Chart datum

[1] Common chart datums are lowest astronomical tide (LAT)[1] and mean lower low water (MLLW).One advantage of using LAT for chart datums is that all predicted tidal heights must then be positive (or zero) avoiding possible ambiguity and the need to explicitly state sign.The 19-year recording period is the nearest full year count to the 18.6-year cycle of the lunar node regression, which has an effect on tides.[10] This is an average of lowest low waters taken over a fixed period of tidal predictions, as opposed to actual observations.This is the datum used for coastal charts published by the Canadian Hydrographic Service,[11] with the average taken from the lowest tides, one from each 19 year period of tidal predictions.
U.S. civil and maritime uses of tidal data
water levelorigindepthsnautical charttidal phaseBaltic Seamean sea levelvertical datumhorizontal datumtidal phasesUnited Kingdom Hydrographic OfficeAustralian Hydrographic Serviceaveragenormal tidal limitNational Tidal Datum EpochAustraliaNational Oceanic and Atmospheric Administrationlunar nodeCanadian Hydrographic Serviceair draftSpring tidesextra-tropics placesdeclinationBanda Seatidal rangehigh watershigh water markamphidromic pointsdrying heightsnautical chartsvertical clearancesmean high water springsTide tablesgeodetic datumGeodetic Reference System 1980satellite navigationVertical Offshore Reference FramesNaval Hydrographic and Oceanographic ServiceReference water levelsProudman Oceanographic Laboratory