Reading the water: tidal datums and the survey words that trip up newcomers

The survey launch idles over the channel and the new hire reads the depth off the screen: 8.2 metres. The chart taped to the console says 6.5. "Chart's wrong," he announces. The party chief does not look up. "Chart's fine. The chart is measured from chart datum. You're measuring the water you're floating on, and the tide's up a metre seven." Some version of that exchange happens on every new survey crew, at every fuel dock and on every job that puts a pier into the sea. Water level words are precise, and nearly all the trouble comes from mixing up which zero a number is measured from.

Zero is a choice

A tidal datum is a reference height defined by averaging some phase of the tide over a long record. Chart datum is the one that sits under every depth on a nautical chart, and it is set low on purpose, so most of the time the water a mariner finds is more than the chart shows. In the United States that zero is MLLW, mean lower low water, an average that roughly half of all lower lows dip below. Many other countries use LAT, the lowest level astronomy alone can produce, which the tide almost never goes under without help from the weather.

Set the zero low and some tides still dip below it. That is a minus tide, printed with a minus sign in the tide book and greeted with joy by clam diggers. Ground that covers and uncovers with the tide is charted with a drying height, a height above chart datum rather than a depth below it, underlined so nobody misreads it. A rock awash sits right at about the zero, breaking the surface at low water.

The family of means

Chart datum has relatives, and they are easy to confuse because their names are all three or four letters starting with M. MHW averages every high water, and in many places it fixes the legal shoreline, so property lawyers care about it as much as sailors do. MHHW averages only the higher high water of each day, which matters on coasts with a mixed tide, where the two daily highs differ noticeably. That difference is the diurnal inequality, and it is why West Coast boaters talk about the low-low, the lower of the day's two low waters. MSL is the average of all hourly heights, and MTL is simply halfway between mean high and mean low.

None of these is a quick average. Each is computed over a tidal datum epoch, a fixed 19-year window chosen to cover the nodal cycle, the slow wobble in the Moon's orbit that nudges tidal ranges up and down.

Then there is the land side. Engineers and land surveyors work in NAVD, the North American Vertical Datum, a geodetic datum that has nothing to do with local tides. Its zero is not MSL and not MLLW, and the gap between them changes from harbour to harbour. The classic expensive mistake on a waterfront job is a drawing that mixes NAVD elevations with MLLW depths without saying so. "What datum?" is the first question an old hand asks about any number on a plan.

Where the numbers come from

All of it rests on a tide gauge, often living in a small tide house on the end of a pier. Float and acoustic gauges sit in a stilling well that damps out the waves. Others use a bubbler pushing gas through an underwater orifice, a submerged pressure gauge, or a microwave radar sensor looking down at the surface. Many stations log six-minute data, and a flatline in that record means a frozen or dry sensor, not a calm day.

The gauge records against a station datum, an arbitrary zero set safely below the lowest water. What ties that zero to the real world is a ring of bench marks on land, stamped brass caps set in concrete or bedrock. Surveyors run leveling from the marks to the gauge, finishing by closing the loop on the starting mark to see how much error crept in. A tide staff bolted to the pier gives a quick eyeball check, and a staff shot, a set of staff readings logged next to the gauge, proves the gauge still reads true. If a storm takes the pier, the bench marks keep the datums alive.

Predicted versus observed

A tidal prediction is pure astronomy: the tide built from each harmonic constituent, led in most places by M2, the principal lunar wave. Full predictions are published for a reference station; a nearby subordinate station borrows them with time and height corrections.

The sea does not read the tables. The observed level minus the predicted one is the residual, and that is where weather lives. Onshore wind and low pressure pile up a storm surge. Offshore wind drags the water away in a set-down, and a really strong one gives a blowout tide that leaves boats sitting in mud at the dock. A king tide is no storm at all, just a big spring tide with the Moon close to Earth, and it is predicted to the minute.

On the survey boat

A hydrographic survey is where all of this becomes arithmetic. Every sounding the boat measures is taken from a moving water surface, so it needs tide correction to bring it down to chart datum. Far from the gauge, the crew applies tide zoning, splitting the area into zones that each shift the gauge record in time and height. On many jobs the tide gauge is bypassed altogether: an ERS takes heights straight from RTK satellite positioning and converts them to chart datum with a separation model.

Then come the boat's own corrections. The static draft is how deep the transducer sits at rest; squat is how much deeper it rides at survey speed; heave is the bobbing from waves. A bar check or a hand lead line confirms the sounder agrees with reality. Crosslines run across the main lines as an independent check, and any mismatch between the two is a bust that someone has to explain before the data leaves the boat.

What finally reaches the chart is shoal-biased: wherever depths are thinned out, the shallow one wins, and over a wreck or rock the chart shows its least depth. The chart is allowed to be pessimistic. It is never allowed to promise water that is not there.

Five words to learn first

• Chart datum: the zero under every charted depth, set low on purpose.
• MLLW: chart datum in US waters, mean lower low water.
• Minus tide: a low that falls below that zero.
• Residual: what the weather did to the predicted tide.
• Bust: two measurements of the same spot that disagree.

And when in doubt about any height or depth, ask the old hand's question first: "What datum?"