The datum from which the predicted heights of tides are reckoned in the tide tables is the same as that used for the charts of the locality. The depression of the datum below mean sea level for Hampton Roads, Virginia is __________.
• Chart datum vs. mean sea level (MSL) – understand what it means when the datum is "depressed" below MSL • How to read the Tide Tables introduction where it explains the local tidal datum and its relation to MSL • Typical range of mean lower low water (MLLW) below MSL on the U.S. East Coast and in Hampton Roads–type harbors
• First, decide whether the tidal datum in Hampton Roads is slightly below, near, or far below mean sea level based on what you know about MLLW on the East Coast • Compare each choice to what would be a realistic depression of chart/tide datum below MSL for a semi‑diurnal tidal area like Hampton Roads • Think about which option is a very small offset, which are moderate, and which would represent an unusually large difference from mean sea level
• Verify in the Tide Tables for Hampton Roads how many feet the tidal datum (usually MLLW) is below mean sea level for that specific locality • Check that the choice you pick is consistent with typical U.S. East Coast depressions of MLLW below MSL (generally not an extreme number) • Make sure you are choosing a value that represents a depression below MSL (i.e., datum is lower than MSL, not above it)
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