Here’s some great advice from Skipper Tips about how to read a chart – electronic or paper – showing numbers, including some with underlines or enclosed by parentheses. Here’s how to read these symbols to keep off the rocks and navigate safely:
Nautical datums are stated in two ways. Look in the title block area (that area of a chart just beneath the title) to the “Chart Datum”, or soundings (depths). These will be stated as feet, fathoms, meters or a combination such as fathoms and feet or meters and decimeters (tenths of a meter).
Next determine the “Height Datum.” This may be Mean High Water (MHW) for most NOAA charts, or Mean High Water Springs (MHWS) for other charts. Heights are given at mean high water because this offers the safest or “least clearance” when passing under bridges or power lines.
Underlined Numbers On or Near a Shoal (top illustration)
Rocks or shoals that cover and uncover with the tide may show an underlined number atop the danger. If there’s no room to put the underlined number atop the rock or shoal, it will be shown in parentheses adjacent to the danger.
So, if you see (3) on a chart with heights in feet and soundings at Mean Lower Low Water, this means the rock or shelf will visible only at Mean Lower Low Water. At that time, the rock should uncover by three feet. At higher stages of the tide, the rock will be covered and not be visible. Scan your chart and highlight dangers like these. Sail clear by a wide margin for safety’s sake.
Numbers On or Near a Shoal (bottom illustration)
Cartographers may show a number atop an islet (small island). This shows the height of the islet that will be visible even at high water (check your chart for the height datum). If there’s no room to put the number atop the islet, the number will be placed into parentheses adjacent to the islet.
So, if you see a (3) on a chart where heights are in feet above mean high water, this means the islet will always be visible by at least three feet above mean high water. Read more:
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