The notion of the “horizon line” in relation to the “viewpoint” is a tricky thing to understand. I'm stealing from my free drawing lessons here to tell you about this.
We're going to imagine we have three people, whom I have drawn above: from left to right, Mme Martinez, who is a concierge in an apartment building in Paris; retired General Bessaignet, who lives on the fifth floor of that building; and Jean-René, the bank thief, who is digging a tunnel under the building to the bank behind it. All three of them love to draw.
We are going to imagine it's Bastille Day and the General looks at the window at the Eiffel Tower and decides to draw it; and his drawing looks like this:
Mme Martinez, the concierge, is admiring the fireworks display as she stands in the street before the building. She whips out a drawing pad and draws the Eiffel Tower like this:
Jean-René is digging his tunnel underground, but he enjoys a good fireworks show so he shoves a manhole cover aside, pokes his head out, and draws a picture of the Eiffel Tower too:
Why so different?
Each person has a different viewpoint; the General is one the fifth floor and looks down on the Tower; Mme Martinez is standing on the ground, and Jean-René is looking up from ground level.
Take a look at the picture below, in which you can see they each have a different field of vision; the middle arrow in each field of vision is the centric ray we talked about with regard to Principle 1: The Picture Plane.
Let’s take another look at the three drawings by these three amateur artists, and let's consider the line of the horizon, which we usually think of as dividing land from sky.
If we consider that Mme Martinez is standing on the ground plane the way Brunelleschi does in Principle 1: The Picture Plane, looking straight ahead so that her eye's centric ray extends straight out to the horizon. When she does so, that means that the horizon in her field of vision will fall so she sees roughly as much land as sky (although in my rendition of Mme Martinez's drawing it's true I have the line of the horizon a little low).
Now let’s take the General at the top of the building as he looks at the horizon. He will actually be looking down at it, but as far as the centric ray is concerned, it still still goes straight from his eye to the horizon – you see that? That means that when he does his drawing of the scene, there will be more ground than sky in the picture. The horizon will be high in the drawing.
Last, if we imagine Jean-René poking his head out of the ground, as he looks his centric ray also will still go straight from his eye to the line of the horizon. Yet when he does his drawing, there will be much more sky than ground in it. The horizon line will be so low in the drawing as to be just about non-existant - there will be lots of sky.
And this is the strange thing. All three of them are looking at the horizon line - from high up looking down, from one the same level looking straight out at it, from low down looking up. You could say (as artists do) that the horizon line is the same thing as the eye level in the drawing. But what changes is how much sky and how much land appear in the drawing.