Applying Tide Corrections to Charted Depth

TL;DR — US charts sound depths referenced to Mean Lower Low Water (MLLW); to find actual depth at any moment, add the tide height above MLLW from the tide tables to the charted sounding. Always verify the depth unit in the chart title before plotting.

---

What the Rule Says

Charted Soundings and Their Datum

US nautical charts show soundings in feet, fathoms, or metres, and every sounding is referenced to Mean Lower Low Water (MLLW) Bowditch Ch. 4 §403. MLLW is the chart datum — the statistical average of the lower of the two daily low waters over a 19-year tidal epoch. Because it represents a low-water average, the actual water surface is almost always at or above this datum, meaning charted depths are generally the minimum you can expect, not the typical depth.

The depth unit — feet, fathoms, or metres — is stated in the chart title Light List — Depth Unit Notation. This is not a detail to skim. A sounding of "3" means three feet on one chart and three fathoms (18 feet) on another. Misreading the unit is a grounding waiting to happen.

Tides: What They Are

Tides are the periodic vertical rise and fall of sea level caused by 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 §901. This means in a semidiurnal tidal regime you get roughly two high waters and two low waters every 24 hours 50 minutes, and the water surface is constantly moving relative to the chart datum.

Tidal currents — the horizontal flow of water associated with that rise and fall — are a separate consideration. Flood is incoming flow, ebb is outgoing flow, and slack water is the brief period of zero flow at the turn Bowditch Ch. 9 §903. Do not confuse tidal height (a vertical measurement used for depth correction) with tidal current (a horizontal flow used for set-and-drift calculations). The exam tests both, and mixing them up costs points.

Required Publications on Board

The regulations mandate that tide tables be carried and kept current. Under 33 CFR §164.33, every vessel subject to that section must carry, for the area to be transited, the current edition of tide tables published by private entities using data provided by the National Ocean Service 33 CFR §164.33. Under 46 CFR §28.225, commercial fishing vessels must carry the current edition of tide tables promulgated by the National Ocean Service 46 CFR §28.225. Both regulations also require currently corrected marine charts of sufficient scale and detail to make safe navigation possible, currently corrected copies of the U.S. Coast Pilot and the Coast Guard Light List, and current tidal current tables 33 CFR §164.33 46 CFR §28.225.

"Currently corrected," as defined in 33 CFR §164.33(c), means corrected with changes contained in all Notices to Mariners published by the National Geospatial-Intelligence Agency (or an equivalent foreign government publication) that are reasonably available to the vessel and applicable to the transit 33 CFR §164.33. Tide and tidal current tables are treated differently from charts and other publications: they must be the current edition, not merely corrected editions 33 CFR §164.33.

Bridge Clearances: The Inverse Problem

Bridge vertical clearances on US charts are stated as the clearance at Mean High Water (MHW) Light List — Bridge Clearance Annotation. This is the opposite reference datum from soundings. For depth, you add tide height to the charted sounding to get actual depth. For bridge clearance, you must account for the tide stage relative to MHW to get actual overhead clearance at that moment Light List — Bridge Clearance Annotation.

---

Why It Matters on the Exam

OUPV and Master 100 GT written exams routinely present scenario questions that require you to compute actual depth of water or actual bridge clearance at a given time. The arithmetic is straightforward, but the exam is testing whether you know which datum applies to which measurement and which direction to apply the correction.

The Core Depth Correction Formula

Charted soundings are referenced to MLLW Bowditch Ch. 4 §403. The tide tables give you the predicted height of tide above (or occasionally below) the chart datum at a specific time and place. Therefore:

Actual depth = Charted sounding + Height of tide

If the tide table shows a height of +4.2 ft above MLLW at 1430, and the chart shows a sounding of 8 ft, the actual depth at 1430 is 8 + 4.2 = 12.2 ft.

If the tide table shows a height of −0.3 ft (a negative tide, which does occur on the US West Coast), and the charted sounding is 6 ft, the actual depth is 6 + (−0.3) = 5.7 ft. Negative tides produce less water than the chart shows — a critical safety consideration.

The Bridge Clearance Correction

Bridge clearances are referenced to MHW Light List — Bridge Clearance Annotation. At any moment other than MHW, the actual water surface is either above or below MHW, which changes the available overhead clearance.

When the water surface is below MHW, there is more clearance than charted. When the water surface is above MHW (as during a storm surge or spring high tide that exceeds MHW), there is less clearance than charted.

The exam may give you a charted bridge clearance, a predicted tide height, and the MHW value for that station, then ask for actual clearance. Work through it systematically: determine how far the current water surface is above or below MHW, then adjust the charted clearance accordingly.

