Orthographic projection is used mostly for producing working drawings, And usually shoes an object as seen from front to rear, from either end, and from above or below.
Orthographic consists of a set if two or more separate views of an object in different directions, at right angles to each other, arranged relative to one another in a definite way. Each of the views shows the shape of the object in a particular direction; and collectively, all the views describe the object completely. In pictorial views, the object is oriented behind, and projected upon a single plane using orthographic, oblique, isometric or perspective projections.
The methods of projection in common use are known as first angle and third angle. Only in few cases are the two angles of projection combined. The first angle projection method is used in most European countries, while the third angle projection method is in general use in united states of America and Canada. In the first angle projection method, the front elevation drawing is followed by the plan, while in the third angle projection method, the plan drawing is followed by the front elevation drawing.
Third angle projection: in this method, each view is placed so as to represent the object nearer to the adjacent view presented. The plan is usually above the front elevation.
Theory of orthographic drawing: if a transparent plane is set up between an object and the view point of an observer's eye, the intersection of this plane with the rays formed by lined of sight from the eye to all points of the object would give a picture just the same as the image formed in the observer's eye.
Usually, such object is viewed with one eye at a specific distance and height, and with a stiff neck - this is perspective projection; but if the observer walks backwards from the viewpoint until they reach a theoretically 'infinite' distance, the rays formed by lined of sight from their eye to the object would grow longer , and finally become infinite in length, parallel to each other and perpendicular to the picture plane. Now, the image formed on the picture plane is what is generally known as orthographic projection. The rays from the picture, can now be regarded as being obtained by extending perpendiculars to the plane from all points of the object. This projection can now be regarded as frontal view without specifying the distance from front to back.
In addition to frontal plane or front elevation, if another transparent plane is placed horizontally above the object , the projection or view found by extending perpendicular on this plane from the object will give the shape of the object as if viewed directly from above, and will show the distance from front to rear. If this horizontal plane is turned in line with frontal plane, the two views will now be in the same plane showing top and front views. If the third view is perpendicular to the first two, this view can be projected as if viewed from the side of the object and the distance from bottom to top and front to rear.
This is called profile plane. Again , if the profile plane is turned in line with the first two planes as if on sheet of paper, they correctly give the three dimensional shapes of the object. In orthographic or orthogonal projection, the picture planes are called planes of projection and the perpendiculars are called projecting lines or projectors.
