Brief History of North American Vertical Datums

Mean Sea Level Datum 1900

United States Lake Survey 1903

*Mean Sea Level Datum 1929

United States Lake Survey 1935

International Great Lakes Datum 1955

*National Geodetic Vertical Datum 1929

**International Great Lakes Datum 1985

**North American Vertical Datum 1988

Early vertical datums were established for charting & hydrographic surveys of harbors

along the eastern seaboard of the U.S.

The first leveling route in the United States of geodetic quality was established in 1856 – 1857 by

the U.S. Coast Survey.

• This leveling survey was required to support river current & tide studies in the Hudson River & New York harbor area, to assist in maritime commerce.

• By 1900 the vertical control network had grown to 21,095 km of geodetic leveling routes along the eastern seaboard of the United States.

• A reference surface was determined in 1900 by holding elevations referenced to local mean sea level fixed at 5 tide stations.

Description of Historical Datums

• Prior to 1900 there were many datums used on the Great Lakes & the U.S. coast for charting, and river & harbor construction projects.

• In 1903 the U.S. Coast & Geodetic Survey made adjustments to these benchmarks based on tide gauge readings & level networks – U.S.Lake Survey Datum of 1903.

• By 1935 movement in the earth’s crust caused tide gauges at the same locations in the lakes, to show differences in the water surface elevations. Adjustments were made to the benchmarks, based on level networks, and this resulted in the U.S. Lake Survey Datum of 1935.

Adjustments to North American vertical datum

based on increased tidal observations

Year of Adjustment

Kilometers of Leveling

Number of Tide Stations

19001903190719121929

21,09531,78938,35946,468

          75,159 (U.S.)              31,565 (Canada)

5889

        21 (U.S.)              5 (Canada)

NGVD - 1929

• NGVD-29 was established by the U.S. Coast & Geodetic Survey in 1929 by constraining the combined United States & Canadian 1st order leveling networks to conform to Mean Sea Level (M.S.L.) The mean sea level was determined at 26 long term tidal gauge stations that were along the East & West Coast of the U.S. in major harbors, and along the Gulf of Mexico.

• 21 of the tide stations were in the United States & 5 were in Canada.

• NGVD-29 was originally named the Mean Sea Level Datum of 1929.

Typical Tide Gauge Diagram

Tide Gauge Recorder – circa 1925

Typical Modern Tide Gauge Station Diagram

Tide Gauge – Anchorage, Alaska - 1913

NGVD - 1929

• It was known at that time that the M.S.L. determinations at the tide gauges would not define a single equipotential surface because of variations of ocean currents, winds, barometric pressures, and other physical causes.

• The name Mean Sea Level Datum – 1929 was changed to National Geodetic Vertical Datum – 1929 in 1973 to eliminate the reference to sea level in the name.

• Since NGVD-29 was established, it has become obvious that the geoid based upon tidal observations would change with each measurement cycle. Sea levels are rising.

Basic Tide Phases & Cycles

Tidal Definitions

• Semi-Diurnal Tides: two sets of high and low water in 24 hours.

• Diurnal: Just one high and low tide per 24 hours. Very few tide stations have diurnal tides.

• If a tide falls between these two extremes it is called a Mixed Tide – there will be two pairs of high & low tides – but one pair will have a greater range than the other tides.

Tidal Observations

• Tidal observations at tide gauges are made over a period of 18.6 years. Over that interval the Moon completes all possible positions with respect to the earth. This period is also called one tidal epoch.

• This 18.6 year period is called the Metronic Cycle.• It is named after Metron, an Athenian who lived in the

5th century B.C. He probably learned of it from Babylonian sources. This lunar cycle was the basis for most calendars until the introduction of the Julian calendar in 45 B.C.

Mixed Tide at Pass Cavallo – May 23rd to 24th , 2005

Mean Sea Level Trend – GalvestonGauge 8771450 - Galveston Pier 21

mean sea level trend is ~ + 6.5 mm – per year

( 2.13 feet per century)Mean Sea Level Data from 1908 to 1999

Change in Global Mean Sea Level

Why change from NGVD–29 to NAVD - 88

• It would be preferable to base measurements of the earth’s shape on a more stable surface that that provided by the constantly changing mean sea level.

