noaa’s center for operational oceanographic products and services tidal datum computation overview

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TIDAL DATUM COMPUTATIONOVERVIEW

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Tidal Datums

DefinitionDefinition

NATIONAL TIDAL DATUM EPOCH NATIONAL TIDAL DATUM EPOCH (NTDE)(NTDE)

ApplicationApplication

ComputationComputation

Geodetic DatumsGeodetic Datums

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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

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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

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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)

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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

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1-76

1-78

1-80

1-82

1-84

1-86

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1-90

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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.

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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

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Types of Tide

Mixed, mainly diurnal

Semidiurnal Tide

Diurnal

Mixed, mainly semidiurnal

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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.

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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

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Equivalent 19-year Epoch Datum at Fort Pulaski

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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

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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)

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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:

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TIDAL DATUM ERROR

(Swanson, 1974)

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http://tidesandcurrents.noaa.gov/pub.html

References

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Geodetic Tie to Tidal Datums at NWLON Stations(GPS Example)

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Published Benchmark Sheet

http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Bench+Mark+Data+Sheets