Tidal Levels are standard heights which are determined from the shape of the graph of tide height against time over a fixed duration at a specific location. See Notes 1,2 below. There are many differing types of tidal levels in use throughout the world. Some are more appropriate for diurnal or mixed tides, and some more relevant to semi-diurnal tides.


Tidal Levels are frequently used as vertical datums by hydrographers and surveyors, in cartography and in planning flood defences etc.


As part of its analysis process GeoTide Analyzer derives the following tidal levels.

HAT: Highest Astronomical Tide

MHWS: Mean High Water Springs

MHHW: Mean Higher High Water

MHW: Mean High Water

MLHW: Mean Lower High Water

MHWN: Mean High Water Neaps

MSL (ML) : Mean Sea Level

MLWN: Mean Low Water Neaps

MHLW:Mean Higher Low Water

MLW: Mean Low Water

MLLW: Mean Lower Low Water

MLWS: Mean Low Water Springs

LAT: Lowest Astronomical Tide.

GeoTide can improve upon the accuracy which would be obtained by a simple examination of past tidal records because i) the harmonic analysis process automatically disregards the meteorological and other noise in the tide and ii) its analysis can extend over the required 18.61 year time period - even though the source tidal data may be considerably shorter than this.


Of course this does not obviate the need for as much accurate historical data as possible. In order to gain a meaningful accuracy for these figures at least one month of very high quality data is required and preferably more.



1. The tide referred to here is the "Astronomical Tide" - which is that part of the tide which is caused by the Moon and the Sun. The other, generally smaller part of the tide, is caused by the weather (e.g. wind and pressure) and is known as the Meteorological Tide". In most places the astronomical tide dominates.

2. Tidal Levels are often defined over a time period of one lunar nodal cycle i.e. over an 18.61 year period.