Sailing Navigation Safety: Nautical Chart “Accuracy Indicators”

Here’s another great tip from our friend Captain John of skippertips.com

Turn on your chart plotter or study your nautical chart and you will see objects on land or water. But how accurate are these to use for navigation. That ship wreck or piling you will pass may not be where it appears on your chart or plotter. And that could mean an cracked or impaled hull, keel, rudder or propeller shaft! Read on to discover how to use this vital information for sailing safety.

1. Look for More Accurate “Circles ‘n Dots”

Objects surveyed to a high degree of precision show a dot surrounded by a circle. On NOAA charts, this indicates a survey accuracy of +/- 10 feet (3 meters). Note the Micro Tower and Monument show a dot surrounded by a circle and show ALL CAPS (see more on this below).

Objects not surveyed to the same degree of precision show a small circle without the central dot. On NOAA charts, this indicates a survey accuracy of +/- 100 feet (30 meters). Note the Tower and Tank to the south (red arrows). Favor objects that show both dot and circle if practicable.

2. Make Landfall on Prominent or Conspicuous Objects

Objects that are prominent or conspicuous rate “ALL CAPS” to signify this. For example, a prominent TANK or SPIRE or CUPOLA (dome shaped roof).But all caps can also apply to land features that are prominent. Imagine that you are making landfall on the west coast. You want to sight CAPE FLATTERY. Note on the chart how this high, lofty landmass rates all caps. Look for objects or land points that carry the privilege of ALL CAPS when making landfall.

3. Beware of These “Two-Letter” Chart Abbreviations

Scan your nautical chart or electronic chart for the two-letter designations “PA”, “PD”, or “ED”. These could be shown next to a buoy, daybeacon (also called “daymark”), light structure, submerged or visible wreck, rock, reef (of any type), breaker line, or submerged object.

“PA” means Position Approximate, labeled next to objects that have a higher margin of survey error than those indicated earlier. Look at the illustration and note two wrecks.

“PD” means Position Doubtful and refers again to the position of the object. This could be a wreck, submerged post or piling, stump, or some other obstruction. Realize that the object will be somewhere in the vicinity of the symbol. Mark these dangers and avoid them like the plague to prevent damage to your hull.

“ED” means Existence Doubtful. This refers to the object itself–not the position. Sail defensive; set your course to sail far away from any object marked with the “ED” abbreviation.

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Use these sailing navigation tips to keep your small sailboat in safe water and avoid hazards that lie in your path. Sail with confidence on the waters of the world–wherever you choose to sail or cruise!

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