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NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
TIDAL DATUM COMPUTATIONOVERVIEW
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Tidal Datums
DefinitionDefinition
NATIONAL TIDAL DATUM EPOCH NATIONAL TIDAL DATUM EPOCH (NTDE)(NTDE)
ApplicationApplication
ComputationComputation
Geodetic DatumsGeodetic Datums
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
VERTICALVERTICAL DATUMDATUM: :
•A base elevation used as a reference from which to reckon A base elevation used as a reference from which to reckon heights or depths. heights or depths. TIDAL DATUM: TIDAL DATUM:
•Defined by a certain phase of the tide (ex. MHW) Defined by a certain phase of the tide (ex. MHW)
•Are local datums Are local datums
•In order that they may be recovered and updated when In order that they may be recovered and updated when needed, such datums are referred to fixed points on land needed, such datums are referred to fixed points on land known as Bench Marksknown as Bench Marks
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Day 1 Day 2
Higher High Higher High
High
MHHW
MHW
Low
Lower Low Low
Lower LowMLLW
MLW
ObservedWater Level
LMSL = avg. of hourly levels MTL = avg. of MHW and MLWDTL = avg. of MHHW and MLLW
Water level record of 19 years (Tidal Epoch) 1983-2001
TIDAL DATUMS FROM TIME SERIESTIDAL DATUMS FROM TIME SERIES
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
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SEATTLE, PUGET SOUND, WAVARIATIONS 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”
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RELATIVE SEA LEVEL CHANGE AT SEVERAL LOCATIONS IN THE U.S.
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IDEALIZED CHANGE OF TIDAL EPOCHACTUAL
1983-01 EPOCH
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New London, CTMean Sea Level 1940-2001
1.200
1.300
1.400
1.500
1.600
1.700
1.800
1-40
1-44
1-48
1-52
1-56
1-60
1-64
1-68
1-72
1-76
1-80
1-84
1-88
1-92
1-96
1-00
Years
Ele
va
tio
n (
m)
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
New London, CTMean Sea Level 1960-2001
1.200
1.300
1.400
1.500
1.600
1.700
1.800
1-60
1-62
1-64
1-66
1-68
1-70
1-72
1-74
1-76
1-78
1-80
1-82
1-84
1-86
1-88
1-90
1-92
1-94
1-96
1-98
1-00
Years
Ele
va
tio
n (
m)
60-78 Epoch 83-01 Epoch
MSL Linear TrendMSL= 1.542MSL= 1.490
0.051 m Epoch Change
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Seldovia, Cook Inlet, AKTidal Datum Update Schematic
Drop inDrop inWater Water LevelLevel
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Galveston Pier 21, Galveston Channel, TX
Rise inRise inWater Water LevelLevel
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General Requirements for Datum Determinationat 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.
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
TIDAL DATUMS ComputationPrimary 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 MnMHW = MLW + MnMLLW = MLW - DLQMHHW = 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 MnMHW = MLW + MnMLLW = DTL - 1/2 GtMHHW = MLLW + Gt
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Types of Tide
Mixed, mainly diurnal
Semidiurnal Tide
Diurnal
Mixed, mainly semidiurnal
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.
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
STATION HIERARCHYSTATION 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
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Monthly Mean Comparison using the
Modified-Range Ratio Method
East Coast, Gulf Coast and Caribbean Island(Semidiurnal and Diurnal Tides)
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Port Pulaski
Charleston
Subordinate
Control
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Simultaneous Comparison of MTL
1983-2001
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Computation Flow of Monthly Mean Comparison
Monthly Mean of each datum at SubordinateMonthly 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
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Accepted 19-year Epoch Datum at Charleston
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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
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Equivalent 19-year Epoch Datum at Fort Pulaski
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
9415144 PORT CHICAGO, SUISUN BAY CA9414811 BRADMOOR ISLAND, NURSE SLOUGH CA
1.000
1.500
2.000
2.500
3.000
3.500
05-01-04 00:00 05-02-04 00:00 05-03-04 00:00 05-04-04 00:00 05-05-04 00:00 05-06-04 00:00 05-07-04 00:00 05-08-04 00:00
Me
ters
BradmoorPt. Chicago
Tide-By-Tide (TBYT) Simultaneous ComparisonTBYT Method -The TBYT Method is usually used when a full calendar month data are not available
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Direct Method Simultaneous ComparisonDirect 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.
Comparison of Low-water Cut-off Station with Control Sation Requiring Use of Direct Method
-1.600
-1.400
-1.200
-1.000
-0.800
-0.600
-0.400
-0.200
0.000
0.200
03-25-00 00:00 03-26-00 00:00 03-27-00 00:00 03-28-00 00:00 03-29-00 00:00 03-30-00 00:00 03-31-00 00:00 04-01-00 00:00
Time (UTC)
Ele
vati
on
Rel
ativ
e to
MH
W (
met
ers)
Hamilton AFBRichmond
Gulf Coast Example 2 – Tide by Tide ComparisonGulf Coast Example 2 – Tide by Tide Comparison
Panama City, FL – Subordinate StationPanama City, FL – Subordinate Station
Gulf Coast Example 1 – Comparison of Monthly MeansGulf Coast Example 1 – Comparison of Monthly Means
Panama City, FL – Subordinate StationPanama City, FL – Subordinate Station
Gulf Coast Example 1 – Comparison of Monthly MeansGulf Coast Example 1 – Comparison of Monthly Means
Panama City, FL – Subordinate StationPanama 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)
Monthly Means - control station (Pensacola) (Pensacola) Monthly Means - uncorrected (Panama City) (Panama City)
Gulf Coast Example 1 – Comparison of Monthly MeansGulf Coast Example 1 – Comparison of Monthly Means
Panama City, FL – Subordinate StationPanama City, FL – Subordinate Station
Panama City NTDE Equivalent datumsMLW = 8.396 - (0.5*0.363) = 8.214 MHHW = 8.188 + 0.419 = 8.607MHW = 8.214 + 0.363 = 8.577 DHQ = 8.607 - 8.577 = 0.03MLLW = 8.397 - (0.5*0.419) = 8.188 DLQ = 8.214 - 8.188 = 0.026
Modified-Range Ratio MethodNeed corrected MTL, DTL, Mn, & GT from comparison to appropriate control station
MLW = MTL - (0.5*Mn) MHHW = MLLW + GTMHW = MLW + Mn DHQ = MHHW - MHWMLLW = 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
Gulf Coast Example 2 – Tide by Tide ComparisonGulf Coast Example 2 – Tide by Tide Comparison
Panama City, FL – Subordinate StationPanama 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)
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:
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
TIDAL DATUM ERROR
(Swanson, 1974)
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
http://tidesandcurrents.noaa.gov/pub.html
References
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Geodetic Tie to Tidal Datums at NWLON Stations(GPS Example)
NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES
Published Benchmark Sheet
http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Bench+Mark+Data+Sheets