touch the other leg above. - If you measure 4 minutes and 36 seconds, the distance is 4.6 nautical
miles (36 divided by 6 equals 0.6 mile). Round off seconds that are not
exactly divisible by 6. For example, 27 seconds rounds off to 0.5 mile,
and 11 seconds rounds off to 0.2 mile.
True versus Magnetic Directions and Two Ways to Find Variation
True north points to the geographic North Pole. Due to the magnetic fi eld
surrounding the earth, the needle on your compass points to the magnetic
North Pole, which is some distance from the geographic North Pole (see more,
below).
True directions. For example, imagine pointing one arm toward the North
Pole and another in the direction you want to go. Measure the angle clockwise
from the arm pointing north to the arm pointing toward the direction you want