11
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Vertical DatumsVertical Datums
A vertical datum is used for measuring the elevations of points on the earth's surface.
– Geodetic Datums (NGVD29, NAVD88) – Hydrologic Datums (LWRP, IGLD)– Tidal Datums (MSL or LMSL, MLLW, MHW)– Local (Cario, Memphis Datum) – Ellipsoid (Space-based) Surfaces
Influenced by Gravity
Not Influenced by Gravity
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Orthometric HeightsComparison of Vertical Datum Elements
NGVD 29 NAVD 88
DATUM DEFINITION 26 TIDE GAUGES FATHER’S POINT/RIMOUSKI IN THE U.S. & CANADA QUEBEC, CANADA
BENCH MARKS 100,000 450,000
LEVELING (Km) 102,724 1,001,500
TIDAL EPOCH None 1960 - 1978
GEOID FITTING Distorted to Fit MSL Gauges Best Continental Model
Vertical DatumsVertical Datums
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The North American Vertical Datum of 1988 is referenced to 1 tide
gage on the St. Lawrence Seaway
1,000,000+ km
The Sea Level Datum of 1929 was
referenced to 26 tide gages in the US and
Canada~100,000 km
The Sea Level Datum of 1929’s name was
changed to the National Geodetic
Vertical Datum in 1973
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4 cm
125 cm
70 cm
85 cm 102 cm
NGVD 29Referenced to 26 Tide Gages
NAVD 88Referenced to 1 Tide Gage
(Father’s Point)
NAVD88 minus LMSL(1960-1978)
-23 cm
-23 cm
-11 cm
-11 cm
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• NOAA's National Geodetic Survey (NGS) defines and manages a national coordinate system.
• This network, the National Spatial Reference System (NSRS), provides the foundation for our spatial infrastructure
• NGS develops Federal standards for geodetic surveys and helps to coordinate surveying methods.
• NGS State Geodetic Advisors are stationed in several states to work with local communities to expand surveying capabilities.
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Changing Elevations in a Dynamic EnvironmentChanging Elevations in a Dynamic Environment
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Changing Elevations in a Dynamic EnvironmentChanging Elevations in a Dynamic Environment
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Additional Information Available at:
http://crunch.tec.army.mil/information/SM_CoP/ndsp
