Basic orthographic projection

Figure 5-2.-Types of projections. (fig. 5-2) is further classified into subtypes according to the direction of its projection lines relative to the plane of projection. If the projection lines, in addition to being parallel to each other, are perpendicular (normal) to the plane of projection, the result is an orthographic projection. If they are parallel to each other but oblique to the plane of projection, the result is an oblique projection. To better understand the theory of projection, you must become familiar with certain elements that are common to each type of projection. Some of these elements are defined below. The POINT OF SIGHT (or STATION POINT) is the position of the observer in relation to the object and the plane of projection (fig. 5-2). It is from this point that the view of the object is taken. The point of sight is changed to give different views of the same object; hence, there must be a different point of sight for each view. Imagine yourself looking first at the front of an object, then down at the top, and then at the right or left side, as the case may be. Each additional view requires a new point of sight. The observer views the features of the object through an imaginary PLANE OF PROJEC- TION (or IMAGE PLANE). In parallel projec- tion, this theoretical transparent plane is placed between the point of sight and the object, as shown in figure 5-2. For perspective pictorials, it Figure 5-3.-Basic orthographic projection. 5-2