fig. 3-3. It is called central projection because the linesof sight, or projectors, meet at a central point; the eyeof the observer.You can see that the projected view of the objectvaries considerably in size, according to the relativepositions of the objects and the plane of projection. Itwill also vary with the distance between the observerand the object, and between the observer and the planeof projection. For these reasons, central projection isseldom used in technical drawings.If the observer were located a distance away fromthe object and its plane of projection, the projectorswould not meet at a point, but would be parallel to eachother. For reasons of convenience, this parallelprojection is assumed for most technical drawings andis shown in figure 3-4. You can see that, if theprojectors are perpendicular to the plane of projection,a parallel projection of an object has the samedimensions as the object. This is true regardless of therelative positions of the object and the plane ofprojection, and regardless of the distance from theobserver.ORTHOGRAPHIC AND OBLIQUEPROJECTIONAn ORTHOGRAPHIC projection is a parallelprojection in which the projectors are perpendicular tothe plane of projection as in figure 3-4. An OBLIQUEprojection is one in which the projectors are other thanperpendicular to the plane of projection. Figure 3-5shows the same object in both orthographic andoblique projections. The block is placed so that itsFigure 3-3.—Central projection.3-2Figure 3-4.—Parallel projections.Figure 3-5.—Oblique and orthographic projections.front surface (the surface toward the plane ofprojection) is parallel to the plane of projection. Youcan see that the orthographic (perpendicular)projection shows only this surface of the block, whichincludes only two dimensions: length and width. Theoblique projection, on the other hand, shows the frontsurface and the top surface, which includes threedimensions: length, width, and height. Therefore, anoblique projection is one way to show all threedimensions of an object in a single view. Axonometricprojection is another and we will discuss it in the nextparagraphs.