altitude—The vertical angle between the plane of the horizon and the line to the object which is being observed or defined.  In astronomy and surveying, the altitude is positive if the object is above the horizon, and negative if it is below it.  In surveying, positive altitude may be described as the angle of elevation; negative altitude as the angle of depression.  In astronomy, altitude is the complement of the zenith distance.   In photogrammetric surveying and airport zoning, altitude applies to the elevation above a datum of points in space.  Altitude is sometimes used synonymously with elevation; angular elevation, angle of inclination.  See also angle of inclination.

altitude, apparent— The observed vertical angle of a celestial object corrected for instrumental errors, personal errors, and inaccuracies in the reference level (principally dip), but not for the refraction or parallax.

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Source: NSPS “Definitions of Surveying and Associated Terms“, used with permission.

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