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Datums Computation Overview

Datums Computation Overview. Center for Operational Oceanographic Products and Services January 8, 2009. Tidal Datums Defined. MHHW: Mean Higher High Water The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch.

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Datums Computation Overview

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  1. Datums Computation Overview Center for Operational Oceanographic Products and Services January 8, 2009

  2. Tidal Datums Defined • MHHW: Mean Higher High Water The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch. • MHW: Mean High Water The average of all the high water heights observed over the National Tidal Datum Epoch. • DTL: Diurnal Tide Level The arithmetic mean of mean higher high water and mean lower low water. • MTL: Mean Tide Level The arithmetic mean of mean high water and mean low water. • MSL: Mean Sea Level The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch. • MLW: Mean Low Water The average of all the low water heights observed over the National Tidal Datum Epoch. • MLLW: Mean Lower Low Water The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch.

  3. Tidal Datums Defined • GT: Great Diurnal Range The difference in height between mean higher high water and mean lower low water. • MN: Mean Range of Tide The difference in height between mean high water and mean low water. • DHQ: Mean Diurnal High Water Inequality The difference in height of the two high waters of each tidal day for a mixed or semidiurnal tide. • DLQ: Mean Diurnal Low Water Inequality The difference in height of the two low waters of each tidal day for a mixed or semidiurnal tide. • HWI: Greenwich High Water Interval The average interval (in hours) between the moon's transit over the Greenwich meridian and the following high water at a location. • LWI: Greenwich Low Water Interval The average interval (in hours) between the moon's transit over the Greenwich meridian and the following low water at a location. • Tropic intervals—Tropic higher high water interval (TcHHWI) is the lunitidal interval pertaining to the higher high waters. Tropic lower low water interval (TcLLWI) is the lunitidal interval pertaining to the lower low waters.

  4. Tidal Datums Defined • Station Datum : unique to each station and established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless of changes to the water level gauge or tide staff. • National Tidal Datum Epoch: Specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present NTDE is 1983 through 2001 and is actively considered for revision every 20-25 years. Tidal datums in certain regions with anomolous sea level changes (Alaska, Gulf of Mexico) are calculated on a Modified 5-Year Epoch.

  5. General Requirements for Datum Determination at a Short-Term Tide Station • Selection of an appropriate short-term station location and installation of sensors and bench marks. • Selection of an appropriate NOS primary or secondary control station. • Collection and processing of simultaneous data from control and short-term stations. • Obtain accepted datums from the control stations based on latest NTDE from NOS. • Computation of equivalent Epoch tidal datums using accepted procedures for simultaneous comparison. • Determine elevations of the tidal datums for each tidal bench mark.

  6. Types of Tides Semidiurnal Tide Mixed Tide Diurnal Tide

  7. TIDAL DATUM COMPUTATIONS Primary Determination - The first reduction mean of all the tidal heights for a particular phase (HW or LW) of the tide over a specific 19-year period or National Tidal Datum Epoch to obtain Accepted Values for the station. Secondary or Tertiary Determination - The computation of equivalent 19-year mean values using a short series of observations using the method of comparison of simultaneous observations. Either monthly mean comparison or tide-by-tide comparisons are used.

  8. Standard Method (Mixed Signal) - Values needed are MTL, Mn, DHQ, and DLQ as determined by comparison with an appropriate control. MLW = MTL - 1/2 Mn MHW = MLW + Mn MLLW = MLW - DLQ MHHW = MHW + DHQ Modified Range Ratio Method (Semi-diurnal or Diurnal) - Values needed are MTL, Mn, DTL, and GT as determined by comparison with an appropriate control. MLW = MTL - 1/2 Mn MHW = MLW + Mn MLLW = DTL - 1/2 Gt MHHW = MLLW + Gt Direct Method - The Direct Method is usually used only when a full range of tidal values are not available. For example, direct MHW can be computed for situations when low waters are not recorded, such as in the upper reaches of a marsh. Since MTL, DTL, and Mn and Gt cannot be determined if low waters are cut-off, equivalent NTDE values for MHW and MHHW datums are determined directly by comparison of high tides with an appropriate control using the available part of the tidal cycle.

