Perspective drawings are used to illustrate the appearance of the object so that it is completely understandable to a third person. Actual measurements are not taken from such drawings, as they are only concerned with the mechanics of vision in which receding lines appear to converge and diminish. Various methods are used, many of which are highly technical and require special study, but 334 shows a simple compromise which should be sufficient for most practical purposes. Figure 334:2 shows the elevation of a simple chest of drawers, and in 334:1 the plan ABCD is drawn to scale at an angle to the horizon (eye-line) dependent on the viewpoint required (in this case 45°). The station point (SP) is placed to represent the eye of the beholder, and only trial and error can give the most satisfactory position. While the angle or cone of vision represented by lines drawn from the extremities dB of the plan ABCD to the station point (SP) should be between 40° and 60° for a normal aspect (the normal camera lens takes in about 55°), a narrower angle, even though it increases the distortion, helps to contain the vanishing points within the drawing, as a little experiment will show. Lines are now drawn to form a right angle at SP and cut the horizon at VPl and VP2, which are the vanishing points to which all receding lines converge. A base - or ground-line
measurement below the eye-line or horizon, while a perpendicular dropped from A to H will provide a height-line on which all significant measurements can be transferred from the elevation (334:2). Lines are then drawn from all the salient points on the plan ABCD to the station point SP, and from where they cut the horizon verticals are then dropped as shown. If from the line AH (334:1) the various heights are connected to VPl and VP2 the completed drawing will then be automatically developed.
plan has to be drawn. If the sizes are known, they can be used directly on the horizontal and vertical scales and a perspective drawing produced. Once the SP has been used to fix the measuring points (335:A) it can be ignored.
Remember that the measuring points fix measurements of length and width, read off directly from the height scale, the lines from which are taken to the vanishing points.
As the object of such a perspective drawing is to convey the visual appearance only, and not to
serve as an accurate scaled representation from which critical measurements can be taken, small details should be drawn free-hand rather than rigidly adhering to mechanical projection with its inevitable distortion. Curves are also better drawn as free-hand ellipses after plotting the general position.