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.

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.

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.

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.

General Requirements for Datum Determination at a Short-General Requirements for Datum Determination at a Short-Term Tide StationTerm 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.

Types of Tides

Mixed Tide

Semidiurnal Tide

Diurnal Tide

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.

Standard Method (Mixed Signal) - Values needed are MTL, Mn, DHQ, and DLQ as determined by comparison with an appropriate control.

MLW = MTL - 1/2 MnMHW = MLW + MnMLLW = MLW - DLQMHHW = 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 MnMHW = MLW + MnMLLW = DTL - 1/2 GtMHHW = 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.

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.

DATUMS Computation

ACCEPTED WATER LEVEL

MONTHLY

ACCEPTED STATION DATUM

ACCEPTED WATER LEVEL

HOURLY

ACCEPTED HI LO

Tide by Tide (TBYT)

Monthly Means (MMSC)

First Reduction (FRED)

STATION DATUM

Data Available Method Used

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

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.

TIDAL DATUM COMPUTATIONS

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

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

Simultaneous Comparison of Mean Tide Level (MTL) for San Francisco and Alameda, CA

1.000

1.500

2.000

2.500

3.000

3.500

3-97 4-97 5-97 6-97 7-97 8-97 9-97 10-97 11-97 12-97 1-98 2-98

time (months)

elev

atio

n re

lativ

e to

sta

tion

datu

m

(met

ers)

MTL - San Francisco

1960-78 MTL - San Francisco

MTL - Alameda

TIDE-BY-TIDE SIMULTANEOUS COMPARISON(Graphical Comparisons)

First Determine the DHQ, DLQ, GT, MN, MHW, MLW, DTL, MTL for Subordinate

Second Determine the differences and ratios for DHQ, DLQ, GT, MN, MHW, MLW, DTL, MTL for Subordinate and Control

Finally using the accepted datums at the control determine the accepted datums at the subordinate.

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

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.

TIDAL DATUM COMPUTATIONS

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

Integrated Bathy/Topo

DEM

NOAA Bathymetry

USGS Topography

EllipsoidModel

TidalModel

GeoidModel

VDatum ( Vertical Datum Transform

Tool )

NAD 83 (86)

NAVD 88 LMSL

MHHW

MHW

MTL

DTL

MLW

MLLW

WGS 84 (G873)

WGS 84 (G730)

WGS 84 (orig.)

ITRF97

ITRF94

ITRF96

ITRF93ITRF92ITRF91

ITRF90

ITRF89

ITRF88

SIO/MIT 92

NEOS 90

PNEOS 90

NGVD 29

GEOID99(USHG2003USGG2003)

TSS

ITRF2000 WGS 84 (G1150)

Ellipsoids Datums Datums

3-D Datum Orthometric Tidal

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

http://vdatum.noaa.gov/

To use Vdatum, go to website, download the software and the transformation gridsTo use Vdatum, go to website, download the software and the transformation grids

An example of Vdatum transformation in New York Harbor

Implies that NADV88 value is 0.8302 m above MLLW at this location

THE END

QUESTIONS?