chart plotting and preparation 35
run in a powerboat from Key West across the Gulf of Mexico to Houston,
the variation will change around 6 degrees. To account for these changes
you must periodically steer a different trackline. Otherwise you’ll end up
somewhere other than your intended destination.
So how do we fi nd the variation at our location? Fortunately, cartographers
have worked this out for us, and all we need to do is know where to look and
how to apply it. Th ere are two sources from which you can learn the variation
for the waters in which you are boating. (Note: a GPS receiver can give you
bearings and courses in degrees true or degrees magnetic—the choice is yours.
And if you choose the latter, the GPS receiver will adjust for local variation
automatically. We’ll discuss GPS navigation in Chapter 4.)
Compass rose. Every chart has at least one compass rose overprinted on it,
and most have three or four. Printed in magenta, most roses have two concentric
rings; the outer one shows true directions and the inner shows magnetic direc-
tions. Locate the closest compass rose to your location, and just above its center
crosshairs you will fi nd a label noting the local variation (as of the stated year)
and its name—east or west. Record this in your log. Just below the crosshairs
you will fi nd a note of the annual increase or decrease in variation. Take the
diff erence between the year shown and the current year. Multiply this diff er-
ence by the minutes of increase or decrease. If increasing, add the diff erence
to variation. If decreasing, subtract the diff erence. Round your answer to the
closest whole degree of variation.
Variation: 18 degrees east (2002)
Annual decrease: 5 minutes
The current year is 2009; what is variation at this location?
7 (years) × 5 = 35 minutes
18 degrees east – 35 minutes = 17 degrees 25 minutes east
Round the answer to the closest whole degree. The variation in this instance
is therefore 17 degrees east.Sometimes the annual change in variation is noted as east or west, rather
than increasing or decreasing. If the annual change has the same name (east or
west) as the variation, variation is increasing. If the names are opposite, variation
is decreasing. For example: If your compass rose says 12°14'W (10'E), this indi-
cates a decrease of 10 minutes a year because the names, W and E, are opposite.
If it says 12°14'W (10'W), that would indicate an increase by 10 minutes a year
because the names are the same.
Isogonic lines. On small-scale charts covering large areas over which
variation changes markedly—most notably, on offshore charts—look for
curved magenta lines (dashed or solid) running across the chart. Using the
curve closest to your position, scan along it to find the noted variation.