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ALTITUDE, height or eminence, and particularly the height above the ground or above sea-level. In geometry, the altitude of a triangle is the length of the perpendicular from the vertex to the base. In astronomy, the altitude of a heavenly body is the apparent angular elevation of the body above the plane of the horizon (see ASTRONOMY : Spherical). Apparent altitude is the value which is directly observed ; true altitude is deduced by correcting for astronomical refraction and dip of the horizon; geocentric altitude by correcting for parallax.