In general, a datum is a baseelevation used as a reference from which to reckon heights or depths. Atidal datum is a standard elevation defined by a certain phase of thetide. Tidal datums are used as references to measure local water levelsand should not be extended intoareas having differing oceanographic characteristics withoutsubstantiating measurements. In order that they may be recovered whenneeded, such datums are referenced to fixed points known as benchmarks. Tidal datums are also the basis for establishingprivately owned land, state owned land, territorial sea, exclusiveeconomic zone, and high seas boundaries. Below are definitions of tidaldatums maintained by the Center for Operational Oceanographic Productsand Services.
Highest Astronomical Tide
Mean Higher High Water
Mean High Water
Diurnal Tide Level
Mean Tide Level
Mean Sea Level
Mean Low Water
Mean Lower Low Water
Lowest Astronomical Tide
Great Diurnal Range
Mean Range of Tide
Mean Diurnal High WaterInequality
Mean Diurnal Low Water Inequality
Greenwich High Water Interval
Greenwich Low Water Interval
Highest Observed Tide
Lowest Observed Tide
*Some locations have diurnal tides--one high tide and one low tide per day. At most locations, thereare semidiurnal tides--the tide cycles through a high and low twiceeach day, with one of the two high tides being higher than the otherand one of the two low tides being lower than the other.
The Datums page (example at right) provides access to established datums for the station indicated in the upper left corner of the page. The tidal datums on this page are referenced to an arbitrary station datum. In order to apply these datums for surveyingor coastal management they must be reducedto Mean Lower Low Water (MLLW), which is the reference datum for predictions, benchmark publication and nautical charting. Other tidal (Mean High Water(MHW)) and geodetic (North American Vertical Datum 1988 (NAVD 88)) datums may also be used for specific reasons. The time meridian (TM) isthe reference meridian used to calculate time. An epoch is a 19-year tidal cycle used to calculate datums. The present National Tidal Datum Epoch (NTDE) is 1983 through 2001. Tidal datums in certain regions with anomalous sea level changes (Alaska, Gulf of Mexico) are calculated on a Modified5-Year Epoch.
The data can be displayed in either feet or meters. Click the Submit button to apply the selected unit to the table of values for tidal datums. The heading of the table will reflect the units currently applied to the table. Link to other types of data for the same station using the links on the top. Links are highlighted only for data types that are available for the station.
The National Geodetic Survey (NGS) defines a geodetic datum as: 1. "A set of constants used for calculating the coordinates of points on the Earth." Generally a datum is a reference from which measurements are made. In surveying and geodesy, a datum is a reference point on the earth's surface against which position measurements are made, and an associated model of the shape of the earth for computing positions. Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude. Vertical datums are used to measure elevations or underwater depths.
Geodetic datum relationships to tidal datums are established at tide stations byconnecting tidal bench mark networks to the National Spatial ReferenceSystem (NSRS) maintained by NGS. There are two survey procedures usedto make this connection. The first is to connect the tidal bench markswith traditional differential levels to nearby geodetic bench markswith known geodetic elevations. The second is to occupy the tidalbench marks using a static GPS survey to determine the geodeticelevations of the bench marks directly. In all cases it is advised tomake the connections to more than one bench mark, preferably to threemarks, in order to confirm the connection and identify unstable benchmarks. The elevation relationship between geodetic datums andtidal datums should not be extrapolated away from a particular locationwithout correction or interpolation as the relationships vary withparameters such as variations in range of tide, bathymetry, topography,geoid variations, and vertical land movement. Any interpolation shouldbe done carefully, and where possible guided by the use of the National Ocean ServiceVDatum tool which can be obtained at: http://vdatum.noaa.gov.
Knowledge of tides and currents is still critical because today's vessels are much larger than the old sailing ships. The depths and widths of the channels in which they sail, and the increased marine traffic leaves very little room for error.
Tide and current data is available from NOAA's Center for Operational Oceanographic Products and Services website.
The tide tables provided by the National Oceanic and Atmospheric Administration (NOAA) are the most accurate tidal forecasts available.
