10.1 Hydrographic and Tidal Datum Surveys

The January 2025 California Professional Land Surveyor test plan places hydrographic surveys in Domain III, Field Operations and Investigations, as professional activity 7, with bathymetric work and tidal-datum recovery as the examples. This OpenExamPrep chapter is independent study material for that field work. It is not a Board publication and does not claim official approval. Legal holdings on navigability, public trust, accretion, and ownership of the bed belong in a later analysis chapter. Here the job is to measure water surface, bottom, and tidal-datum elevations so someone else can apply the law to a complete record.

Why the water-level datum is the survey

A depth reading from an echo sounder is the two-way travel time of a pulse through water, scaled by a sound-speed assumption, from a transducer that sits some distance below a moving water surface. Until you name the water-level surface you reduced to, the number is not a map elevation. California harbor, wetland, and shoreline jobs mix three families of surfaces:

• Tidal datums defined from observed water levels at a tide station over a 19-year National Tidal Datum Epoch (NTDE)
• Geodetic datums such as NAVD88, a leveled orthometric surface that is not a phase of the tide
• Project or chart datums named in the scope (often MLLW for navigation products, MHW or MHHW for some coastal mapping, NAVD88 for flood, levee, and engineering work)

NOAA's datum glossary is the vocabulary the exam expects you to use. Mean higher high water (MHHW) is the average of the higher high-water height of each tidal day over the NTDE. Mean high water (MHW) is the average of all high-water heights over the NTDE. Mean tide level (MTL) is the arithmetic mean of MHW and MLW. Mean sea level (MSL) is the arithmetic mean of hourly heights over the NTDE. Mean low water (MLW) averages all low waters. Mean lower low water (MLLW) averages the lower low of each tidal day. For a short series, NOAA derives an equivalent NTDE datum by simultaneous comparison with a control tide station. Do not invent a 30-day average and call it MHW.

MSL is a tidal statistic at a station. NOAA states that it should not be confused with NAVD88. The relationship between any tidal datum and NAVD88 is station-specific. You recover it from the published datum sheet and bench-mark sheet, or from a documented transform such as VDatum, not from a California-wide rule of thumb.

Published California examples (verify the live sheet on the job)

The numbers below come from NOAA CO-OPS accepted datum pages referenced to MLLW, NTDE 1983–2001, in feet. They illustrate the local nature of the transform; they are not a substitute for the sheet of the station that actually controls your reach.

At San Francisco, MHW sits 5.29 ft above NAVD88 (5.23 − (−0.06)). At San Diego, MHW sits 4.55 ft above NAVD88 (4.98 − 0.43). That is about 0.74 ft of difference between two California ports in the MHW-to-NAVD88 offset. Transferring San Francisco's MLLW-to-NAVD88 value into San Diego Bay is a datum error, not a rounding choice.

NOAA also warns that a tidal datum is a local reference and should not be extended into waters with different oceanographic characteristics without substantiating measurements. A Golden Gate sheet does not automatically control a back-bay slough, a riverine reach with a slope, or a lagoon with a restricted inlet. If the project needs a local MHW or MLLW, occupy a subordinate gauge or staff, collect simultaneous observations with the control station, and reduce the short series by the NOAA comparison method (or an equivalent documented procedure).

The present NTDE is 1983 through 2001. NOAA is preparing the next 19-year epoch from water-level data spanning 2002 through 2020, with a currently proposed release of new datum products in 2029. Field notes must name the epoch on the sheet you used. After the switch, the same physical bench mark will carry new tidal elevations. Highest astronomical tide (HAT) on NOAA sheets is a predicted extreme over a 40-year window (present values based on 2000–2040), not a mean datum and not a property line.

Bathymetry: what the field crew actually measures

A California PLS hydrographic crew typically builds a reduced sounding from four concurrent observations:

1. Horizontal position of the sounding, usually GNSS on the vessel with lever arms to the transducer, often with an inertial measurement unit for heading, pitch, and roll.
2. Raw depth from a single-beam echo sounder, multibeam sonar, bathymetric LiDAR in clear shallow water, or a pole/lead line where sonar is unreliable.
3. Sound speed in the water column (probe, profiler, or surface sound-speed sensor plus casts), because a wrong speed tilts multibeam rays and scales every depth.
4. Water-level corrector from a gauge, staff, or GNSS buoy reduced to the named datum, plus vessel draft, squat/settlement, and latency.

Work the reduction in this order in the notes even if software applies it in a pipeline: apply sensor offsets and attitude; convert travel time with the sound-speed profile; reduce the instantaneous water surface to the project datum; then grid. NOAA's Hydrographic Surveys Specifications and Deliverables treat sound-speed, tide, and offset correctors as part of the sounding's pedigree. Cross-lines, bar checks or patch tests, and a comparison against independent pole soundings in the shoal zone are how you prove the pipeline, not optional extras.

Predicted tides from a harmonic station are a planning tool. They are not a substitute for a water-level time series when the contract or the physics of the site (wind setup, river discharge, seiche) can move the surface by more than the depth tolerance. If you must zone tides between two gauges, document the time offset and range ratio from simultaneous data, not from a guessed lag.

Field sequence that survives review

• Recover or set onshore control; level or GNSS-level the staff zero to published tidal bench marks or to NAVD88, and write both numbers.
• Name the reduction datum in the first line of the sounding log (example: "depths reduced to MLLW, NTDE 1983–2001, using subordinate staff compared to 9414290").
• Record transducer draft at the start and end of the day, and after fuel or passenger changes.
• Cast for sound speed when the water mass changes (harbor mouth, thermocline afternoon, freshwater lens after rain).
• Run check lines that cross mains at 45–90 degrees and compare residuals to the project total vertical uncertainty.
• Keep the unreduced time series. A sounding that cannot be recomputed from raw range, attitude, and water level is not a defensible hydrographic survey.

California mixed semi-diurnal tides produce two highs of unequal height. That is why MHHW is not MHW. A crew that sets a "high tide" shot on a single afternoon high has captured an event, not a datum. Datum recovery is either a published elevation on a tidal bench mark or a reduced series. Event shots still belong in the notes as physical water evidence; they just must be labeled as events.

When the product is a shoreline at MHW, the hydrographic step is to realize the MHW elevation in the project vertical system, then intersect that elevation with a dense shore profile (rod, scanner, or photogrammetry) collected at a known stage. Walking the wrack line is not a tidal-datum survey. Section 10.3 covers those physical marks as evidence. This section's product is a named surface with a published or observed definition.