VERTICAL DATUMS

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VERTICAL DATUMS

• APRIL 08, 2008
• By
• Ronnie L. Taylor
• Chief, Geodetic Advisor Branch
• NOAA, National Geodetic Survey

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VERTICAL DATUMS

• Local/Regional
• Assumed
• City, County
• International Great Lakes Datum 1955 (IGLD55)
• International Great Lakes Datum 1985 (IGLD85)
• National
• Sea Level Datum 1929 (SLD29)
• National Geodetic Vertical Datum of 1929
  (NGVD29) (As of July 2, 1973)
• North American Vertical Datum of 1988 (NAVD88)
  (As of June 24, 1993)
• Tidal Datums

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VERTICAL DATUMS

• What types of heights are there?
• Orthometric heights
• Ellipsoid heights
• Geoid heights

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NATIONAL SPATIAL REFERENCE SYSTEM(NSRS)

• The National Spatial Reference System (NSRS) is
  the name given to all geodetic control contained
  in the National Geodetic Survey (NGS) Data Base.
  This includes A, B, First, Second and
  Third-Order horizontal and vertical control,
  Geoid models such as GEOID 99, precise GPS orbits
  and Continuously Operating Reference Stations
  (CORS), observed by NGS as well as data submitted
  by other Federal, State, and local agencies,
  academic institutions and the private sector

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NATIONAL SPATIAL REFERENCE SYSTEM(NSRS)

• ACCURATE -- CM accuracy on a global scale
• MULTIPURPOSE -- Supports Geodesy, Geophysics,
  Land Surveying, Navigation, Mapping, Charting and
  GIS activities and derived products
• ACTIVE -- Accessible through Continuously
  Operating Reference Stations (CORS)
• INTEGRATED -- Related to International services
  and standards (e.g. International Earth Rotation
  and Reference System Service, International GPS
  Service etc.)

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WHAT IS A GEODETIC DATUM?

• Geodetic Datum
• A set of constants specifying the coordinate
  system used for geodetic control, i.e., for
  calculating coordinates of points on the Earth
• above together with the coordinate system and
  the set of all points and lines whose
  coordinates, lengths, and directions have been
  determined by measurement or calculation.
• Definitions from the Geodetic Glossary,
  September 1986
• Characterized by
• A set of physical monuments and
• Published coordinates (horizontal and/or
  vertical) on
  the monuments

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GEODETIC DATUMS

• Classical
• Horizontal 2 D (Latitude and Longitude, e.g.
  NAD 27, NAD 83(1986))
• Vertical 1 D (Orthometric Height, e.g. NAVD 88)
• Contemporary
• Practical 3 D (Latitude, Longitude and
  Ellipsoid Height) Fixed and Stable Coordinates
  seldom change, e.g. NAD 83 (1991)
• Scientific
• 4 D (Latitude, Longitude, Ellipsoid Height,
  Velocities) Coordinates change with time, e.g.
  ITRF00

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VERTICAL CONTROL DATUMS

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

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VERTICAL DATUMS OF THE UNITED STATES
First General Adjustment - 1899
Second General Adjustment - 1903
Third General Adjustment - 1907
Fourth General Adjustment - 1912
Mean Sea Level 1929 National Geodetic Vertical
Datum of 1929 (NGVD 29)
North American Vertical Datum of 1988 (NAVD 88)
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METADATAData About Data

• DATUMS
• NAD 27, NAD 83(1986), NAD83 (199X), NGVD29,
  NAVD88, MLLW, MSL, MHW etc.
• UNITS
• Meters, U.S. Survey Feet, International Feet,
  Chains, Rods, Poles, Links, Varas (CA or TX),
  Smokes, Smoots.
• ACCURACY
• A, B, 1st, 2nd, 3rd, 3cm, Scaled etc.

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METADATA??
Horizontal Vertical Datums??
Plane Coordinate System ??
Units of Measure ??
How Accurate ??
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METADATA??
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NGVD 29 Versus NAVD 88

• Datum Considerations NGVD 29
  NAVD 88
• Defining Height(s) 26 Local MSL
  1 Local MSL
• Tidal Epoch Various 1960-78
• 
  (18.6 years)

• Treatment of Leveling Data
• Gravity Correction Ortho Correction
  Geopotential Nos.
• (normal gravity) (observed
  gravity)
• Other Corrections Level, Rod, Temp.
  Level, Rod, Astro,
• Temp, Magnetic,
• and Refraction
• 
  

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NGVD 29 Versus NAVD 88 (continued)

• Adjustments Considerations NGVD 29 NAVD
  88
• Method Least-squares
  Least-squares
• Technique Condition Eq.
  Observation Eq.
• Units of Measure Meters
  Geopotential Units
• Observation Type Links Between
  Height Differences
• Junction Points
  Between Adjacent BMs

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NGVD 29 Versus NAVD 88 (continued)

• Adjustments Statistics NGVD 29
  NAVD 88
• No. of Bench Marks 100,000 (est)
  450,000 (US only)
• Km of Leveling Data 75,159 (US)
  1,001,500
• 31,565 (Canada)

