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.

'[8] Mean lower low water (MLLW) is the average height of the lowest tide recorded at a tide station each day during a 19-year recording period, known as the National Tidal Datum Epoch as used by the United States' National Oceanic and Atmospheric Administration.

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.

Spring tides are those when the moon is in a direct alignment with the sun (thus new or full) and in many extra-tropics places when its declination is 23.5°, its maximum.