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Geodetic Control and Datums

Geodetic Control and Datums. Where is it? How Accurately can we map it?. Mission Statement for the South Carolina Geodetic Survey.

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Geodetic Control and Datums

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  1. Geodetic Control and Datums Where is it? How Accurately can we map it?

  2. Mission Statement for the South Carolina Geodetic Survey • The primary mission of the Geodetic Survey is to establish horizontal and vertical geodetic control throughout the state to allow land and land-related items to be referenced to the national horizontal and vertical coordinate system, to ensure the integrity of new geodetic data, and to maintain geodetic files.  • Mapping Coordination supports the development of an accurate, uniform statewide mapping system on a county-by-county base.  These mapping systems will provide a large-scale map base for county and state land information systems that will enhance land records, engineering, land planning and economic development.

  3. Spatial Data • Data that identifies the geographic location of features • Data is generally stored as coordinates and topology • Data is often overlaid onto some form of a map • Data is often accessed, manipulated or analyzed through Land/Geographic Information Systems

  4. Coordinate System • A reference system for defining points in space or on a particular surface by means of distances or angles or both, with relation to designated axes, planes or surfaces. • Generally refer to two common types of systems • Spherical • Rectangular

  5. Coordinates Defined • Datum, Geodetic Datum, Geocentric Datum • A set of 8 constants specifying the coordinate system used for calculating coordinates of points on the Earth • 3 Constants to Define the Coordinate Origin • 3 Constants to Define the Coordinate Orientation • 2 Constants to Define the Ellipsoid Shape and Size

  6. Datums • "A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating the coordinates of points on the Earth." • "The datum, as defined in (1), 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." • When the first definition is used, the published coordinates of control points can change when better measurements allow better determinations. With the second definition, a change in coordinates should result in a new datum. NGS has used the first definition for NAD 1983.

  7. Vertical Datums • NGVD 29 • A vertical datum renamed in 1973 from the Sea Level Datum of 1929. Held 21 tide gauges in the US fixed. 19 along the East Coast. The datum did not represent mean sea levels on East and West coasts of the U.S. • NAVD 88 • Established in 1991. A refined datum, much like NAD 83. Factors addressed with NAVD 88 included crustal motion, post glacial rebound, and subsidence due to withdrawal of underground water.

  8. Gotta Love Science?

  9. Why Should I change Datums? • More players than just the United States • Improved technology and Surveying techniques • EDMs, VLBI Sites, Introduction of Satellite Navigation • Improved understanding of the shape and mass of mother Earth

  10. Why Survey with Satellites?

  11. Okay – Lets look at the Geodesy

  12. OMG Another Datum WGS 84 World Geodetic System of 1984 • Reference frame used by DoD • Defined in 1987 Using Doppler Satellites • Was used as the reference frame for broadcast GPS Ephemerides • Differed very little from NAD 83 (86) in its infancy • Has been revised 3 times since inception . The latest was using the International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) • Differs from NAD 83 by ~2 meters

  13. What do I do now? • Corrections need to be applied to data collected with GPS receivers. What is available? • DGPS • WAAS • Differential Corrections • SC RTN

  14. Reference/Base Stations

  15. IERS and ITRF

  16. The Differences between Datums

  17. Coordinate Shift 2 ft 0.61 m 1.2 ft 0.36 m 0.8 ft 0.24 m

  18. Improving Positional Accuracy TIME NETWORK LOCAL NETWORK SPAN ACCURACY ACCURACY • NAD 27 1927-1986 10 Meters First-Order • NAD 83 1986-1991 1 Meter First-Order • NAD 83 (1995) 1991-1997 0.1 Meter B-Order • (CORS96) A-Order • NAD 83 (2001) 2001 0.01 Meter 0.01 Meter • (HARN) • NAD 83 (2007) 2007 0.01 Meter 0.01 Meter • (SC RTN)

  19. SC Height Modernization Projects

  20. Explanations of Datums from the NGS • OK-your situation is a little more complicated than most because of the desire of South Carolina and North Carolina not to publish the results of any HARN readjustments until there was a full national adjustment (which happened in 2007). • Passive control was never tagged as NAD83 (CORS)-only CORS coordinates had this tag. This I believe would have the projection tag of (CORS96) • No coordinates were published with the tag of NAD83 (HARN). This was the 2001 adjustment. This was adjustment was adopted in SC. • When the 2007 national adjustment was completed, the 2007 coordinates for both states were published and any previous values in the database showed up as superseded.

  21. Questions?Thank-you! Matt Wellslager 803-896-7715 or 803 528 4857 Matt.wellslager@scgs.sc.gov

  22. Considerations for the Future

  23. Light Squared New York Times

  24. More Light Squared Washington Post

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