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Author
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Topic: Chart Manual Errata on bridge clearances
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David Burch
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posted April 03, 2004 04:03 PM
Page 3-1, column A of the Nautical Chart User's Manual
The method they give for figuring bridge clearance is not exactly right and could lead to discrepancies of 3 feet or so in extreme cases.
Their suggesion to use "mean range" of the tide in the example given is not right. One should use mean high water, which is not in the tide tables but must be looked up on the chart. Every chart inlcudes this crucial data as a table, printed somewhere on the chart.
Their method likely works ok on the east coast, but on the west coast it does not. One could use diurnal range and get closer, but better to do it right. They reference the Hobbs text as their source, but we have not seen the particular discussion referred to.
From: Starpath, Seattle, WA
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