Ellipse P F 2 O F 1 a b X Z An ellipse is defined by Focal length Distance F1 P

Ellipse p f 2 o f 1 a b x z an ellipse is defined by

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is
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Ellipsoid or Spheroid Rotate an ellipse around an axis O X Z Y a a b Rotational axis
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Standard Ellipsoids Ellipsoid Major axis, a (m) Minor axis, b (m) Flattening ratio, f Clarke (1866) 6,378,206 6,356,584 1/294.98 GRS80 6,378,137 6,356,752 1/298.57 Ref: Snyder, Map Projections, A working manual, USGS Professional Paper 1395, p.12
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Horizontal Earth Datums An earth datum is defined by an ellipse and an axis of rotation NAD27 (North American Datum of 1927) uses the Clarke (1866) ellipsoid NAD83 (NAD,1983) uses the GRS80 ellipsoid on a geocentric axis of rotation WGS84 (World Geodetic System of 1984) uses GRS80, almost the same as NAD83
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Definition of Latitude, f (1) Take a point S on the surface of the ellipsoid and define there the tangent plane , mn (2) Define the line pq through S and normal to the tangent plane (3) Angle pqr which this line makes with the equatorial plane is the latitude f , of point S O f S m n q p r
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Cutting Plane of a Meridian P Meridian Equator Prime Meridian
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