An inventory of tide prediction machines

This report provides an update to two ‘inventories’ made many years ago of tide prediction machines (or ‘Kelvin machines’). It provides a list of all the machines that were made between the late-19th and mid-20th centuries, up until the 1960s when the advent of digital computers in tide table production made these remarkable analogue computers redundant. Photographs and brief descriptions of each machine are given. It will be seen that most of the machines still exist, and many are displayed in museums around the world, where they can be inspected by people interested in the history of tidal science

Differences between mean tide level and mean sea level

This paper discusses the differences between mean tide level (MTL) and mean sea level (MSL) as demonstrated using information from a global tide gauge data set. The roles of the two main contributors to differences between MTL and MSL (the M4 harmonic of the M2 semidiurnal tide, and the combination of the diurnal tides K1 and O1) are described, with a particular focus on the spatial scales of variation in MTL–MSL due to each contributor. Findings from the tide gauge data set are contrasted with those from a state-of-the-art global tide model. The study is of interest within tidal science, but also has practical importance regarding the type of mean level used to define land survey datums. In addition, an appreciation of MTL–MSL difference is important in the use of the historical sea level data used in climate change research, with implications for some of the data stored in international databanks. Particular studies are made of how MTL and MSL might differ through the year, and if MTL is measured in daylight hours only, as has been the practice of some national geodetic agencies on occasions in the past

Seiches in the eastern Caribbean

Data from a network of tide gauges in the eastern Caribbean have been used to investigate the periods and amplitudes of seiches at each station, and the coherence in seiche activity between stations. Seiches in this area have very small amplitudes, but they can be readily identified in the sea level spectra obtained from high-quality, high-frequency tide gauges. Most of their periods are found to be determined by the quarter-wavelength response of the adjacent shelf, and their Q values are generally consistent with them being relatively undamped. At several stations, the forcing responsible for the seiches is clear. At others, the forcing is harder to pin down, seiches appearing seemingly randomly, and resulting in what has been called ‘continuous seiche’. Overall, it is demonstrated that seiches (or at least their statistics) need to be better understood and to be included effectively in studies of extreme sea levels. Also related to shelf resonance, two possible examples of ‘tidal ringing’ have been identified

Manuel Johnson's tide record at St. Helena

The astronomer Manuel Johnson, a future President of the Royal Astronomical Society, recorded the ocean tides with his own instrument at St. Helena in 1826–1827, while waiting for an observatory to be built. It is an important record in the history of tidal science, as the only previous measurements at St. Helena had been those made by Nevil Maskelyne in 1761, and there were to be no other systematic measurements until the late 20th century. Johnson's tide gauge, of a curious but unique design, recorded efficiently the height of every tidal high and low water for at least 13 months, in spite of requiring frequent re-setting. These heights compare very reasonably with a modern tidal synthesis based on present-day tide gauge measurements from the same site. Johnson's method of timing is unknown, but his calculations of lunar phases suggest that his tidal measurements were recorded in Local Apparent Time. Unfortunately, the recorded times are found to be seriously and variably lagged by many minutes. Johnson's data have never been fully published, but his manuscripts have been safely archived and are available for inspection at Cambridge University. His data have been converted to computer files as part of this study for the benefit of future researchers

Why we must tie satellite positioning to tide gauge data

Accurate measurements of changes in sea and land levels with location and time require making precise, repeated geodetic ties between tide gauges and satellite positioning system equipment

A century of sea level measurements at Newlyn, SW England

The Newlyn Tidal Observatory is the most important sea level station in the UK. It commenced operations in 1915 as part of the Second Geodetic Levelling of England and Wales, and the mean sea level determined from the tide gauge during the first six years (May 1915-April 1921) defined Ordnance Datum Newlyn (ODN) which became the national height datum for the whole of Great Britain. The 100 years of sea level data now available have contributed significantly to many studies in oceanography, geology and climate change. This paper marks the centenary of this important station by reviewing the sea level (and, more recently, detailed land level) measurements and Newlyn’s contributions to UK cartography, geodesy and sea-level science in general. Recommendations are made on how sea and land level measurements at Newlyn might be enhanced in the future