  9. Tidal Datum Calculation from Observed Data Method of Comparison of Simultaneous Observations is a two-step process: 1. Compute the differences and/or ratios in the tidal parameters between short-term and control stations over the period of simultaneous comparison. 2. Apply the differences and ratios computed above to the NTDE Accepted Values at the control station. This provides equivalent NTDE values for the short-term station.

  10. DATUMS Computation Data Available Method Used ACCEPTED WATER LEVEL MONTHLY Monthly Means (MMSC) First Reduction (FRED) STATION DATUM Tide by Tide (TBYT) ACCEPTED WATER LEVEL HOURLY ACCEPTED STATION DATUM ACCEPTED HI LO

  11. Datum Computation • Monthly Mean Comparison (> 1 month of data) • Modified Range Ratio (Semi-diurnal or Diurnal) • Standard (Mixed tides) • Direct (Incomplete tidal signal, i.e. missing LW) • Tide-By-Tide Comparison (<1 month of data) • Modified Range Ratio • Standard • Direct • First Reduction (FRED) • When no control is available • Or when > 19 years of data

  12. TIDAL DATUM COMPUTATIONS Primary Determination - The first reduction (FRED) mean of all the tidal heights for a particular phase (HW or LW) of the tide over a specific 19-year period or National Tidal Datum Epoch to obtain Accepted Values for the station.

  13. TIDAL DATUM COMPUTATIONS Modified Range Ratio Methodusing Monthly Mean Comparison - Values needed are MTL, DTL, Mn, and GT as determined by comparison with an appropriate control. (Semidiurnal and Diurnal Tides)

  14. TIDAL DATUM COMPUTATIONS Standard Methodusing Monthly Mean Comparison- Values needed are MTL, Mn, DHQ, and DLQ as determined by comparison with an appropriate control. (Mixed Tides)

  15. TIDE-BY-TIDE SIMULTANEOUS COMPARISON (Graphical Comparisons) First Determine the DHQ, DLQ, GT, MN, MHW, MLW, DTL, MTL for Subordinate Finally using the accepted datums at the control determine the accepted datums at the subordinate. Second Determine the differences and ratios for DHQ, DLQ, GT, MN, MHW, MLW, DTL, MTL for Subordinate and Control

  16. TIDAL DATUM COMPUTATIONS Direct Method using Monthly Mean Comparison -The Direct Method is usually used only when a full range of tidal values are not available. For example, direct MHW can be computed for situations when low waters are not recorded, such as in the upper reaches of a marsh. Since MTL, DTL, and Mn and Gt cannot be determined if low waters are cut-off, equivalent NTDE values for MHW and MHHW datums are determined directly by comparison of high tides with an appropriate control using the available part of the tidal cycle.

  17. Vertical Datum Transformation Tool The Vertical Datum (VDatum) program is a cooperative effort between the CO-OPS, NGS/RSD, and the OCS/CSDL groups to develop a web-interface and software tool to allow a seamless transform of elevations between approximately 27 vertical datums of three categories: tidal (MSL), orthometric (NAVD88) and ellipsoidal (NAD83) vertical datums

  18. Geoid Model Tidal Model Ellipsoid Model USGS Topography NOAA Bathymetry VDatum ( Vertical Datum Transform Tool ) Integrated Bathy/Topo DEM

  19. WGS 84 (G873) ITRF2000 WGS 84 (G1150) MHHW WGS 84 (G730) WGS 84 (orig.) ITRF97 MHW NGVD29 ITRF96 ITRF94 MTL ITRF93 NAD 83 (86) NAVD 88 LMSL ITRF92 ITRF91 TSS GEOID99 (USHG2003 USGG2003) DTL ITRF90 ITRF89 MLW ITRF88 SIO/MIT 92 NEOS 90 MLLW PNEOS 90 3-D Datum Orthometric Tidal Ellipsoids Datums Datums

  20. VDatum is currently available in: • The Great Lakes • Rhode Island to North Carolina • Chesapeake Bay • Tampa Bay • Pensacola to Mobile Bay • Part of Louisiana • Strait of Juan de Fuca • Southern California

  21. http://vdatum.noaa.gov/ To use Vdatum, go to website, download the software and the transformation grids

  22. An example of Vdatum transformation in New York HarborImplies that NADV88 value is 0.8302 m above MLLW at this location

  23. THE END QUESTIONS?

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