• Published Information
• Orthometric Height Type Normal Helmert
  
• Orthometric Height Units Meters
  Meters
• Gravity Value Normal Actual

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First-Order Leveling Network NGVD 29
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NGVD 29 TIDE CONTROL
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NAVD 88 Origin Point
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PRESENT NETWORK FOR NAVD 88

• ORIGINAL LEVELING 700,000 KM
• REPEAT LEVELING 200,000 KM
• NEW BNA LEVELING 81,500 KM
• NEW OUTSIDE LEVELING 20,000 KM
• TOTAL FOR NAVD 88 1,001,500 KM
• (620,000
  MILES)

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National Geodetic Vertical Datum 1929(NGVD 29)

• Defined by heights of 26 tidal stations in U.S.
  and Canada
• Tide gages were connected to the network by
  leveling from tide gage staffs to bench marks
• Water-level transfers used to connect leveling
  across Great Lakes
• Normal Orthometric Heights
• H C / ?
• C model (normal) geopotential number
• ? from normal gravity formula
• H 0 level is NOT a level surface

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NORTH AMERICAN VERTICAL DATUM 88

• WHAT IS A VERTICAL CONTROL NETWORK?
• An Interconnected System of Bench Marks
• Each Bench Mark Is Assigned A height Referenced
  To A Common Surface

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VERTCON - Vertical Datum Transformations
Published 4.896 m Difference 0.000 m / 0.00 ft
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HEIGHT SYSTEMS DATUMS

• INTERNATIONAL Great Lakes Datum (IGLD) 1985
• Same as NAVD 88, except published in Dynamic
  Heights
• Dynamic Heights
• Hdym C/Go Go 980.6199 gals
• (Normal gravity at 45 degrees latitude as defined
  in 1985)

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NORTH AMERICA VERTICAL DATUM 88

• WHY DO WE NEED A VERTICAL CONTROL NETWORK?
• Reduces The Amount Of Future Leveling Required
• Enables Surveyors To Check Their New Leveling
• Provides Backups For Destroyed Or Disturbed Bench
  Marks
• Assists In Monitoring Changes In Local Areas
• Provides A Common Framework

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Vertical Datums
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Vertical Datums
Vertical Datums

• MSL elevation is roughly equivalent to
  orthometric height (H), the technical name for
  height above the geoid.
• Geoid height (N) is the separation between the
  geoid and the ellipsoid. It can be plus or minus.
  
• Ellipsoid height (h) is the distance above or
  below the ellipsoid (plus or minus). Ellipsoid
  height is also called geodetic height.

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

H
TOPOGRAPHIC SURFACE
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What is the GEOID?

• The equipotential surface of the Earths gravity
  field which best fits, in the least squares
  sense, mean sea level.
• Cant see the surface or measure it directly.
• Modeled from gravity data.
• Definition from the
  Geodetic Glossary, September 1986

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Level Surfaces and Orthometric Heights
Earths
Surface
WP
Level Surfaces
P
Plumb
Line
Mean Sea Level
Geoid
WO
PO
Level Surface Equipotential Surface (W)
Ocean
Geopotential Number (CP) WP -WO
H (Orthometric Height) Distance along plumb
line (PO to P)
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Orthometric Heights

• Using Optical or Digital/Bar Code Leveling

B
Topography
A
C

• Adjusted to Vertical Datum using existing control
• Achieve 3-10 mm relative accuracy

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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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National Water Level Observation Network (NWLON)
175 Permanent Stations
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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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MHHW 10.27 feet (3.130 meters)
MHW 9.83 feet (2.996 meters)
Boston Harbor, Massachusetts
NAVD88 5.51 feet (1.679 meters)
Feet
MTL 5.08 feet (1.548 meters)
NGVD29 4.70 feet (1.433 meters)
MLW 0.34 feet (0.104 meters)
MLLW 0.00 feet (0.000 meters)
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Importance of Shoreline
AL, AK, CA, CT, FL, GA, LA, MD, MS, NJ, NY, NC,
OR, RI, SC, WA
Territorial Seas
Privately Owned Uplands
State Owned Tidelands
Contiguous Zone
Exclusive Economic Zone
State Submerged Lands
Federal Submerged Lands
High Seas
3 n. mi.
12 n. mi.
MHHW
200 n. mi.
MHW
MLLW
Chart Datum
Privately Owned
State Owned
Privately Owned
State Owned
TX
DE, MA, ME, NH, PA, VA
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Vertical Datum Relationships
3-D Datums
Orthometric Datums
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Leveling and Datums
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National Spatial Reference System(NSRS)

• Consistent National Coordinate System
• Latitude
• Longitude
• Height
• Scale
• Gravity
• Orientation
• and how these values change with time.

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Before you build . . .
D E F I N I T I O N
. . . inspect your foundation.
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Questions?
Ronnie L. Taylor Chief, Geodetic Advisor
Branch 301-713-1054 Ronnie.Taylor_at_noaa.gov