Vertical Datums and Heights


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Vertical Datums and Heights

  • Daniel J. Martin

  • National Geodetic Survey

  • VT Geodetic Advisor

  • VTrans Monthly Survey Meeting

  • October 06, 2008


Can You Answer These Questions?

  • What is the current official vertical datum of the United States?

  • What’s the difference between ellipsoid, orthometric and geoid and dynamic heights?

  • The difference between NGVD 29 and NAVD 88 in most of Vermont is?

  • A point with a geoid height of -28.86 m means what?





GEODETIC DATUMS

  • A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating coordinates of points on the Earth. Specific geodetic datums are usually given distinctive names. (e.g., North American Datum of 1983, European Datum 1950, National Geodetic Vertical Datum of 1929)



GEODETIC DATUMS

    • CLASSICAL
    • Horizontal – 2 D (Latitude and Longitude) (e.g. NAD 27, NAD 83 (1986))
    • Vertical – 1 D (Orthometric Height) (e.g. NGVD 29, NAVD 88)
    • Contemporary
    • PRACTICAL – 3 D (Latitude, Longitude and Ellipsoid Height) Fixed and Stable – Coordinates seldom change (e.g. NAD 83 (1992) or NAD 83 (NSRS 2007))
    • SCIENTIFIC – 4 D (Latitude, Longitude, Ellipsoid Height, Velocity) – Coordinates change with time (e.g. ITRF00, ITRF05)


Vertical Datums

  • A set of fundamental elevations to which other elevations are referred.

  • Datum Types

  • Tidal – Defined by observation of tidal variations over some period of time

  • (MSL, MLLW, MLW, MHW, MHHW etc.)

  • Geodetic – Either directly or loosely based on Mean Sea Level at one or more points at some epoch

  • (NGVD 29, NAVD 88, IGLD85 etc.)



TYPES OF HEIGHTS





VERTICAL DATUMS OF THE UNITED STATES



NGVD 29 TIDE CONTROL



Orthometric Heights Comparison of Vertical Datum Elements

  • NGVD 29 NAVD 88

  • DATUM DEFINITION 26 TIDE GAUGES FATHER’SPOINT/RIMOUSKI

  • IN THE U.S. & CANADA QUEBEC, CANADA (BM 1250-G)

  • TIDAL EPOCH Varies from point-to-point 1970-1988

  • BENCH MARKS 100,000 450,000

  • LEVELING (Km) 106,724 1,001,500

  • GEOID FITTING Distorted to Fit MSL Gauges Best Continental Model







What is the GEOID?

  • “The equipotential surface of the Earth’s gravity field which best fits, in the least squares sense, mean sea level.”*

  • Can’t see the surface or measure it directly.

  • Modeled from gravity data.

  • *Definition from the Geodetic Glossary, September 1986





Relationships

  • Geoid = global MSL

  • Local MSL is where the average ocean surface is with the all the disturbing forces (i.e., what is seen at tide gauges).

  • Dynamic ocean topography (DOT) is the difference between MSL and LMSL:

    • LMSL = MSL + DOT


ELLIPSOID - GEOID RELATIONSHIP







Tectonic Motions



PRELIMENARY Vertical Velocities: CORS w/ <2.5 yrs data



PRELIMENARY North American Vertical Velocities



High Resolution Geoid Models GEOID03 (vs. Geoid99)

  • Begin with USGG2003 model

    • 14,185 NAD83 GPS heights on NAVD88 leveled benchmarks (vs 6169)
    • Determine national bias and trend relative to GPS/BMs
    • Create grid to model local (state-wide) remaining differences
    • ITRF00/NAD83 transformation (vs. ITRF97)
    • Compute and remove conversion surface from G99SSS


High Resolution Geoid Models GEOID03 (vs. Geoid99)

  • Relative to non-geocentric GRS-80 ellipsoid

  • 2.4 cm RMS nationally when compared to BM data (vs. 4.6 cm)

  • RMS  50% improvement over GEOID99 (Geoid96 to 99 was 16%)

  • GEOID06 ~ By end of FY07









VERTCON - Vertical Datum Transformations





Using the Differential Form

  • Using the difference eliminates bias

  • Assumes the geoidal slopes “shape” is well modeled in the area.

  • “Valid” Orthometric constraints along with “valid” transformation parameters removes additional un-modeled changes in slope or bias (fitted plane)





What is OPUS?

  • On-Line Positioning User Service

  • Processes Dual-Frequency GPS data

  • Global availability (masked)

  • 3 goals:

    • Simplicity
    • Consistency
    • Reliability


How Does OPUS Compute Position?











Spring-based relative gravimeters Example: LaCoste & Romberg land meter





Changes for the Better Improve Gravity Field Modeling

  • NGS will compute a pole-to-equator, Alaska-to-Newfoundland geoid model, preferably in conjunction with Mexico and Canada as well as other interested governments, with an accuracy of 1 cm in as many locations as possible

  • NGS redefines the vertical datum based on GNSS and a gravimetric geoid

  • NGS redefines the national horizontal datum to remove gross disagreements with the ITRF





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