<Dl> <Dd> κ = (1 + h R) 2 − 1 . (\ displaystyle \ kappa = (\ sqrt (\ left (1 + (\ frac (h) (R)) \ right) ^ (2) - 1)) \ .) </Dd> </Dl> <Dd> κ = (1 + h R) 2 − 1 . (\ displaystyle \ kappa = (\ sqrt (\ left (1 + (\ frac (h) (R)) \ right) ^ (2) - 1)) \ .) </Dd> <P> The curvature is the reciprocal of the curvature angular radius in radians . A curvature of 1 appears as a circle of an angular radius of 57.3 ° corresponding to an altitude of approximately 2640 km above the Earth's surface . At an altitude of 10 km (33,000 ft, the typical cruising altitude of an airliner) the mathematical curvature of the horizon is about 0.056, the same curvature of the rim of circle with a radius of 10 m that is viewed from 56 cm directly above the center of the circle . However, the apparent curvature is less than that due to refraction of light in the atmosphere and because the horizon is often masked by high cloud layers that reduce the altitude above the visual surface . </P> <P> The horizon is a key feature of the picture plane in the science of graphical perspective . Assuming the picture plane stands vertical to ground, and P is the perpendicular projection of the eye point O on the picture plane, the horizon is defined as the horizontal line through P . The point P is the vanishing point of lines perpendicular to the picture . If S is another point on the horizon, then it is the vanishing point for all lines parallel to OS . But Brook Taylor (1719) indicated that the horizon plane determined by O and the horizon was like any other plane: </P>

When does curvature of the earth affect line of sight
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