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G EODESY

G EODESY. OUTLINE: definition and history three major models how are reference shapes used geodetic systems. G EODESY. mapping involves determining locations on earth, transforming positions onto flat map, graphically symbolizing those positions

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G EODESY

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  1. GEODESY OUTLINE: definition and history three major models how are reference shapes used geodetic systems

  2. GEODESY • mapping involves • determining locations on earth, • transforming positions onto flat map, • graphically symbolizing those positions • to establish a system of geographic coordinates, we first have to know the Earth’s size and shape

  3. GEODESY Geodesy: discipline that deals with measurement and representation of earth in a three-dimensional time varying space • uses spaceborne and airborne remotely sensed, and ground-based measurements

  4. GEODESY study of: • size and shape of earth • measurements of the position and motion of points on the earth’s surface • configuration and area of large portions of earth’s surface

  5. HISTORYOF GEODESY • has been around since approximately 500 BC, not taken seriously until 100 AD (Ptolemy) • Toscanelli (1397-1482) famous incorrect vision of the world

  6. HISTORYOF GEODESY • notion of the earth’s gravity started to form in the late 1500s and early 1600s • mid 18th C to present- position determined by terrestrial and astronomical means was needed for making maps

  7. IMPORTANCEOF GEODESY • geodesy has a symbiotic relationship with some other sciences

  8. MAJOR MODEL OF GEODESY • Sphere: • based on observations of ships going over horizon and astronomical phenomena • Authalic sphere – sphere with the same surface area of earth as a more regular surface called ellipsoid • used today for short range navigation and global distance approximations

  9. MAJOR MODEL OF GEODESY • Ellipse: • earth is ellipse with a slightly larger equatorial axis than polar axis (~20 km) due to spinning of earth • most commonly used model to calculate position on earth

  10. sphere ellipse MAJOR MODEL OF GEODESY

  11. MAJOR MODEL OF GEODESY

  12. at least 20 different determination of parameters for ellipsoid since 1800 • geodesists use semi-major axis and flattening to identify ellipsoids

  13. MAJOR MODEL OF GEODESY • Geoid • surface where strength of gravity equals that at mean sea level • variations in rock density and topography causes deviations up to 100 m • irregular (geoid) vs regular (ellipsoid)

  14. rises over continents, depressed in oceanic areas

  15. highest point over New Guinea; lowest point, south of India

  16. MAJOR MODEL OF GEODESY • Current geoid model available in Canada • Canadian Gravimetric Geoid 2000 (CGG2000) • A geoid model to be released in 2006 is being considered as a new vertical datum to replace CGVD28 Geoid representation from GRACE satellite gravity mission data

  17. MAJOR MODEL OF GEODESY • small scale maps: locations defined by authalic sphere • detailed large scale: • horizontal positions – ellipsoid • elevations – geoid surface

  18. GEODETIC SYSTEMS • Datum – any numerical or geometrical quantity which serve as a reference or base of other quantities (coordinates of points) • in GIS – geodetic datums define the reference system that describes the size and shape of the earth

  19. GEODETIC SYSTEMS • two types: horizontal and vertical datum • horizontal datum - forms the basis for the computations of horizontal control surveys in which the curvature of the earth is considered • vertical datum – elevations are referred

  20. HORIZONTAL DATUMS • may consist of: • latitude and longitude of an initial point (origin), • azimuth of a line (direction) to some other triangulation station, • parameters (semi-major axis and flattening) of the ellipsoid selected for the computations • geoid separation at the origin • change in any of these quantities affects every point on the datum

  21. DATUM TRANSFORMATION • must perform a datum transformation to take a latitude, longitude and/or height from one system to a second system

  22. COMMONLY USED DATUMS North American Datum (NAD27) • origin is in Kansas and includes all of the contiguous US, Canada and Mexico • based on Clarke Ellipsoid (1966) North American Datum (NAD83) • based on the GRS80 Ellipsoid • as great as 300 m shifts are possible from locations mapped using NAD27 World Geodetic System (WGS84) • based on GRS80 Ellipsoid • basis for all GPS based maps • very similar to, and more precise than NAD83

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