1. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES TIDAL DATUM COMPUTATION OVERVIEW

2. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Tidal Datums l Definition l NATIONAL TIDAL DATUM EPOCH (NTDE) l Application l Computation l Geodetic Datums

3. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES VERTICAL DATUM : A base elevation used as a reference from which to reckon heights or depths.A base elevation used as a reference from which to reckon heights or depths. TIDAL DATUM: Defined by a certain phase of the tide (ex. MHW)Defined by a certain phase of the tide (ex. MHW) Are local datumsAre local datums In order that they may be recovered and updated when needed, such datums are referred to fixed points on land known as Bench MarksIn order that they may be recovered and updated when needed, such datums are referred to fixed points on land known as Bench Marks

4. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES

5. Day 1 Day 2 Higher High High MHHW MHW Low Lower LowLow Lower Low MLLW MLW Observed Water Level LMSL = avg. of hourly levels MTL = avg. of MHW and MLW DTL = avg. of MHHW and MLLW Water level record of 19 years (Tidal Epoch) 1983-2001 TIDAL DATUMS FROM TIME SERIES

6. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES  A specific 19 year period that includes the longest periodic tidalvariations 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.  The present NTDE is 1983 through 2001. NATIONAL TIDAL DATUM EPOCH (NTDE) A common time period to which tidal datums are referenced

7. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES SEATTLE, PUGET SOUND, WA VARIATIONS IN MEAN RANGE OF TIDE: 1900 – 1996 Variations in Mean Range of tide due to the 18.6-year cycle of “Regression of the Moon’s Nodes”

8. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES RELATIVE SEA LEVEL CHANGE AT SEVERAL LOCATIONS IN THE U.S.

9. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES IDEALIZED CHANGE OF TIDAL EPOCH ACTUAL 1983-01 EPOCH

10. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES

11. 60-78 Epoch 83-01 Epoch MSL Linear Trend MSL= 1.542 MSL= 1.490 0.051 m Epoch Change

12. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Seldovia, Cook Inlet, AK Tidal Datum Update Schematic Drop in Water Level

13. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Galveston Pier 21, Galveston Channel, TX Rise in Water Level

14. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES General Requirements for Datum Determination at a Short-Term Tide Station 1.Selection of an appropriate short-term station location and installation of sensors and bench marks. 2.Selection of an appropriate NOS primary or secondary control station. 3.Collection and processing of simultaneous data from control and short-term stations. 4.Obtain accepted datums from the control stations based on latest NTDE from NOS. 5.Computation of equivalent Epoch tidal datums using accepted procedures for simultaneous comparison. 6.Determine elevations of the tidal datums for each tidal bench mark.

15. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES TIDAL DATUMS Computation 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. Standard Method - Values needed are MTL, Mn, DHQ, and DLQ as determined by comparison with an appropriate control ( West Coast and Pacific Island, Mix Tide) MLW=MTL -1/2 Mn MHW =MLW+Mn MLLW=MLW-DLQ MHHW =MHW + DHQ Modified Range Ratio Method - Values needed are MTL, DTL, Mn, and GT as determined by comparison with an appropriate control ( East Coast, Gulf Coast and Caribbean Island, Semidiurnal and Diurnal) MLW= MTL -1/2 Mn MHW= MLW +Mn MLLW = DTL - 1/2 Gt MHHW= MLLW + Gt

16. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Types of Tide Mixed, mainly diurnal Semidiurnal Tide Diurnal Mixed, mainly semidiurnal

17. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Tidal Datum Calculation Method of Comparison of Simultaneous Observations is a two-step process: 1.Compute the differences and/or ratios in the tidal parameters and differences in mean values 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.

18. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES STATION HIERARCHY SECONDARY TIDE STATION: minimum of 1 yr but less than 19 yrs PRIMARY TIDE STATION: minimum of 19 years allows statistical and datum control for short term stations TERTIARY TIDE STATION: more than 30 days but less than a year

19. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Monthly Mean Comparison using the Modified-Range Ratio Method East Coast, Gulf Coast and Caribbean Island (Semidiurnal and Diurnal Tides)

20. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Port Pulaski Charleston Subordinate Control

21. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Simultaneous Comparison of MTL 1983- 2001

22. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Computation Flow of Monthly Mean Comparison Monthly Mean of each datum at Subordinate Monthly Mean of each datum at Control Average difference/Ratios between Monthly Mean of each datum between subordinate and control Use the average difference/ratios as corrector to adjust accepted 19-year datums at control station to derive 19-year datums at subordinate

23. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Accepted 19-year Epoch Datum at Charleston

24. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Compute tidal datums at Fort Pulaski, Ga with Charleston, SC as a control by Monthly Mean Comparison using Modified-Range Ratio Method  MLW = MTL - (0.5 x Mn)  MHW = MLW + Mn  MLLW= DTL - (0.5 x Gt)  MHHW = MLLW + Gt  MTL, MN, DTL and GT have to be determined before computing MLW, MHW, MLLW, and MHHW

25. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Equivalent 19-year Epoch Datum at Fort Pulaski

26. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Bradmoor Pt. Chicago Tide-By-Tide (TBYT) Simultaneous Comparison TBYT Method -The TBYT Method is usually used when a full calendar month data are not available

27. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Direct Method Simultaneous Comparison 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.

