The eye level is the height of the viewer’s eye above the ground, usually represented by a horizontal line running across the picture, even if the horizon itself is not visible in the scene.
The drawing to the above is a compositional sketch for a scene in Dinotopia: Journey to Chandara. I drew grid lines on it so that it could be enlarged to the 24x30 inch final painting.
“Eye level” is basically synonymous with “horizon.” They’re one and the same thing in the preliminary line drawing above, where the ocean is in view. But in most scenes, you don’t have such a far vista. Either you’re in a forest, or inside a room, or the view is hemmed in with buildings. But you still have to draw the imaginary line in the same place it would have been if you could see all the way to infinity.
The eye level is the very first line you put into your drawing—even a figure drawing at a sketch group. I mark it with the letters “EL” to remind me what it is. If your scene is more of an upshot, the EL is toward the bottom of the scene. Everything that you draw above the line is something you’re looking up at. In a view that’s more of a downshot, the EL is high in the composition because almost everything in the scene is below your level gaze.