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Tide and Water Level Datums May 01, 2012

Tide and Water Level Datums May 01, 2012.

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Tide and Water Level Datums May 01, 2012

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  1. Tide and Water Level Datums May 01, 2012

  2. The hydrographer must be able to relate all measured depths, regardless of the stage of tide or level of water at the time of sounding, to a common plane or datum. The datum used to reckon heights or depths for marine applications is a vertical datum called a ‘water level datum’ The water level datum to which the soundings on any particular survey are referred is known as the ‘sounding datum’. The datum to which depths on a chart are referred is known as the ‘chart datum’. The Chart Datum should generally be a datum plane such that: - the water level will but seldom fall below it. - it will not be so low as to cause the charted depths to be unrealistically shallow - it should vary only gradually form area to area and from chart to adjoining chart, so as to avoid significant discontinuities

  3. A water level datum is called a ‘tidal datum’ when defined in terms of a certain phase of tide. In United States coastal waters, Mean Lower Low Water (MLLW) is used for both sounding and chart datums. For tidally derived datums, most datums are computed over, or referred to, specific 19 year periods or tidal datum epochs. The 19 year period is important because of the 19 year modulation of lunar constituents by the long-term variation in the plane of the Moon’s orbit called the regression of the moon’s nodes.

  4. CORRECTIONS TO ECHO SOUNDINGS (HYDRO)

  5. In United States coastal waters, Mean Lower Low Water (MLLW) is used for both sounding and chart datums. MLLW is computed from tabulation of the observations of the tide, in this case the average of the lower low waters each tidal day over a 19 year period. The United States presently refers all tidal datums computed from tide observations to the 1983-2001 National Tidal Datum Epoch (NTDE) and updates to new NTDEs only after analysis of relative mean sea level change. In contrast, some Chart Datums are derived from harmonic analyses of observations and constructing time series of tide predictions over 19 year periods. The Canadian Chart Datum is the surface of Lower Low Water, Large Tide or LLWLT which encompasses the previously used datum of Lowest Normal Tide (LNT). British Charts now use a Chart Datum of Lowest Astronomical Tide (LAT) based on the lowest predicted tide expected to occur in a 19 year period. LAT is determined at a particular location by performing a harmonic analysis of the observations, then using the resulting harmonic constituents in a prediction equation to predict the elevation of the lowest predicted tide to occur over a 19 year period. Use of LAT has been adopted for international use by the International Hydrographic Organization (IHO).

  6. Computation of LLWLT Dochet Island 8410834 Step 1: Gather hourly heights for the current 19 year epoch period based on predictions Step 2: Identify the lowest value for each individual year during the 19 year time period Step 3: average the 19 year values The Canadian Chart Datum of Lower Low Water, Large Tide (LLWLT) is not equivalent to the US. Mean Lower Low Water (MLLW).  LLWLT is a much more conservative datum (lower) than MLLW.  By definition from the Canadian Tidal Manual (1983 ed.), LLWLT is determined from the average of the lowest low waters, one from each of 19 years of predictions.  Thus it is determined by predicting the tide for each year over a 19-year time period, selecting the individual lowest low water elevation predicted during that year, and averaging the individual lowest predicted tide each year for 19 years.  It is the average of 19 values.  The specific 19-year period is not significant when using predicted tides.MLLW is based on observed tides and is this average of the lower low waters observed each tidal day over a 19-year period.  In practice it is derived from the average of monthly mean lower low waters or the average of 228 values.

  7. NATIONAL TIDAL DATUM EPOCH • A specific 19 year period that includes the longest periodic tidal variations caused by the astronomic tide-producing forces. • Averages out long term seasonal meteorological, hydrologic, and oceanographic fluctuations. • Provides a nationally consistent tidal datum network (bench marks) by accounting for seasonal and apparent environmental trends in sea level that affects the accuracy of tidal datums. • The NWLON provides the data required to maintain the epoch and make primary and secondary determinations of tidal datums.

