Tides, Datums, and Water-Level Correction

Why Datums Matter in Hydrographic Surveying

When you measure water depth with an echo sounder, the number you record is the distance from the transducer to the bottom at that exact moment. The problem is that water surfaces move constantly due to tides, wind setup, and seasonal lake-level changes. Two soundings taken at the same location six hours apart during a tidal cycle will produce different raw readings even though the bottom has not moved. To make those readings meaningful and comparable, you must reduce them to a common reference surface called a vertical datum. This lesson explains the datums used in coastal and inland hydrography, shows you how to read a tide gauge record, and walks through the zonal tidal correction procedure.

Common Vertical Datums in US Hydrography

Mean Lower Low Water (MLLW) is the standard chart datum for US coastal and tidal waters. It is defined as the average of the lower of the two daily low-water heights, computed over the National Tidal Datum Epoch (a 19-year averaging period maintained by NOAA). Because MLLW represents a low-water surface, charted depths referenced to it tend to be conservative — a navigator who trusts a MLLW-referenced chart will not be surprised by shallower water than expected during normal tidal conditions. All NOAA nautical charts of coastal and tidal waters use MLLW as the zero reference.

Mean Low Water (MLW) is the average of all low-water heights over the same epoch. In areas with a mixed semidiurnal tide (two unequal low waters per day), MLW is slightly higher than MLLW. Understanding this distinction matters when comparing historical charts that may have used different conventions.

Mean High Water (MHW) is the average of all high-water heights. It is commonly used as a legal boundary for shoreline ownership in many US states and appears in coastal property descriptions.

North American Vertical Datum of 1988 (NAVD 88) is the standard land-based vertical datum for the contiguous United States, Alaska, and Canada. It is defined by a single fixed point at Pointe-au-Père, Québec, and is realized through a network of bench marks tied by geodetic leveling. NAVD 88 is the datum used for topographic maps, digital elevation models (DEMs), and most land surveying work. It is not the same as MLLW. The separation (offset) between NAVD 88 and MLLW varies by location and must be determined from NOAA CO-OPS tidal benchmark data for your specific project area before you can merge bathymetric and topographic datasets.

International Great Lakes Datum of 1985 (IGLD 85) applies to the Great Lakes system and connecting waters. Because the Great Lakes are not tidal, there is no tidal datum like MLLW. Instead, IGLD 85 is a dynamic-height datum tied to mean water level at Pointe-au-Père and is used for water-level regulation, navigation charts on the Lakes, and hydrographic surveys in that region. Chart datum on the Great Lakes is set at Low Water Datum (LWD), a fixed plane based on historical low-water records for each lake, referenced through IGLD 85.

Reading a NOAA CO-OPS Tidal Gauge Record

NOAA's Center for Operational Oceanographic Products and Services (CO-OPS) maintains hundreds of water-level stations around the US coast and Great Lakes. At tidesandcurrents.noaa.gov you can download verified water-level data as a CSV file. A typical CSV contains columns for date, time (UTC or local), and water level in meters or feet referenced to MLLW.

When you import that file into a spreadsheet and plot water level versus time, you see the familiar sinusoidal tidal curve. Key features to identify are:

• High water peaks and low water troughs
• The time of each peak and trough (needed for interpolation)
• Any anomalies caused by storms, which can raise or depress water levels well above or below predicted tides

Always note whether the downloaded data are predicted (astronomical model) or verified (actual gauge readings). Use verified data whenever available for tidal corrections; predicted data may differ from observed water levels by significant amounts during weather events.

The Zonal Tidal Correction Method

A tidal correction converts a raw (observed) sounding depth to a depth referenced to the chart datum (MLLW for coastal work). The relationship is:

Corrected depth = Raw sounding depth + Water-level correction

where the water-level correction at the time of the sounding equals the gauge reading at that moment referenced to MLLW. If the gauge reads +1.2 m above MLLW when a sounding of 4.5 m is recorded, the corrected depth is 4.5 + 1.2 = 5.7 m below MLLW. (Be careful with sign conventions; always define positive as up for water level and positive as down for depth.)

When you do not have a gauge reading at the exact second of each sounding, you linearly interpolate between the two bracketing gauge readings:

Water level at time T = WL₁ + [(T − T₁) / (T₂ − T₁)] × (WL₂ − WL₁)

where T₁ and T₂ are the times of the two nearest gauge records and WL₁, WL₂ are the corresponding water levels.

The "zonal" part of the method refers to dividing a large survey area into zones, each controlled by the nearest appropriate gauge, so that spatial variation in tidal phase is handled correctly. For a small survey near a single gauge, one zone is usually sufficient.

Safety Notes

Hydrographic survey fieldwork involves working near or on water. Always wear a properly fitted US Coast Guard–approved personal flotation device (PFD) when operating from a vessel or working near the water's edge. Establish a float plan before departure and file it with a responsible person ashore. Be aware of boat traffic, submerged hazards, and weather changes. These fieldwork rules are reinforced in the companion field sessions; the datum and correction concepts in this lesson are foundational before you go on the water.