Tidal and Geodetic Vertical Datums State Geodetic Advisor, NGS National Ocean Service, NOAA Sacramento, CA October, 2005 Workshop

Vertical Control Datums A set of fundamental elevations to which other elevations are referred. A set of fundamental elevations to which other elevations are referred. Vertical Datum Types Vertical Datum Types Assumed – Self-explanatory Assumed – Self-explanatory Tidal – Defined by observation of tidal variations over some period of time Tidal – Defined by observation of tidal variations over some period of time (MSL, MLLW, MLW, MHW, MHHW, etc.) (MSL, MLLW, MLW, MHW, MHHW, etc.) Geodetic Orthometric– Either directly or loosely based on Mean Sea Level at one or more points at some tidal epoch (NGVD 29, NAVD 88, IGLD85, etc.) (NGVD 29, NAVD 88, IGLD85, etc.)

Types of Tides Mixed Tide Semidiurnal Tide Diurnal Tide

MHHW

RELATIVE SEA LEVEL CHANGE AT SEVERAL LOCATIONS IN THE U.S.

IDEALIZED CHANGE OF TIDAL EPOCH EPOCH

Tidal Datum Boundary Jurisdictions

NAVD 88 PROGRAM DEFINITION NAVD 88 was an initiative which combined 1.1 million kilometers of leveling surveys in the NGS National Spatial Reference System (NSRS) database into a single least squares adjustment, to provide users with improved heights for over 500,000 vertical control points distributed throughout the United States, on a common datum.

RATIONALE FOR BASIS OF NAVD 88  IN ORDER TO MINIMIZE THE EFFECTS ON USGS NATIONAL MAPPING PRODUCTS (NMP), AS REQUESTED BY USERS, NGS SELECTED THE NEW INTERNATIONAL GREAT LAKES DATUM OF 1985 (IGLD 85) LOCAL MEAN SEA LEVEL HEIGHT VALUE AS the MINIMUM-CONSTRAINT CONTROL POINT FOR NAVD 88. THE DATUM POINT IS LOCATED AT THE MOUTH OF THE ST. LAWRENCE RIVER IN QUEBEC, CANADA.  USING FATHER POINT/RIMOUSKI AS THE DATUM POINT FOR BOTH IGLD 85 AND NAVD 88 MINIMIZES THE IMPACT ON NMP, AND ALLOWS NAVD 88 TO REPLACE BOTH NGVD 29 AND IGLD 55.

LEVELED NETWORK FOR NAVD 88  ORIGINAL LEVELING 700,000 KM  REPEAT LEVELING 200,000 KM  NEW NGS LEVELING 81,500 KM  NEW OUTSIDE LEVELING 20,000 KM  TOTAL FOR NAVD 88 1,001,500 KM (620,000 MI) (620,000 MI)

NAVD 88 DATUM DEFINITION  Vertical datum based upon an equipotential surface  Minimally constrained adjustment held fixed at Father Point (Point-au-Pere), in Rimouski  1.1 million km of leveling data used  Heights of 585,000 permanent bench marks estimated  Both orthometric heights and geopotential numbers published

NGVD 29 TIDE CONTROL

COMPARISON OF VERTICAL DATUM ELEMENTS NGVD 29NAVD88 DATUM DEFINITION 26 TIDE GAGESFATHER POINT IN THE U.S. & CANADA QUEBEC, CANADA DATUM DEFINITION 26 TIDE GAGESFATHER POINT IN THE U.S. & CANADA QUEBEC, CANADA BENCH MARKS 100, ,000 BENCH MARKS 100, ,000 LEVELING (Km) 102,724 1,001,500 LEVELING (Km) 102,724 1,001,500 GEOID FITTING Distorted to Fit Best Continental MSL GaugesModel GEOID FITTING Distorted to Fit Best Continental MSL GaugesModel

NGVD 29 versus NAVD 88

What is the GEOID? “The equipotential surface of the Earth’s gravity field which best fits, in the least squares sense, mean sea level.”* “The equipotential surface of the Earth’s gravity field which best fits, in the least squares sense, mean sea level.”* Its surface can’t be seen or measured directly Its surface can’t be seen or measured directly It is modeled from gravity data It is modeled from gravity data *Definition from the Geodetic Glossary, September 1986 *Definition from the Geodetic Glossary, September 1986

H H = Orthometric Height (NAVD 88) H = h - N TOPOGRAPHIC SURFACE h = Ellipsoidal Height (NAD 83) N = Geoid Height (GEOID 03) h Ellipsoid GRS80 N Geoid GEOID 03 Ellipsoid, Geoid, and Orthometric Heights

Geodetic Control Networks Northern California Caltrans Observations Leveling Hwy 20 Caltrans Observations Leveling Hwy 20

6A specific 19 year period that includes the longest periodic tidal variations caused by the astronomic tide-producing forces. 6Averages out long term seasonal meteorological, hydrologic, and oceanographic fluctuations. 6 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. 6The NWLON provides the data required to maintain the epoch and make primary and secondary determinations of tidal datums. NATIONAL TIDAL DATUM EPOCH

Comparison of NAVD88 ‘datum’ from Geodetic and Tidal viewpoints

NOAA MARSH AND HABITAT RESTORATION PROGRAM Marsh Study Site Planning Before Planting Marine Boundaries are Essential for Successful Marsh Restoration Efforts

SUMMARY TIDAL DATUMS are based on a phase of the tide (eg, MHW, MLLW). TIDAL DATUMS are based on a phase of the tide (eg, MHW, MLLW). Station datums are based on a 19 year mean called an National Tidal Datum Epoch (NTDE). Station datums are based on a 19 year mean called an National Tidal Datum Epoch (NTDE). Tide stations are referenced to a bench mark system (NAVD 88) for stability checks & long term maintenance. Tide stations are referenced to a bench mark system (NAVD 88) for stability checks & long term maintenance. Datum applications: PORTS, chart depths, marine boundaries, hydrographic surveys, nautical charting, dredging, storm surge, modeling, wetland restoration, etc. Datum applications: PORTS, chart depths, marine boundaries, hydrographic surveys, nautical charting, dredging, storm surge, modeling, wetland restoration, etc.

APPLICATIONS SUPPORTED BY THE NATIONAL WATER LEVEL AND TIDAL CURRENT PROGRAMS FunctionApplication Navigation/SafetyReal-Time Navigation, Hydrographic Surveys, Airborne Hydrography, HAZMAT Response...etc. Environmental Assessment/Wetland Delineation, Coastal Monitoring Zone Management, Enforcement, Insurance, Storm Surge, Tsunami Warning…etc. Basic & Applied ResearchModeling, Wetland Studies, Global Warming Studies, Sea Level Change, Estuarine/ Hydrodynamic Studies…etc.

TIDES SUPPORT TO NAUTICAL CHARTING HYDROGRAPHY APPLICATIONS HEIGHTS ARE REFERRED TO MHW SOUNDINGS ARE REFERRED TO MLLW

Importance of defining shoreline 95,000 miles in US 95,000 miles in US Marine territorial limits, incl EEZ Marine territorial limits, incl EEZ Law passed by US Congress in 1998 gave NOAA explicit authority Law passed by US Congress in 1998 gave NOAA explicit authority Nautical/navigation charts demarcate MHW Nautical/navigation charts demarcate MHW NGS’ shoreline products depict MLLW NGS’ shoreline products depict MLLW Broader adoption of NOAA’s shorelines is encouraged to alleviate some confusion Broader adoption of NOAA’s shorelines is encouraged to alleviate some confusion

Thank you for attending! NOS, NOAA