Tidal Datums and Their Applications

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Tidal Datumsand TheirApplications
Thomas F. Landon, NOAA
Center for
Operational Oceanographic Products and Services
(CO-OPS)
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http//co-ops.nos.noaa.gov
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DATUM (VERTICAL)
CHART DATUM

• For marine applications
• 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)
  
• are local datums
• in order that they may be recovered when needed,
  such datums are referred to fixed points known
  as BENCH MARKS

• The tidal datum to which soundings on a nautical
  chart are referred.
• USA chart datum Mean Lower Low Water (MLLW)
• For non tidal waters (including Great Lakes)
  chart datum is Low Water Datum (LWD)

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Types of Tides
Semidiurnal Tide
Mixed Tide
Diurnal Tide
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MHHW
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APPLICATIONS SUPPORTED BY THE NATIONAL WATER
LEVEL AND TIDAL CURRENT PROGRAMS
Function Application Navigation/Safety Real-Time
Navigation, Hydrographic Surveys, Airborne
Hydrography, HAZMAT Response ...etc. Environmen
tal Assessment/ Wetland Delineation,
Coastal Monitoring Zone Management
Enforcement, Insurance, Storm Surge, Tsunami
Warningetc. Basic Applied Research Modeling,
Wetland Studies, Global Warming Studies, Sea
Level Change, Estuarine/ Hydrodynamic
Studiesetc.
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Datums Boundary Applications
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CORRECTIONS TO ECHO SOUNDINGS (HYDRO)
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TIDES SUPPORT TO NAUTICAL CHARTING HYDROGRAPHY
APPLICATIONS
SOUNDINGS ARE REFERRED TO MLLW
HEIGHTS ARE REFERRED TO MHW
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National Water Level Observation Network (NWLON)
175 Permanent Stations
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NATIONAL TIDAL DATUM EPOCH

• A specific 19 year period that includes the
  longest periodic tidal variations 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.

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SEATTLE, PUGET SOUND, WA VARIATIONS IN MEAN RANGE
OF TIDE 1900 - 1996
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ACTUAL
IDEALIZED CHANGE OF TIDAL EPOCH
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RELATIVE SEA LEVEL CHANGE AT SEVERAL LOCATIONS
IN THE U.S.
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60-78 Epoch
83-01 Epoch
MSL 1.490
MSL 1.542
MSL Linear Trend
Epoch Change
0.051 m
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0.000m
0.016m
0.042m
0.038m
0.044m
0.057m
0.051m
0.050m
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NORTH
SOUTH
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National Water Level Observation Network (NWLON)
175 Permanent Stations
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STATION HIERARCHY

• TERTIARY TIDE STATION
• more than 30 days but less than a year

• PRIMARY TIDE STATION
• minimum of 19 years
• allows statistical and datum control for short
  term stations

• SECONDARY TIDE STATION
• minimum of 1 yr but less than 19 yrs

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GENERALIZED ACCURACY OF TIDAL DATUMS FOR SHORT
SERIES (BASED ON ONE STD DEV) Series
East Gulf West Length
Coast Coast Coast Mo
FT FT FT 1
0.13 0.18 0.13 3
0.10 0.15 0.11 6
0.07 0.12 0.08 12
0.05 0.09 0.06
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Leveling and Datums
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NOS BENCHMARK LEVELING
Distances vary but usually several hundred meters.
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Relative to NAVD 88
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Summary

• Tidal datums are based on a phase of the tide
  (ex. MHW, MLLW).
• Station datums are based on a 19 year mean called
  an National Tidal Datum Epoch (NTDE).
• Tide stations are referenced to bench mark system
  for stability checks long term maintenance.
• Datum applications PORTS, chart depths , marine
  boundaries, hydrographic surveys, dredging, storm
  surge, modeling, wetland restoration, etc.

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Why Are We Here?
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ships out there...
BIG
There are REALLY
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QUESTIONS?