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 to establish a system of geographic coordinates, we first have to know the Earth’s size and shape G EODESY
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
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
H ISTORY OF G EODESY has been around since approximately 500 BC, not taken seriously until 100 AD (Ptolemy) Toscanelli ( ) famous incorrect vision of the world
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
I MPORTANCE OF G EODESY geodesy has a symbiotic relationship with some other sciences
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
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
sphere ellipse M AJOR M ODEL OF G EODESY
at least 20 different determination of parameters for ellipsoid since 1800 geodesists use semi-major axis and flattening to identify ellipsoids
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
rises over continents, depressed in oceanic areas
highest point over New Guinea; lowest point, south of India
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
small scale maps: locations defined by authalic sphere detailed large scale: horizontal positions – ellipsoid elevations – geoid surface M AJOR M ODEL OF G EODESY
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
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
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
D ATUM T RANSFORMATION must perform a datum transformation to take a latitude, longitude and/or height from one system to a second system
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