29. 1.Make Observations WL Data collected at 6-minute intervals, downloaded to DMS, CORMS & Auto QC 2.Preliminary Analysis Check station parameters, initialize data into working table, fill gaps, QC 3.Tabulation Tabulate Highs & Lows, QC & edit Hourly Heights & Hi/Lo picks, Compute Monthly Means *** This example - Data series too short to compute monthly means, or data starts & ends in middle of months <1 month 4.Compute Datums - Subordinate Station with <1 month of data Make secondary determinations using simultaneous comparison of High & Low waters Choose appropriate control station Pensacola, FL Choose appropriate method (Gulf coast=Diurnal) Modified-Range Ratio Method Gulf Coast Example 2 – Tide by Tide Comparison Panama City, FL – Subordinate Station

30. 1.Make Observations WL Data collected at 6-minute intervals, downloaded to DMS, CORMS & Auto QC 2.Preliminary Analysis Check station parameters, initialize data into working table, fill gaps, QC 3.Tabulation Tabulate Highs & Lows, QC & edit Hourly Heights & Hi/Lo picks, Compute Monthly Means 4.Compute Datums - Subordinate Station with <19ys of data Make secondary determinations using simultaneous comparison of Monthly Means Choose appropriate control station Pensacola, FL Choose appropriate method (Gulf coast=Diurnal) Modified-Range Ratio Method Gulf Coast Example 1 – Comparison of Monthly Means Panama City, FL – Subordinate Station

31. Gulf Coast Example 1 – Comparison of Monthly Means Panama City, FL – Subordinate Station A.Compare monthly means between Subordinate & Control to calculate corrected MTL, DTL, Mn, & GT for subordinate that will be used to calculate remaining datums (MLW, MHW, MLLW, MHHW, DHQ, DLQ) Choose the time frame of data to use for comparison (in this example – 1 yr) Get control station data from Tides & Currents website (Monthly means & Accepted datums) (Pensacola) Monthly Means - control station (Pensacola) (Panama City) Monthly Means - uncorrected (Panama City)

32. Gulf Coast Example 1 – Comparison of Monthly Means Panama City, FL – Subordinate Station Panama City NTDE Equivalent datums MLW = 8.396 - (0.5*0.363) = 8.214 MHHW = 8.188 + 0.419 = 8.607 MHW = 8.214 + 0.363= 8.577DHQ = 8.607 - 8.577 = 0.03 MLLW = 8.397 - (0.5*0.419) = 8.188DLQ = 8.214 - 8.188 = 0.026 Modified-Range Ratio Method Need corrected MTL, DTL, Mn, & GT from comparison to appropriate control station MLW = MTL - (0.5*Mn) MHHW = MLLW + GT MHW = MLW + MnDHQ = MHHW - MHW MLLW = DTL - (0.5*GT)DLQ = MLW - MLLW B.Apply corrected MTL, DTL, Mn, & GT to formulas for Modified-Range Ratio Method to calculate remaining NTDE equivalent datums for the subordinate

33. Gulf Coast Example 2 – Tide by Tide Comparison Panama City, FL – Subordinate Station A.Compare highs & lows between Subordinate & Control to calculate corrected DTL, & GT for subordinate that will be used to calculate remaining datums (MLW, MHW, MLLW, MHHW, DHQ, DLQ) 1.Plot highs and lows to ensure that designations agree –If they do not, a type conversion is done for the subordinate (ie. If sub. value is designated as LL, where control is L, then sub. is changed from LL to L so that the pair remains matched in designation)

34. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES ACCURACY OF TIDAL DATUM COMPUTATIONS  DATA QUALITY: MEASUREMENT SYSTEM ACCURACY, DATA CONTINUITY, QUALITY OF TABULATION OF THE TIDE, VERTICAL STABILITY OF THE STATION  SIMILARITY OF TIDAL CHARACTERISTICS: SIMILARITY IN TYPE OF TIDE, SIMILAR SHAPE OF THE TIDE CURVE (LITTLE SHALLOW WATER DISTORTION), SMALL DIFFERENCES IN TIME OF TIDE (HIGH AND LOW WATERS), SMALL DIFFERENCES IN MEAN RANGE OF TIDE AND IN THE DIURNAL INEQUALITIES  GEOGRAPHIC DISTANCE: DIFFERENTIAL EFFECTS OF THE WEATHER ON THE WATER LEVELS, DIFFERENCES IN SEA LEVEL TRENDS, DIFFERENTIAL EFFECTS OF SEICHE, EFFECTS OF RIVER FLOW DEPENDS UPON:

35. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES TIDAL DATUM ERROR (Swanson, 1974)

36. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES http://tidesandcurrents.noaa.gov/pub.html References

37. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Geodetic Tie to Tidal Datums at NWLON Stations (GPS Example)

38. NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Published Benchmark Sheet http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Bench+Mark+Data+Sheets