Applying tide corrections to charted depth

Bowditch Ch. 9 §901

Tides and Tidal Currents Tides are the periodic vertical rise and fall of the sea level due to the gravitational interaction of the moon, sun, and earth. The principal lunar tide has a period of approximately 12 hours 25 minutes.

Bowditch Ch. 9 §902

Tides and Tidal Currents Spring tides occur near new and full moon and have the greatest range. Neap tides occur near first and last quarter moon and have the smallest range.

Bowditch Ch. 9 §903

Tides and Tidal Currents Tidal currents are the horizontal flow of water associated with the rise and fall of the tide. Flood is incoming, ebb is outgoing, slack water is the brief period of zero flow at the turn.

Bowditch Ch. 9 §904

Tides and Tidal Currents Charted soundings on US charts are referenced to Mean Lower Low Water (MLLW), the datum from which the tide tables predict the height of tide. Actual depth of water equals the charted sounding plus the predicted height of tide, and underkeel clearance equals that actual depth minus the vessel's draft. Heights of features such as bridges and lights are referenced instead to Mean High Water, so vertical clearance shrinks as the tide rises.

Bowditch Ch. 9 §905

Tides and Tidal Currents The NOAA Tide Tables predict the times and heights of high and low water, and the Tidal Current Tables the times of slack water and of maximum flood and ebb, at reference stations. Predictions for a nearby subordinate station are obtained by applying the tabulated time and height differences to its reference station. Always check the correct time zone and daylight time before applying a prediction.

Bowditch Pub. 9 Ch. 2 §229 ¶1

Working from top to bottom the entries are: Date: The UT date of the sight. Body: The name of the body whose altitude was measured. Indicate whether the upper or lower limb was measured if the body was the Sun or the Moon. GMT (Greenwich Mean Time): The UT (GMT is an Figure 1904. The basis for the line of position from a celestial observation. 3 SIGHT REDUCTION outdated name for UT) of the observation. The UT is the Watch Time of the observations adjusted for the Watch Correction and Zone Description (see Chapter 20). IC (Index Correction): The instrumental correction for the sextant used. Chapter 18 discusses determining its magnitude and sign. D (Dip): Dip correction is a function of the height of eye of the observer and atmospheric refraction. Its magnitude is determined from the Dip …