I understand the above sea level contours, but my navionics map shows elevations at maybe a big 0 and a small 6. (1 digit big, 1 digit small. maybe im a little slow, but I dont get it. Someone please explain, these are not soundings or elevations are they? kinda confused…
umbers printed on the water areas of the chart indicate the depth of the water at that spot. However, a two on the chart might mean two feet, two fathoms, or two meters. It is essential to know which unit of measurement is used. This is always shown on the face of the chart in large print-SOUNDINGS IN FATHOMS AND FEET, for example. A fathom, by the way, is six feet, so on a chart with soundings in fathoms, a number such as 03 means zero fathoms and three feet. The water at this spot is three feet deep at MLLW. A sounding of 55 would be 35 feet (five fathoms and five feet). In 1992 U.S. charts began switching from feet and fathoms to soundings in meters. If your brain hasn’t made that switch, you can divide soundings in meters by 0.3 to get feet.
The soundings printed on the chart normally represent the depth at mean lower low water (MLLW), so the actual depth is usually more than the charted depth. However, when the tide table shows a negative low-tide entry, actual depths will be less than the chart indicates. When the depth of the water is a concern, you will need to know the height of the tide.
Contour lines
In addition to numerical depths, charts show depth contour lines. The 10-fathom contour line on a small scale chart, for example, indicates that all the waters inside this line are no more than 60 feet deep. On larger scale charts, contour lines are often shown for three fathoms, two fathoms, one fathom, and sometimes even three feet. Typically, the area inside either the three fathom or one fathom contour line is tinted blue to provide quick identification of shoal water.
Mike Crouch
Sea Tow Charleston
843-881-8949
Sea School Charleston
843-747-1000
www.seatow.com
www.seaschool.com
Right On! Excellent response Mike!
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