azimuth of the principal plane
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ii. The arc of the observer's rational horizon or the angle at his zenith contained between the observer's celestial meridian and the vertical circle through that body. It is the distance, measured in degrees, along the horizon westward from the south point of the horizon to the place where the vertical circle through an object intersects the horizon.
iii. As it pertains to aerial photography, the azimuth of a photograph is the clock-wise horizontal angle measured about the ground nadir point from the ground survey north meridian to the principal plane of the photograph. Also called azimuth of the principal plane.