Tide Height vs Tidal Range vs Current: Read Each Value

Tide height is a water level relative to a stated zero or datum. Tidal range is the difference between a specified high and low water level. Tidal current describes horizontal water motion associated with tides, including its speed and direction. A high-water time does not universally identify slack current.

Keep those quantities in separate records even when an app displays them together. The fictional datum and timing examples below show why changing a reference affects a height, why a shared offset cancels from a range, and why a height curve cannot supply a missing current prediction.

Give each quantity a complete label

Consider these three entries in an imaginary notebook:

The numbers are hypothetical. Notice that metres appear in both of the first two rows, while the last row has metres per second. Subtracting two heights can produce a range. It cannot produce a horizontal current speed without further information about the actual water system.

NOAA publishes tide-height predictions and tidal-current predictions as separate products. 1 That separation belongs in your notes even when a single application places them in adjacent cards.

A height depends on the chosen zero

A datum supplies the reference elevation for a reported height. NOAA’s datum documentation distinguishes several tidal references and emphasizes their local basis. 2 “Metres above sea level” is incomplete if the underlying product actually uses a named tidal datum.

Imagine one physical high level and one physical low level. In our invented Datum A, their heights are 3.2 m and 0.8 m. Now choose Datum B exactly 1.0 m higher than Datum A. The same surfaces become 2.2 m and −0.2 m above B. No water moved during this change of labels.

The minus sign in −0.2 m describes the surface’s position relative to B. It does not describe the distance from surface to seabed. That distinction is useful whenever a graph crosses its zero line: first identify the reference before interpreting the sign.

For a real comparison, do not create a conversion by subtracting two datum names or by assuming a familiar offset from another harbour. Retain the station’s published reference information. Our example works because the relationship between its two invented zeros was explicitly supplied.

Range cancels a shared datum offset

Using Datum A, our pair gives 3.2 − 0.8 = 2.4 m. Using Datum B, it gives 2.2 − (−0.2) = 2.4 m. The unchanged difference is the point of this example. Adding or subtracting the same constant from both levels changes the labels but preserves their separation.

The cancellation fails if you silently combine heights with different zeros. Taking 3.2 m from A and −0.2 m from B produces 3.4 m, which is not the separation of our physical high and low surfaces. The arithmetic succeeds while the comparison fails.

Label the chosen high-low pair and time interval too. A range for one pair is not automatically a daily maximum-to-minimum range or a long-term mean. NOAA’s glossary distinguishes event quantities from named mean tidal measures. 3 In a mixed sequence with unequal highs or lows, changing which pair you select changes the question.

A high-water time cannot fill an empty current column

NOAA explains that the relationship between water-level extrema and current events varies by location. Its examples include standing-wave and progressive-wave cases with different timing. 1 This provides a direct counterexample to the idea that all high-water times identify slack current.

In the drawing, H = cos(πt/6) represents relative height. Two possible relative-current curves are A = sin(πt/6) and B = cos(πt/6). At t = 0, height is at a maximum. A equals zero, while B equals its positive maximum. Both comparisons use exactly the same height curve.

At t = 3, the roles change: A is at its positive maximum and B is zero. The example shows why height alone cannot choose between these current records. It does not claim that every real site matches either simple curve.

The sign on a current graph needs its own convention. In this illustration, plus and minus merely denote opposing directions. For an actual product, preserve its stated direction definition and location rather than assuming that positive always means the direction you have in mind. NOAA defines slack in terms of current speed near zero, rather than the height graph’s maximum. 3

Compare two products without mixing their metadata

Build two rows rather than squeezing everything into a single “tide” field. In the water-level row, retain station identity, datum, units, date, time zone and whether the value is predicted or observed. In the current row, retain the relevant current location, measurement or prediction depth where provided, units, direction convention and valid time.

Suppose an application shows high water at 11:20 and a current maximum at 12:10. Before deciding that the displays disagree, check whether they describe the same location and the same physical quantity. Preserve both timestamps. Replacing the current entry with the high-water entry would remove information rather than reconcile it.

Keep a missing current record visibly missing. A note reading “current timing not obtained” is more useful than an inferred slack time that later looks like a published prediction.

Preserve predictions and observations as different records

NOAA’s astronomical tide predictions do not include every short-term effect of weather and runoff. 1 If an observation differs from a prediction, retain the original prediction, the observation time and the source of each. Do not overwrite the earlier value and lose the comparison.

For example, a fictional prediction of 1.8 m and an observation of 2.0 m give an observed-minus-predicted difference of +0.2 m, provided time, location and datum match. That residual is a calculation, not an identification of its cause. A single pair cannot tell you which environmental process produced it.

The surf-forecast guide keeps offshore wave components separate from local coastal context. The wave-period explanation separates seconds from spacing and propagation speed. Together, these distinctions help retain the meaning of a water-related number before drawing conclusions from it.