Norling asserts that you only include in a drawing what you can see without shifting your eyes. Trying to include objects that you can see out of the corner of your eyes results in warped perspective. Thus, it is very important to establish the picture frame around whatever object you wish to be the center of interest. In the first drawing, the tracks are the center of interest and there is only one vanishing point. The tree is at the corner of vision; therefore, it is largely not within the picture frame. In the second drawing, the tree is the center of interest and there are two vanishing points.
When a group of roofs have the same slope, then the lines of these roofs meet at two points along a line, drawn through one of the vanishing points, that is perpendicular to the horizon. All roofs that slope to the left go through the two points on the line that is extended from VP A. All roofs that slope to the right go through the two points on the line that is extended from VP B. It seems that the two points on these perpendicular lines are equidistant from their respective VP point (A or B). This lesson was confusing at first, but once I began to draw the roofs as demonstrated, it began to make a lot of sense. I'm really enjoying that perspective is so methodical, not some esoteric mystery.