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

2.  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 G EODESY

3. 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 G EODESY

4. 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 G EODESY

5. H ISTORY OF G EODESY  has been around since approximately 500 BC, not taken seriously until 100 AD (Ptolemy)  Toscanelli (1397-1482) famous incorrect vision of the world

6.  notion of the earth’s gravity started to form in the late 1500s and early 1600s  mid 18 th C to present - position determined by terrestrial and astronomical means was needed for making maps H ISTORY OF G EODESY

7. I MPORTANCE OF G EODESY  geodesy has a symbiotic relationship with some other sciences

8. M AJOR M ODEL OF G EODESY 1. 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. 2. 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 M AJOR M ODEL OF G EODESY

10. sphere ellipse M AJOR M ODEL OF G EODESY

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

13. 3. 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) M AJOR M ODEL OF G EODESY

15.  rises over continents, depressed in oceanic areas

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

17.  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 M AJOR M ODEL OF G EODESY

18.  small scale maps: locations defined by authalic sphere  detailed large scale:  horizontal positions – ellipsoid  elevations – geoid surface M AJOR M ODEL OF G EODESY

19. G EODETIC S YSTEMS  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

21.  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 G EODETIC S YSTEMS

22. H ORIZONTAL D ATUMS  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

23. D ATUM T RANSFORMATION  must perform a datum transformation to take a latitude, longitude and/or height from one system to a second system

24. 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 C OMMONLY U SED D ATUMS