Although the details vary around the world’s hydrographic agencies, the representation, on a chart or in a sailing pilot, of the height of the tide follows similar conventions throughout. A number of representative levels can be identified with reference to tidal curves at Flat Holm along the Bristol Channel in the South West of England (Figure 5.3) and to Figure 5.4:
• Highest astronomical tide (HAT). The height of the highest occurring water level under tidal forcing (i. e. excluding freshwater, meteorological, and storm surges).
• Mean high water Springs (MHWS). The height of an average Spring tide high water level.
• Mean high water Neaps (MHWN). The height of an average Neap tide high water level.
• Mean sea level (MSL). The mean water depth at the site.
• Mean low water Neaps (MLWN). The height of an average Neap tide low water level.
• Mean low water Springs (MLWS). The height of an average Spring tide low water level.
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Lowest astronomical tide (LAT). The height of the lowest occurring water level under tidal forcing (i. e. excluding freshwater, meteorological, and storm surges). LAT usually corresponds to the chart datum (CD) on hydrographic charts.
Figure 5.4 Tidal heights on a vertical scale (left) and currents on a speed scale (right). The height data are for Flat Holm in the Bristol Channel, and the currents are for the nearby tidal diamond.
There is then the local topographic datum to which maps are referenced. This would usually correspond to mean sea level, so that hydrographic charts display the difference between chart datum and topographic datum. In the United Kingdom, the topographic datum is called the ordnance survey datum Newlyn, and is referenced to mean sea level at Newlyn in Cornwall (Pugh, 2006).
For the present purposes, we define a corresponding set of parameters to describe the tidal currents at a site (Figure 5.4):
• Peak flood current (PFC). The speed of the fastest occurring flood current under tidal forcing (i. e. excluding freshwater, meteorological, and storm surges).
• Mean Spring peak flood (MSPF). The speed of an average Spring tide peak flood current.
• Mean Neap peak flood (MNPF). The speed of an average Neap tide peak flood current.
• Slack water current (SWC). Usually zero.
• Mean Neap peak ebb (MNPE). The speed of an average Neap tide peak ebb.
• Mean Spring peak ebb (MSPE). The speed of an average Spring tide peak ebb current.
• Peak ebb current (PEC). The speed of the fastest occurring ebb current under tidal forcing (i. e. excluding freshwater, meteorological, and storm surges).
In North America, the National Ocean Survey (NOS) is responsible for tidal measurements and for the definition of tidal datums. In San Francisco, for example, the average height of the higher of the two daily high tides is called local mean higher high water (MHHW) (CERES, 2007). The average of all the high tides is called local mean high water (MHW). There are many other datums, including mean lower low water (MLLW), mean low water (MLW), and mean tide level (MTL), which is midway between MHW and MLW. Mean sea level (MSL) is the average of all the tide measurements. Local MLLW is the zero datum of the tides. Values for mean sea level in 1929 were adopted as the national geodetic vertical datum (NGVD 29), or zero elevation for measuring land height. Benchmarks were established throughout the United States, marking local elevations relative to NGVD 29. Since then, disturbance and loss of many benchmarks has warranted a new datum. The NGVD 29 is being replaced by the North American vertical datum of 1988 (NAVD 88), and new geodetic datums are being planned to make use of improved surveying technology.
