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Old 08-06-2013, 17:26   #1
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Rocks which cover and uncover?

Could someone please explain to me:

Chart No. 1 - Section K "rock which covers and uncovers; height above chart datum"

There are no depth/soundings around the examples; only symbols for rock with height nomenclature. Does this mean that if the depth near the rock is indicated at 5 fathoms and the rock height is 2, that the rock will be uncovered 2 feet above the water at MLW or MLLW of 5 fathoms but potentially covered at high tide?
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Old 08-06-2013, 17:32   #2
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Re: Rocks which cover and uncover?

There are a number of symbols for rocks. There though are only two basic categories of rocks, those that are above the surface at MLLW (for semi-diurnal tides) and those rocks that are always below the surface at MLLW. They each have their own distinct chart symbols.

Rocks that don't get covered are a land mass and are colored like a land mass even though you and I would call it a rock.

They don't give heights for rocks that are above any datum, only how far they are below the surface they are at MLLW.

Any heights relating to rocks is in reference to MLLW.

So if a rock is listed at 1 foot and the tide is a minus 2 then the rock will be sticking one foot out of the water at that time.

Rocks that are awash when the tide is above MLLW don't have any heights listed.

Nautical Chart Symbols: Rocks, Wrecks and Obstructions
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Old 08-06-2013, 17:54   #3
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Re: Rocks which cover and uncover?

Quote:
Originally Posted by David M View Post
There are a number of symbols for rocks. There though are only two types of rocks, those that are above the surface at MLLW (for semi-diurnal tides) and those rocks that are always below the surface at MLLW. They each have their own distinct chart symbols.

Rocks that don't get covered are a land mass and are colored like a land mass even though you and I would call it a rock.

They don't give heights for rocks that are above any datum, only how far they are below the surface they are at MLLW.

Any heights relating to rocks is in reference to MLLW.

So if a rock is listed at 1 foot and the tide is a minus 2 then the rock will be sticking one foot out of the water at that time.

Rocks that are awash when the tide is above MLLW don't have any heights listed.

Nautical Chart Symbols: Rocks, Wrecks and Obstructions
I am referring to the same page. Specifically the second example down from the top which mentions covers and uncovers, above chart datum.
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Old 08-06-2013, 17:58   #4
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Re: Rocks which cover and uncover?

That is the height of the landmass above MHHW, in the NOS column. Land masses have color...rocks do not....well black, yes.

The height of land masses and landmarks are measured from MHHW.
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Old 08-06-2013, 21:40   #5
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Re: Rocks which cover and uncover?

A rock which covers and uncovers, AKA a drying rock, includes its height above chart datum in parentheses and underlined. At a Higher High Water Large Tides it would be covered. At chart datum (what ever standard is used) it would be exposed. It is also shown as a "*" on the chart.

As an American your chart datum is likely Mean Lower Low Water.

Read the chart title block to determine chart datum and heights of land, bridges, etc.
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Old 09-06-2013, 18:59   #6
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Re: Rocks which cover and uncover?

David Burch Navigation Blog: Chart Sounding Datums… who needs um?
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