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? 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
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 1250G) TIDAL EPOCH Varies from pointtopoint 19701988 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:
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 (statewide) remaining differences
 ITRF00/NAD83 transformation (vs. ITRF97)
 Compute and remove conversion surface from G99SSS
High Resolution Geoid Models GEOID03 (vs. Geoid99) Relative to nongeocentric GRS80 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 Assumes the geoidal slopes “shape” is well modeled in the area. “Valid” Orthometric constraints along with “valid” transformation parameters removes additional unmodeled changes in slope or bias (fitted plane)
What is OPUS? OnLine Positioning User Service Processes DualFrequency GPS data Global availability (masked) 3 goals:  Simplicity
 Consistency
 Reliability
How Does OPUS Compute Position?
Springbased relative gravimeters Example: LaCoste & Romberg land meter
Changes for the Better Improve Gravity Field Modeling NGS will compute a poletoequator, AlaskatoNewfoundland 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 national horizontal datum to remove gross disagreements with the ITRF
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