Unit Verification Is Not Optional

Because US charts may express soundings in feet, fathoms, or metres, and the unit is stated only in the chart title Bowditch Ch. 4 §403 Light List — Depth Unit Notation, exam questions sometimes embed a unit-conversion trap. A charted sounding of 2 fathoms is 12 feet — a vessel drawing 10 feet has 2 feet of clearance, not negative 8 feet. Always confirm the unit before performing any calculation.

---

Common Pitfalls

1. Subtracting tide height instead of adding it. Charted depth is referenced to MLLW, which is a low-water datum. Tide height from the tables is the height above that datum. Adding a positive tide height increases the available depth. Candidates who confuse the direction of correction will consistently get wrong answers on depth problems.

2. Applying the depth datum to bridge clearances. Bridge clearances reference MHW, not MLLW Light List — Bridge Clearance Annotation. Using the tide table height above MLLW directly against a bridge clearance without accounting for the MHW reference will produce an incorrect answer. Know which datum applies to which measurement.

3. Confusing tidal height with tidal current. Tidal height is a vertical measurement used to correct charted depths. Tidal current (flood, ebb, slack) is a horizontal flow used in set-and-drift and course-made-good problems Bowditch Ch. 9 §903. The exam tests both; do not import current data into a depth correction problem or vice versa.

4. Using an outdated tide table. Both 33 CFR §164.33 and 46 CFR §28.225 require the current edition of tide tables 33 CFR §164.33 46 CFR §28.225. Last year's tables are not compliant. On the exam, if a question asks what publications must be on board, "current edition" is the operative phrase for tide and tidal current tables.

5. Ignoring the chart title for depth units. The depth unit is stated in the chart title Light List — Depth Unit Notation Bowditch Ch. 4 §403. Exam scenarios may mix units deliberately. Convert everything to a common unit before computing.

6. Treating a negative tide as zero. On the US West Coast, predicted tide heights can be negative — the water surface falls below MLLW. A negative tide height reduces actual depth below the charted sounding. This is not a rounding artifact; it is a real navigational hazard and a testable concept.

---

Quick Check

Q1 — A chart titled "Soundings in Fathoms" shows a sounding of 3 at a shoal. The tide table predicts a height of +2.0 ft above MLLW at the time of transit. What is the actual depth in feet?

3 fathoms = 18 feet. Actual depth = 18 ft + 2.0 ft = 20.0 feet. The key steps are (1) convert fathoms to feet using the chart title unit, and (2) add the tide height above MLLW to the charted sounding. Bowditch Ch. 4 §403 Light List — Depth Unit Notation

Q2 — A charted bridge has a vertical clearance of 45 ft at MHW. The current tide height is +1.5 ft above MLLW, and MHW at this station is +4.0 ft above MLLW. What is the actual overhead clearance?

The water surface is 4.0 − 1.5 = 2.5 ft below MHW. When the water is below MHW, there is more clearance than charted. Actual clearance = 45 + 2.5 = 47.5 feet. Light List — Bridge Clearance Annotation

Q3 — What datum are US chart soundings referenced to, and where is the depth unit stated?

US chart soundings are referenced to Mean Lower Low Water (MLLW). The depth unit (feet, fathoms, or metres) is stated in the chart title. Bowditch Ch. 4 §403 Light List — Depth Unit Notation

Q4 — A vessel subject to 33 CFR §164.33 is preparing for a coastal transit. The master has last year's tide tables aboard. Is the vessel in compliance?

No. 33 CFR §164.33 requires the current edition of tide tables for the area to be transited. Last year's edition does not satisfy this requirement. 33 CFR §164.33

Q5 — The tide table shows a predicted height of −0.5 ft at 0600. The charted sounding at a bar is 5 ft. What is the actual depth at 0600, and is this more or less water than the chart shows?

Actual depth = 5 + (−0.5) = 4.5 feet. This is less water than the charted sounding. A negative tide means the water surface has fallen below MLLW, reducing depth below what the chart indicates. Bowditch Ch. 4 §403 Bowditch Ch. 9 §901

Q6 — What is the difference between tidal height and tidal current, and why does the distinction matter for chart plotting?

Tidal height is the vertical rise and fall of sea level relative to the chart datum, used to correct charted soundings and bridge clearances. Tidal current is the horizontal flow of water (flood, ebb, slack) associated with that rise and fall, used in set-and-drift calculations. Applying current data to a depth correction problem, or vice versa, produces an incorrect answer. Bowditch Ch. 9 §901 Bowditch Ch. 9 §903