An incoming tide, or rising tide, is considered one of the best fishing tide times. Water that enters an estuary area from the ocean can have a lower temperature, contain more oxygen, and have better clarity than the water that exists in the estuary during low tide or slack water periods.
Predicting and measuring tides and currents is important for things like getting cargo ships safely into and out of ports, determining the extent of an oil spill, building bridges and piers, determining the best fishing spots, emergency preparedness, tsunami tracking, marsh restoration, and much more.
Flooding and Generators
Spring tides, or especially high tides can sometimes endanger buildings and people near the shore, often flooding houses or wharfs. This is not a common occurrence since most buildings are constructed beyond the normal tidal range.
NOAA Tide Predictions allows users of the Tides and Currents website (http://tidesandcurrents.noaa.gov) to generate tide predictions for up to 2 years in the past or future, at any of 3000+ locations around the United States.
A tide gauge, which is one component of a modern water level monitoring station, is fitted with sensors that continuously record the height of the surrounding water level. This data is critical for many coastal activities, including safe navigation, sound engineering, and habitat restoration and preservation.
The Hydrographic Office EasyTide web site will provide free tidal predictions for the next 7 days for the locations shown on the map. The TideTimes web site provides the same predictions and is possibly easier to use. It will also provide free tidal data for past dates and sells printed tide tables.
- Marine Weather Forecast Pro. Weather.
- Buoycast: NOAA Marine Forecast. Weather.
- BoatUS - Boat Weather & Tides. Weather.
- NOAA Marine Weather. Weather.
- Nautide. Weather.
- Windfinder: Wind & Weather map. Weather.
- Diurnal Tide. ••• A diurnal tide has one episode of high water and one episode of low water each day. ...
- Semi-diurnal Tide. ••• A semi-diurnal tide has two episodes of equal high water and two episodes of low equal water each day. ...
- Mixed Tide. ••• ...
- Meteorological Tide. •••
You can look for visual clues like seaweed or wet sand in dry weather to tell if the tide is coming in or out. If the sand above the waterline is wet, you can usually assume that the tide is on its way out. If everything is dry, the tide is probably on its way in.
Typically, the best times to fish are when the tide is “running” — that is, when it's on the move from high to low or vice versa. Once the water approaches the extremes of high or low tide, it slows down and stops moving, becoming what's known as “slack,” and the fish are less likely to bite.
Because water is in movement when a tide is changing, these times are the best times to fish. A falling tide occurs when a tide changes from high tide to low tide and is the best time of the day to fish. The best time to take advantage of a falling tide is two hours before low tide.
Fish do bite when it rains, but their activity is generally lower during rain. Rain both oxygenates and colors the water, which are two very positive aspects when it comes to fishing, but the fish seem to need time to adjust to the sudden shift. That is why you should focus on fishing right after a rainfall instead!
Tidal currents are strongest in large water depths away from the coastline and in straits where the current is forced into a narrow area. The most important tidal currents in relation to coastal morphology are the currents generated in tidal inlets.
The incoming tide along the coast and into the bays and estuaries is called a flood current; the outgoing tide is called an ebb current. The strongest flood and ebb currents usually occur before or near the time of the high and low tides.
Tides, waves and currents are completely different. They form under different conditions and are influenced by different factors. Waves are somewhat more noticeable than tides and currents while tides can often be seen on the shore.
What do you think the cause of having tides around the Earth and why it affect the seafaring industry? ›
The moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides. High tide (left) and low tide (right) in the Bay of Fundy in Canada.
Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth (Sumich, J.L., 1996).
By moving heat from the equator toward the poles, ocean currents play an important role in controlling the climate. Ocean currents are also critically important to sea life. They carry nutrients and food to organisms that live permanently attached in one place, and carry reproductive cells and ocean life to new places.
Tides help trade by making rivers navigable for trading ships. During high tide, the depth of water at the mouth of some rivers increase. This helps the ships to reach ports located on those rivers. Tides help the fishermen too.