• Since the establishment of NGVD-29, it has become obvious that the geoid (i.e. the shape of the earth based on mean sea level - the equipotential surface of the earth’s gravity field which best fits global mean sea level); would continue to change with each new set of tidal observations, per 18.6 year tidal epoch.

• Technology existed that allowed for making measurements of the Earth’s size & shape – (such as satellite observations – with doppler & GPS) which allowed for a more accurate & stable definition of the Earth’s dimensions.

NAVD-88

• The datum – NAVD-88 is based on the mass or density of the Earth, instead of the varying values of mean sea level.

• Measurements of the acceleration of gravity are made at control points within the vertical network.

• Only one tide gauge is held as the origin – that is at Pointe-au-Pere/Rimouski, Quebec, Canada, on the Saint Lawrence River.

• The vertical reference surface is then defined by the surface on which the gravity values are equal to the control point gravity values at the tide station Rimouski, Quebec.

NAVD-88

• The mean water level at Pointe-au-Pere/Rimouski, Quebec was observed and recorded from 1970 to 1988. ( ~ one tidal epoch of 18.6 years)

• This site was chosen for the tidal station because it is tectonically sound, and is located on the stable bedrock of the Canadian Shield, and not likely to move.

• NAVD-88’s reference surface is an abstract surface like that of the geoid, and it is not referenced to any sea level surface.

NGVD-29 to NAVD-88 Differences

• In the United States – differences in elevation range from – 0.40 m to + 1.50 m.

• In Alaska – differences in elevation range from + 0.94 m to + 2.40 m.

• In more stable areas of the United States – differences in elevation are less than 0.1 m.

NGVD-29 to NAVD-88 Differences

NGVD-29 to NAVD-88 Height Differences

units in centimeters

KP0037 DESIGNATION - A 42 KP0037 PID - KP0037 KP0037 STATE/COUNTY- NV/WHITE PINE KP0037 USGS QUAD - HOGUM (1987) KP0037 KP0037 *CURRENT SURVEY CONTROL KP0037 ___________________________________________________________________ KP0037* NAD 83(1986)- 39 04 07. (N) 114 27 07. (W) SCALED KP0037* NAVD 88 - 1747.784 (meters) 5734.19 (feet) ADJUSTED KP0037 ___________________________________________________________________ KP0037 GEOID HEIGHT- -20.69 (meters) GEOID03 KP0037 DYNAMIC HT - 1745.927 (meters) 5728.10 (feet) COMP KP0037 MODELED GRAV- 979,504.0 (mgal) NAVD 88 KP0037 KP0037 VERT ORDER - FIRST CLASS II KP0037.The orthometric height was determined by differential leveling KP0037.and adjusted by the National Geodetic Survey in June 1991. KP0037 KP0037.The geoid height was determined by GEOID03. KP0037 KP0037.The modeled gravity was interpolated from observed gravity values. KP0037 SUPERSEDED SURVEY CONTROL KP0037 NGVD 29 (??/??/92) 1746.576 (m) 5730.22 (f) ADJ UNCH 1 2 KP0037 KP0037.Superseded values are not recommended for survey control.

Gravitational Effects on Leveling – Distortions in the NGVD-29 network

Leveled Height Differences

B

A C

Topography

Transferring the elevation from a tide gauge to nearby benchmarks – Alaska - 1915

Leveling crew - Navajo Springs, Arizona - 1921

Benchmark crew - Utah, 1934

Leveling crew - rural Mississippi - 1935

Leveling crew – Whisky Pass, Colorado - 1931

Leveling crew – Salmon River, Idaho - 1945

Extending the Network – NGVD- 29 – Vertical control in the United States as of 1936

Closure Standards for Vertical Control[ error of closure - closed level loops ]

• First Order – Class I = 4mm*(SQRT km)• First Order – Class II = 5mm*(SQRT km)• Second Order – Class I = 6mm*(SQRT km)• Second Order – Class II = 8mm*(SQRT km)• Third Order . . . . . . . . . =12mm*(SQRT km)