UK NATIONAL TIDE GAUGE NETWORK - Report on Mean Sea Level and Z0 results from tidal analyses of data for the period Jan 1995 to Dec 2025, incl

The National Oceanography Centre (NOC) has provided national and international tidal prediction services for almost 100 years. Modern tidal prediction methods used around the world are based on the pioneering work of Dr Arthur Doodson, a former director of the institute. NOC has continued to develop and refine these analytical techniques and now makes them available through its software products. Poltips is NOC's commercially available tidal prediction software, designed to generate fully formatted tide tables.

Tides are the regular and predictable movements of water caused by astronomical forces, specifically the relative positions and motions of the Earth, Moon and Sun. These movements generate a series of known frequencies within the tide, each referred to as a tidal harmonic constituent.

Harmonic analysis is used to identify these constituents and determine their amplitude (size) and phase (timing). These pairs of values, known as harmonic constants, are unique to each location.

The harmonic constants define a series of sine waves centred around a zero reference level. Where observed tidal data is not centred on zero, a difference exists between the zero reference and the mean water level of the observations. This value is known as Z0 and must be added to the predicted tide levels to align them with the observed data.