Missing points of repair

 

Figure 1a : the line from F to VP, divides AV and BW so that a/b = c/d.

Figure 1b and 1c : If VP cannot be created as shown in figure 1, G is best estimated by dividing BW in the relation a/b using e.g. a calculator.

Figure 2a : Missing repair line ?

Draw a small image (figure 2b), create VP1 as crossing-point of a and b, draw HL, create crossing-point with c to obtain VP2. Connect VP2 with Q: The angle in-between PQ and P-VP2 is equal to the angle of inclination of ?.

Or (figure 2c) : Create mini-triangle (blue) with aa parallel to a to define x/y, divide PQ in the relation x/y to create Z. Draw vertical through PQ through Z to create VP2, proceed as above.