  8. TIDES SUPPORT TO NAUTICAL CHARTING HYDROGRAPHY APPLICATIONS SOUNDINGS ARE REFERRED TO MLLW HEIGHTS ARE REFERRED TO MHW

  9. Comparison of US Tidal Datums with Lowest and Highest Astronomical Tide (LAT, HAT)

  10. Other Chart Datums Harmonic analyses have also been used to determine other Chart Datums. Chart Datums used on some older British Admiralty Charts were Mean Low Water Springs (MLWS) and Indian Spring Low Water (ISLW). MLWS and ISLW are derived from summations of the amplitudes of various major harmonic constituents below local Mean Sea Level. For instance: MLWS = Local MSL – (Amplitude M2 + Amplitude S2)

  11. Other Chart Datums In areas where there is very little or no tide, other water level datums are used. For the Black Sea, Mean Sea Level or Mean Water level is used. In the Great Lakes, both Canada and the United States use a separate fixed Low Water Datum (LWD) for each lake based on analyses of monthly means during low water stages. In non-tidal lagoons and bays in the coastal United States where the area transitions from tidal to non-tidal, a LWD is used which is determined by subtracting 0.2 m from the local Mean Sea Level derived from observations and adjusted to a 19 year period. There are a variety of local Chart Datums employed in tidal rivers as well. In the United States, Chart Datums have been derived from analyses of measurements during the low river stages over a period of time and then are held fixed for charting purposes. Examples are the Hudson River Datum and Columbia River Datum derived from MLLW based on observations during the lowest river stages during the year.

  12. Great Lakes Chart Datum (LWD)

  13. NOS CHART DATUM - NONTIDAL AREAS Chart Datum: Low Water Datum (LWD) is used where the tide is negligible or has less than 0.5 foot range. LWD is determined by subtracting one-half foot from the computed Mean Water Level (MWL) based on observations. MWL is determined from the average of the hourly heights measured at nearby water level stations and adjusted to a 19-year period. This 19-year period corresponds to the same 19-year period upon which tidal datums, including Mean Lower Low Water (MLLW), are based (National Tidal Datum Epoch).

  14. Water level datums are completely distinct from geodetic vertical datums. For instance, the United States and Canada use the North American Vertical Datum of 1988 (NAVD 88) and the International Great Lakes Datum of 1985 (IGLD 85) as the vertical datums for geodetic purposes. The relationship between NAVD 88 (or IGLD 85) and local mean sea level or mean water level varies considerably from place to place. In fact, it is impossible to transfer a tidal datum from one place to another with geodetic levelling, without considering local tidal conditions. The geodetic network does, however, establish relationships between the many tide stations and their tidal datum elevations around the North American continent, and it could be used to recover a connected local tidal datum if the tidal bench marks are destroyed. This requires level connections or GPS connections between geodetic and tidal bench mark networks.

  15. VERTICAL DATUMS • A set of fundamental elevations to which other elevations are referred.

  16. THE ELLIPSOIDMATHEMATICAL MODEL OF THE EARTH N b a S a = Semi major axis b = Semi minor axis f = a-b = Flattening a

  17. ELLIPSOID - GEOID RELATIONSHIP H = Orthometric Height(NAVD 88) h = Ellipsoidal Height (NAD 83) H=h- N N = Geoid Height (GEOID 99) H TOPOGRAPHIC SURFACE h N GEOID99 Ellipsoid GRS80 Geoid

  18. UNITED STATESELLIPSOID DEFINITIONS BESSEL 1841 a = 6,377,397.155 m 1/f = 299.1528128 CLARKE 1866 a = 6,378,206.4 m 1/f = 294.97869821 GEODETIC REFERENCE SYSTEM 1980 - (GRS 80) a = 6,378,137 m 1/f = 298.257222101 WORLD GEODETIC SYSTEM 1984 - (WGS 84) a = 6,378,137 m 1/f = 298.257223563

  19. HORIZONTAL DATUMS • BESSEL1841 --------------LOCAL ASTRO DATUMS (1816-1879) • NEW ENGLAND DATUM (1879-1901) • U.S. STANDARD DATUM (1901-1913) • NORTH AMERICAN DATUM (1913-1927) • NORTH AMERICAN DATUM OF 1927 • OLD HAWAIIAN DATUM • CLARKE 1866PUERTO RICO DATUM • ST. GEORGE ISLAND - ALASKA • ST. LAWRENCE ISLAND - ALASKA • ST. PAUL ISLAND - ALASKA • AMERICAN SAMOA 1962 • GUAM 1963 • GRS80 -----------NORTH AMERICAN DATUM OF 1983 • (As of June 14, 1989)

  20. THE GEOID AND TWO ELLIPSOIDS CLARKE 1866 GRS80-WGS84 Earth Mass Center Approximately 236 meters GEOID

  21. HIGH ACCURACY REFERENCE NETWORKS

  22. Continuously Operating Reference Stations

  23. Tidal and water level datum surfaces are sloped surfaces relative to the land

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