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

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

GGEODESYEODESY

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

GGEODESYEODESY

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

GGEODESYEODESY

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

GGEODESYEODESY

HISTORY OF GEODESYHISTORY 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

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

HISTORY OF GEODESYHISTORY OF GEODESY

IMPORTANCE OF GEODESY

IMPORTANCE OF GEODESY

geodesy has a symbiotic relationship with some other sciences

MAJOR MODEL OF GEODESY


1.1. Sphere:Sphere:

based on observations of ships going over horizon based on observations of ships going over horizon and astronomical phenomena and astronomical phenomena

Authalic sphere – sphere with the same surface area Authalic sphere – sphere with the same surface area of earth as a more regular surface called ellipsoidof earth as a more regular surface called ellipsoid

used today for used today for short range navigation and global short range navigation and global distance approximationsdistance approximations

2.2. Ellipse:Ellipse:

earth is ellipse with a slightly earth is ellipse with a slightly larger equatorial axis than polar larger equatorial axis than polar axis (~20 km) due to spinning of axis (~20 km) due to spinning of earthearth

most commonly used model to most commonly used model to calculate position on earthcalculate position on earth



sphere

ellipse





at least 20 at least 20 different different determination of determination of parameters for parameters for ellipsoid since ellipsoid since 18001800

geodesists use geodesists use semi-major axis semi-major axis and flattening to and flattening to identify ellipsoidsidentify ellipsoids

3.3. GeoidGeoid

surface where strength of gravity equals that at mean surface where strength of gravity equals that at mean sea levelsea level

variations in rock density and topography causes deviations up to 100 m

irregular (geoid) vs regular (ellipsoid)



rises over continents, depressed in oceanic areasrises 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



small scale maps: locations defined by authalic small scale maps: locations defined by authalic sphere sphere

detailed large scale: detailed large scale:

horizontal positions – ellipsoidhorizontal positions – ellipsoid

elevations – geoid surfaceelevations – geoid surface



GEODETIC SYSTEMSGEODETIC 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

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

GEODETIC SYSTEMSGEODETIC SYSTEMS

HORIZONTAL DATUMS

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

DATUM TRANSFORMATION

DATUM TRANSFORMATION

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 NAD27World Geodetic System (WGS84)

based on GRS80 Ellipsoid basis for all GPS based maps very similar to, and more precise than NAD83

COMMONLY USED DATUMSCOMMONLY USED DATUMS

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