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Precise Digital Leveling
Section 6Vertical Datum
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Vertical Datum• Local/Regional
– Assumed– City, County– International Great Lakes Datum 1955 (IGLD55)– International Great Lakes Datum 1985 (IGLD85)– Tidal Datums
• National– National Geodetic Vertical Datum of 1929 (NGVD29)– North American Vertical Datum of 1988 (NAVD88)
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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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First-Order Leveling Network NGVD 29
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North American Vertical Datum 1988 (NAVD 88)• Defined by one height (Father Point/Rimouski)
• Water-level transfers connect leveling across Great Lakes
• Adjustment performed in Geopotential Numbers
• Helmert Orthometric Heights:– H = C / (g + 0.0424 H0)– C = geopotential number– g = surface gravity measurement (mgals)– H0 = approximate orthometric height (km)
• H = 0 level is nearly a level surface
• H = 0 level is biased relative to global mean sea level
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Vertical Control Network NAVD 88
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NGVD 29 Versus NAVD 88NGVD 29 Versus NAVD 88Datum 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 Adjustments Considerations:
• 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)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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Tidal DatumTidal Datum• Heights Measured Above Local Mean Sea Level
• National Tidal Datum epoch; 19 year series
• Encompasses all significant tidal periods including 18.6 year period for regression of Moon’s nodes
• Averages out nearly all meteorological, hydrological, and oceanographic variability
• Leveling is used to determine relationship between bench marks and tidal gauges
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http://tidesandcurrents.noaa.gov/
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NAVD 88 minusNAVD 88 minusLMSL (1960-1978)LMSL (1960-1978)
(units = cm)
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Point Reyes, Drakes Bay, CaliforniaFT
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Geodetic and Tidal
Geodetic and Tidal Datum Relationships
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AL, AK, CA, CT, FL, GA, LA, MD, MS, NJ, NY, NC, OR, RI, SC, WA
Privately Owned
Uplands
State Owned
Tidelands
Territorial Seas
State Submerged Lands
Contiguous Zone
Exclusive Economic Zone
Federal Submerged LandsHigh Seas
Privately Owned
State Owned
TX
3 n. mi.12 n. mi.
200 n. mi.
Privately Owned
State Owned
DE, MA, ME, NH, PA, VA
MHHWMHW
MLLW
Importance of Shoreline
Chart Datum