chart plotting and preparation 29
How to Choose the Chart Scale You Need
A chart’s scale indicates the amount of detail it provides for safe navigation.
Think of chart scales like the scale of a model ship. Let’s say you go to a craft
store looking for a model of the tall ship HMS Bounty. The store offers two
different models of the same ship. One box states that the model is on a scale
of 1:48, and the other is 1:96.
If we open the fi rst box, we’ll fi nd rather large pieces of the model. For
instance, the ship’s wheel will be prominent with good detail, and we won’t
lose our eyesight straining to see the intricate details of its spokes. Popping
open the second box, we’ll fi nd much smaller pieces. Th e ship’s wheel on this
model has shrunk to half the size of the other wheel. What happened?
If we look at the second number of each ratio, the mystery resolves itself.
A larger second number means a smaller representation, or scale, than a smaller
second number. This is true regardless of the units of measure.
In a 1:48 scale ship model, each^1 / 4 inch represents 1 foot of the full-size ship
(12" ÷ 48 = 0.25"). HMS Bounty was 91 feet long on deck (small for her day
and her mission), so a 1:48 scale model would be 1.9 feet or 23 inches long. If
her ship’s wheel was 5 feet in diameter, it would be 1^1 / 4 inches in diameter on
the model, and so on. A 1:96 scale model of the Bounty, however, would be just
111 / 2 inches long with a wheel of^5 / 8 -inch diameter.
It is the same with charts. On a 1:80,000 scale chart, 1 inch represents
80,000 inches of actual nautical real estate. On a 1:10,000 scale chart, 1 inch
represents 10,000 inches on the earth’s surface. Th e 1:10,000 chart gives you
more detail for a tricky harbor entrance, while the 1:80,000 chart gives you a
bigger picture for coastal piloting.
When choosing charts, always select the largest scale available. Larger scale
charts enable you to enter harbors and anchorages, pass close to or even over
shoals (if your draft allows), and give the best detail in restricted waters. Large
scale equals larger detail and a smaller area; small scale equals smaller detail
and a larger area.
SCALE SELECTION SIMPLIFIED
Use a baseline of 1:80,000 to select the appropriate scale for navigation. Th is
is the most common chart scale used in U.S. coastal waters. If using British
Admiralty charts, use the largest scale available for safe navigation. A 1:80,000
scale chart provides excellent detail from about 1 to 10 miles off the coast.
Shift to a larger-scale chart—1:60,000; 1:40,000; 1:20,000, or larger—as soon
as possible when making landfall. Farther off the coast, you must change to
a smaller-scale chart. Ocean-crossing charts oft en have scales of 1:1,000,000
or smaller.