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PORP: Tidal and Water Level Datums, Reduction, and Predictions

PORP · Work through your exam, one topic at a time.

All guide sections

Tidal and Water Level Datums, Reduction, and Predictions

Domain focus

This domain tests tidal datums, water-level reduction, and predictions-the difference between a sensor reading and a charted or predicted height.

Core rules

  • Station datum is the local reference for observations; tidal datums (MLLW, MHW, MSL) are computed from a 19-year National Tidal Datum Epoch (NTDE). NAVD88 is a geodetic vertical datum, not a tidal datum.
  • Short-term stations are reduced to tidal datums by simultaneous comparison with a long-term control station; the method transfers the control station's datum relationships using the short record.
  • Harmonic analysis yields constituents (e.g., M2, S2, N2, K1, O1); predictions sum constituents plus node factors. Weather-driven surge and seiche are not in astronomical predictions.
  • Chart datum in U.S. waters is generally MLLW; depths and bridge clearances depend on the correct datum.
  • Always state datum, units, and time zone with a water-level value.

Worked example

A new gauge has 90 days of data. You compare simultaneous hourly heights with a nearby long-term control station, compute mean differences for the common period, and apply the control station's MLLW offset to the short-term station. You do not simply subtract mean sea level from the observed series.

Traps

Calling NAVD88 a tidal datum; using MSL as chart datum; applying a 19-year epoch to a 3-month record without reduction; confusing predicted tide with observed water level; ignoring meteorological effects.