How to Ascertain the Relative Heights of Figures on an Inclined Plane
The three figures to the right marked f, g, b (Fig. 96) are on level ground, and we measure them by the vanishing scale aS, bS. Those to the left, which are repetitions of them, are on an inclined plane, the vanishing point of which is S·; by the side of this plane we have placed another vanishing scale a·S·, b·S·, by which we measure the figures on that incline in the same way as on the level plane. It will be seen that if a horizontal line is drawn from the foot of one of these figures, say G, to point O on the edge of the incline, then dropped vertically to o·, then again carried on to o·· where the other figure g is, we find it is the same height and also that the other vanishing scale is the same width at that distance, so that we can work from either one or the other. In the event of the rising ground being uneven we can make use of the scale on the level plane.
Fig. 96.
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How to Find the Distance of a Given Figure or Point from the Base Line
Let P be the given figure. Form scale ACS, S being the point of sight and D the distance. Draw horizontal do through P. From A draw diagonal AD to distance point, cutting do in o, through o draw SB to base, and we now have a square AdoB on the perspective plane; and as figure P is standing on the far side of that square it must be the distance AB, which is one side of it, from the base line—or picture plane. For figures very far away it might be necessary to make use of half-distance.