Spatial Interrelationships

Click here to view a video that explains an intro orthographic projection.

Orthographic Drawing

Orthographic drawing is the basis of all engineering drawing, and it is also the basis for the study of Descriptive Geometry. A well-trained engineer or technician must be able to pick up a drawing and understand it. This understanding, of necessity, involves the basic principles of orthographic drawing. Generally speaking, a course in Engineering Drawing consists of drawing various objects in two or more views utilizing the principles of orthographic projection. These views may be projected on the three principal planes – horizontal, frontal and profile – or on auxiliary planes. In turn, the views may or may not be sectioned. Many students entering a learnership have had limited experience in orthographic drawing in the high school or technical school which may have prepared them for the learnership. It may have only consisted of several weeks of Mechanical Drawing, but this previous contact with the principles involved in orthographic drawing forms a frame of reference that usually proves valuable in solving Engineering Drawing problems. The question might then be asked, "Well, what is Descriptive Geometry?" Very briefly, Descriptive Geometry is the graphical solution of point, line and plane problems in space. These solutions are accomplished by means of the same principles of orthographic drawing which are involved in making a simple three-view drawing of an object. Orthographic Projection – the use of parallel lines of sight at 90° to an image plane.

Figure 1 is of a 3-dimensional object that is represented by top, front and side views.

Cuts and Fills

Of the many types of problems encountered by farmers, one of the most common is that of reading contour maps. Sometimes soil needs to be removed from one place (cut) to be placed in another spot (fill), for example when land is being levelled (see Fig. 3 below). The following are some of the terms used in locating cuts and fills:

Profile – a vertical section of the earth's surface containing a given line that may be either straight or curved. The length of the profile must be equal to the true length of the given line. (See Fig. 2)

Section – a vertical section at right angles to the profile line.

Cut – earth removed to obtain a required slope or elevation.

Fill – earth added to existing contour in order to obtain a required slope or elevation.