curvature of the Earth - sky and Earth appear to meet. Below this line is the ground; above it, the sky. Objects appear to get smaller as they recede away from us: they diminish in size towards a vanishing point on this line.
On the left is a sequence of three pictures of a group of buildings. In the first picture, they are viewed from high above ground level; in the second they are viewed from rooftop level; in the third they are seen from the eye level of a person standing in the street. Notice that, as our vantage point gets lower, the horizon line likewise gets lower; in fact, it is always at the same level in the picture as our eye level. For this reason, we call the horizon the eye-level line.
Since, normally, everything in one picture is viewed from the same place, the eye-level line is horizontal across the picture. It is not necessarily a line we can actually see as a horizon, for tall objects may obscure our view of the far distance, but we need to establish it so that we can refer everything we draw in the picture to it, and thereby ensure that the whole picture is consistent.
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