OUTLINE:

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GEODESY

• OUTLINE
• definition and history
• three major models
• how are reference shapes used
• geodetic systems

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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 Earths size and shape

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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

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GEODESY

• study of
• size and shape of earth
• measurements of the position and motion of points
  on the earths surface
• configuration and area of large portions of
  earths surface

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HISTORY OF GEODESY

• has been around since approximately 500 BC, not
  taken seriously until 100 AD (Ptolemy)

• Toscanelli (1397-1482) famous incorrect vision of
  the world

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HISTORY OF GEODESY

• notion of the earths 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

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IMPORTANCE OF GEODESY

• geodesy has a symbiotic relationship with some
  other sciences

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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

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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

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MAJOR MODEL OF GEODESY
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MAJOR MODEL OF GEODESY
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• at least 20 different determination of parameters
  for ellipsoid since 1800
• geodesists use semi-major axis and flattening to
  identify ellipsoids

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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)

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• rises over continents, depressed in oceanic areas

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• highest point over New Guinea lowest point,
  south of India

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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
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MAJOR MODEL OF GEODESY

• small scale maps locations defined by authalic
  sphere
• detailed large scale
• horizontal positions ellipsoid
• elevations geoid surface

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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

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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

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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

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DATUM TRANSFORMATION

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

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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