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Geoid
An euipotential surface of the Earth's gravity field, which closely approximates mean sea level.
Cannot be defined mathematically.

Ellipsoid
An arbitrary mathematically defined geometrical construct that closely approximates the earth's surface
It is defined by semimajor, semiminor axes and flattening
f= (ab) / a

Relationship between Geoid & Ellipsoid
An ellipsoid can be mathematically defined where the Geoid cannot as its shape depends on the irregular distribution of the mass inside the Earth.

3 Specific Referenced Ellipsoids
 WGS84  World Geodetic Systems
 GDA84 & GDA94  Geocentric Datum of Aus
 ANS  Australian National Spheroid
 AGD66  Australian Geodetic Datum 66 (not in use)

Projection
A method used to plot a curved surface of the Earth on a flat surface.

5 Types of Projection
 Gnomonic
 Azimuthal
 Stereographic
 Conic
 Cylindrical

Parallels of Latitude
Parallel lines that run north and south of the equator.

Meridians of Longitude
Semicircular lines that run from pole to pole, to the east and west of the Greenwich Meridian

Great Circles
A circle that will cut a sphere into 2 equal halves, which have both the same circumference and radius as the sphere.
They provide the shortest path between points on the Eath's surface.

Great Circle Angles
Are lines that are not given as a length but as an angle of the arc. (meridian of longitude)

Grids
A system of squares superimposed on a projection so that computation & plotting of data can be done using simple trigonometry.

Relationship between Geodesy, Projection & Grids
Geodesy is the study of the shape and size of the earths surface once this is known the curved surface can then be projected on to a flat surface. A grid is then applied to this in order to be able to work out exact positions using simple trigonometry.

