Definitions of Tidal Terms

Standard Ports
Standard Ports are those for which tidal predictions are provided in the form of daily tables giving the times and heights of high and low waters. All times in these tables are in New Zealand Standard Time. Predicted heights are in metres and are based on the Chart Datum of the largest scale chart of the place.
Secondary Ports
Secondary Ports are those for which daily predictions are not provided. Data sufficient for calculating times and heights at these ports and places are given after the Standard Port predictions in this book. Secondary Ports are grouped under Standard Ports with a similar tidal pattern.
Mean Sea Level (MSL)
The average level of the sea surface over a long period or the average level which would exist in the absence of tides.
Mean High Water Springs (MHWS)
& Mean Low Water Springs (MLWS)
The average of the levels of each pair of successive high waters, and of each pair of successive low waters, during that period of about 24 hours in each semi-lunation (approximately every 14 days), when the range of the tide is greatest (Spring Range).
Mean High Water Neaps (MHWN)
& Mean Low Water Neaps (MLWN)
The average of the levels of each pair of successive high waters, and of each pair of successive low waters, during that period of about 24 hours in each semi-lunation (approximately every 14 days), when the range of the tide is least (Neap Range).
Chart Datum (CD)
A water level so low that the tide will but seldom fall below it. When meteorological conditions are such that sea level is lowered, the tide will fall below the predicted low water heights, and at a place where Chart Datum is at a comparatively high level, the actual depths at or near low water may be considerably less than charted.
Highest & Lowest Astronomical Tide (HAT & LAT)
The highest and lowest tidal levels which can be predicted to occur under average meteorological conditions over 18 years. Modern chart datums are set at the approximate level of Lowest Astronomical Tide (LAT) and Tide Tables list the predicted height of tide above Chart Datum. It should be noted that water level may fall below the level of LAT if abnormal meteorological conditions are experienced.
Diagram illustrating tidal terms
Diagram illustrating tidal terms.

CAUTIONS

  • In certain circumstances a tide at low water may fall below the level of Chart Datum thus giving depths less than the charted depth.
  • The times predicted for high and low water can be affected by changes in the force and direction of the wind and by changes in the barometric pressure. It will generally be found that the heights are increased with onshore and decreased with off-shore winds. Sea level rises as the barometer falls, and vice versa, approximately 1 cm for each millibar difference from the average pressure.