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Vertical control
Published in W. Schofield, M. Breach, Engineering Surveying, 2007
In the UK the MSL datum was measured and established by the Ordnance Survey (OS) of Great Britain, and hence it is often referred to as Ordnance Datum (OD). It is the mean level of the sea at Newlyn in Cornwall calculated from hourly readings of the sea level, taken by an automatic tide gauge over a six-year period from 1 May 1915 to 30 April 1921. The readings are related to the Observatory Bench Mark, which is 4.751 m above the datum. Other countries have different datums; for instance, Australia used 30 tidal observatories, interconnected by 200 000 km of levelling, to produce their national datum, whilst just across the English Channel, France uses a different datum, rendering their levels incompatible with those in the UK.
Site Works
Published in Roy Chudley, Roger Greeno, Karl Kovac, Chudley and Greeno’s Building Construction Handbook, 2020
Roy Chudley, Roger Greeno, Karl Kovac
Altitude zero is taken at mean sea level. This varies between different countries, but for UK purposes it was established at Newlyn in Cornwall from tide data recorded between May 1915 and April 1921. Relative levels, defined by bench marks, are located throughout the country. The most common are identified as carved arrows cut into walls of permanent structures. Reference to Ordnance Survey maps of an area will indicate bench mark positions and their height above sea level, hence the name ordnance datum (OD).
Maps, drawings and surveying
Published in Malcolm Copson, Peter Kendrick, Steve Beresford, Roadwork, 2019
Malcolm Copson, Peter Kendrick, Steve Beresford
The term ‘levelling’ is used to describe the operation of finding the difference in height between points on the earth’s surface. These heights are given relative to a plane or datum which is known as the Ordnance Datum (OD). This is the mean level of the sea and is measured at Newlyn, Cornwall. Levelling is only accurate over relatively short distances as the line of sight is a tangent to the earth’s surface and longer sights would need to be corrected to take account of the earth’s curvature as in Figure 2.45.
A framework for assessing the remaining life of storm surge barriers
Published in Structure and Infrastructure Engineering, 2023
Hidde Vader, Alexander M. R. Bakker, Sebastiaan N. Jonkman, Martine van den Boomen, Esther van Baaren, Ferdinand L. M. Diermanse
The functional performance with respect to flood protection was evaluated by comparing the extreme water levels behind the Hollandsche IJssel barrier with a critical water level. This critical water level should not be exceeded to limit the loads on the dikes along the Hollandsche IJssel. For this study, the critical water level was assumed to be the water level above which the dikes along the Hollandsche IJssel may fail. The functional performance of the Hollandsche IJssel barrier must be such that the exceedance probability of this water level is less than 1/30,000 per year. Based on the water level statistics in front of the storm surge barrier and the non-closure probability of the barrier, the exceedance probabilities of the water levels behind the barrier, and thus the critical water level, can be determined using the open-source, probabilistic model Hydra-NL.1 The resulting critical water levels corresponding to a non-closure probability of 1/200 and 1/1000 per closure are NAP +3.04 m and NAP +2.90 m, respectively (NAP = Dutch Ordnance Datum, which approximately corresponds to mean sea level). These critical water levels are further referred to as assessment water levels to avoid any confusion with other critical water levels.
X-band radar as a tool for monitoring natural coastal behaviour and potential development impacts
Published in Journal of Operational Oceanography, 2019
Steven Wallbridge, Tony Dolphin, Colin J. L. Taylor
The base data used in this paper were the 64-image averages collected twice per tide at 0 m ODN (Ordnance Datum Newlyn — approximately mean sea level at Sizewell). The maximum resolution of both detection methods was limited to the pixel size (4.69 m). Methods yielding sub-pixel accuracy, such as the mean water level shoreline position over seven days, and interpolating (curve-fitting) through successive detections over each mid-tide, were also tested. Each method yielded slightly different (sub-pixel scale) shoreline position estimates, but longshore and temporal variability due to normal pixel ‘speckle’ was not reduced — thus, the increased resolution does not translate to increased accuracy.