Definitions of tidal levels and other parameters | National Tidal and Sea Level Facility National Tidal and Sea Level Facility Tides Storm surges Products About About tides Tides – questions and answers Sea level – questions and answers The Doodson-Légé Tide Predicting Machine Tide clocks and watches Tidal river bores The River Dee / Afon Dyfrdwy The River Mersey UK National Tide Gauge Network Data availability Tidal predictions – UK and Ireland UK South Atlantic Network Gibraltar sea level station Tidal predictions – South Atlantic Tide gauge instruments Definitions of tidal levels Chart datum and ordnance datum Permanent Service for Mean Sea Level (PSMSL) (external link) Engineers’ page Proudman Building, Liverpool weather station About storm surges Tide-surge model Surge model forecast Surge model archive Surge model archive: 2004–2019 Surge model ensemble forecasting Skew surges Skew surge history: England – south Skew surge history: England – east Skew surge history: England – west Skew surge history: Wales Skew surge history: Scotland Skew surge history: Northern Ireland Skew surge history: Isle of Man Skew surge history: Channel Islands Sea level trends Sea level trend charts Software Definitions of tidal levels and other parameters Latitude, Longitude and Grid reference The latitude of a point on the earth is its angular distance north or south of the equator, and the longitude is its angular distance east or west of the Greenwich meridian (in degrees). The grid reference of the same location is expressed as eastings and northings (distance in metres from an origin located south-west of the Isles of Scilly) on the British National Grid used by the Ordnance Survey in its mapping of Great Britain. The location referred to is of the measuring point, which can either be a pressure point or stilling well. In some instances the measuring point may be located at a slightly different location to that of the instrumentation. If there is a significant difference in the location of the two latitudes then the position of the measuring point will also be shown. Earliest data This is the earliest record found of a tide gauge being in operation at the site. The charts obtained from the tide gauge may still be held by the operating authority that was responsible at the time, or they may be archived in the local authority’s archives or by the British Oceanographic Data Centre (BODC) , and may have been digitised to form a continuous data set together with data obtained from electronic dataloggers. Data from early gauges may have been lost, be incomplete or have gaps due to changes in ownership over the years. Before the UK Tide Gauge Network was established in 1976, calibration and maintenance of the gauges were the responsibility of the operating authority. Therefore the accuracy of the data may be unknown. Further information may be available in the site histories. HAT – Highest astronomical tide LAT – Lowest astronomical tide Highest astronomical tide (HAT) is the highest level, and Lowest astronomical tide (LAT) the lowest level that can be expected to occur under average meteorological conditions and under any combination of astronomical conditions. HAT and LAT are not extreme levels, as certain meteorological conditions can cause a higher or lower level, respectively. The level under these circumstances is known as a ‘storm surge’ (‘negative surge’ in the case of level lower than LAT). HAT and LAT are determined by inspecting predicted sea levels over a number of years. Note: The values of HAT and LAT may not be the same as in other reference sources. The value given in any source depends on the years being inspected, the period covered and the exact location and calibration of the tide gauge used. The values listed here have been produced from predictions over a 19-year period from the gauges, which are maintained and calibrated to a uniform standard. MHWS – mean high water springs MLWS – mean low water springs The height of mean high water springs is the average throughout the year (when the average maximium declination of the moon is 23.5°) of two successive high waters during those periods of 24 hours when the range of the tide is at its greatest. The height of the mean low water springs is the average height obtained by the two successive low waters during the same period. MHWN – mean high water neaps MLWN – mean low water neaps The height of mean high water neaps is the average throughout the year (when the average maximium declination of the moon is 23.5°) of two successive high waters during those periods of 24 hours when the range of the tide is at its least. The height of the mean low water neaps is the average height obtained by the two successive low waters during the same period. Note : The values of MHWS, MLWS, MHWN and MLWN vary from year to year with a cycle of approximately 18.6 years. Mean spring and neap ranges The mean spring range is the difference between mean high water springs (MHWS) and mean low water springs (MLWS). The mean neap range is the difference between mean high water neaps (MHWN) and mean low water neaps (MLWN). Highest and lowest levels The highest and lowest tides shown for the particular years is an indication of the maximium levels under average meteorological conditions which can be expected during that year. The indicated levels have been calculated from predictions over a 19-year period. Note that the absolute tidal levels for this period will be classified as HAT and LAT. No further dates have been given in the years since tidal levels of the same amplitude may occur in the spring, autumn and on more than one occasion.