Introduction
Orthographic projection is one of the most fundamental methods of representing three-dimensional objects on two-dimensional drawing sheets. It is widely used in engineering drawing, architecture, mechanical design, manufacturing, product development, and technical communication. The purpose of orthographic projection is to represent the exact shape, size, and features of an object by showing it from different directions.
A single view of an object is often insufficient to describe its complete geometry. Therefore, several views are prepared, usually the front view, top view, and side view. These views are projected onto mutually perpendicular planes. Two major systems are used for arranging these views: the First Angle Projection System and the Third Angle Projection System.
Both systems follow the same basic principles of orthographic projection, but the position of the object, observer, and projection planes differs. As a result, the arrangement of views on the drawing sheet is different.
Meaning of Orthographic Projection
The term “orthographic” is derived from words meaning correct or straight representation. In orthographic projection, the projectors are parallel to each other and perpendicular to the plane of projection.
When an object is viewed from the front, the image formed on the vertical plane is called the front view or elevation. When it is viewed from above, the resulting image on the horizontal plane is called the top view or plan. Similarly, a view obtained from the side is known as the side view or end elevation.
The main objective of orthographic projection is to communicate the complete shape of an object without perspective distortion. Therefore, dimensions can be represented accurately.
Principal Planes of Projection
Orthographic projection makes use of two principal planes:
Horizontal Plane (HP)
Vertical Plane (VP)
The horizontal plane is similar to the surface of a floor, while the vertical plane may be imagined as a wall. These two planes intersect along a line known as the XY reference line.
For certain side views, an additional plane called the Profile Plane is used.
The location of an object in relation to the horizontal and vertical planes determines the quadrant in which it lies.
The four quadrants are:
First quadrant: object above HP and in front of VP.
Second quadrant: object above HP and behind VP.
Third quadrant: object below HP and behind VP.
Fourth quadrant: object below HP and in front of VP.
In engineering drawing, the first and third quadrants are primarily used for projection systems.
First Angle Projection
First angle projection is a method in which the object is assumed to be located in the first quadrant. This means that the object is positioned above the horizontal plane and in front of the vertical plane.
In this system, the object lies between the observer and the plane of projection.
Because of this arrangement, the view is projected on the plane situated behind the object.
For example, when the front view is projected, the object lies between the observer and the vertical plane. Similarly, for the top view, the object lies between the observer and the horizontal plane.
Arrangement of Views in First Angle Projection
The arrangement of views in first angle projection may initially appear opposite to the direction from which the object is viewed.
The top view is placed below the front view.
The right-side view is placed on the left side of the front view.
The left-side view is placed on the right side of the front view.
This reverse arrangement is one of the most important characteristics of first angle projection.
If an observer looks at the object from the right side, the image is projected onto the plane on the opposite side. Therefore, the right-side view appears on the left side of the drawing.
First Angle Projection Symbol
Engineering drawings usually include a standard projection symbol that identifies the system being used.
The symbol is based on the projection of a truncated cone or frustum. In the first angle projection symbol, the circular end view is placed in a particular position relative to the side view of the frustum.
This symbol allows engineers and manufacturers to identify the projection convention without examining the entire drawing.
Applications of First Angle Projection
First angle projection is widely used in many countries and is common in engineering and technical education in India and several other regions.
It is frequently used for:
Machine component drawings
Architectural and engineering drawings
Manufacturing drawings
Fabrication plans
Product drawings
Assembly drawings
Technical documentation
Because it follows internationally recognized engineering drawing conventions, understanding first angle projection is essential for students and professionals.
Third Angle Projection
Third angle projection is another major system of orthographic projection. In this system, the object is assumed to be placed in the third quadrant.
The object is located below the horizontal plane and behind the vertical plane.
The most important principle of third angle projection is that the plane of projection lies between the observer and the object.
This is the opposite of the first angle projection system.
The observer looks through the projection plane toward the object, and the view is projected onto the plane closest to the observer.
Arrangement of Views in Third Angle Projection
The arrangement of views in third angle projection is more directly related to the direction of observation.
The top view is placed above the front view.
The right-side view is placed on the right side of the front view.
The left-side view is placed on the left side of the front view.
Therefore, the views appear on the same side from which the object is observed.
For many beginners, this arrangement may appear more intuitive because the right view is placed on the right and the left view is placed on the left.
Third Angle Projection Symbol
Third angle projection also has a standard symbol using the same frustum and circular end view, but their relative arrangement differs from that of the first angle projection symbol.
The projection symbol is usually printed in the title block or another visible part of a technical drawing.
The presence of this symbol is important because the same object can produce different view arrangements depending on the projection method.
Difference Between First Angle and Third Angle Projection
The basic geometry of the object remains unchanged in both systems. The main difference is the relationship between the observer, object, and projection plane.
In first angle projection, the object is between the observer and the plane.
In third angle projection, the projection plane is between the observer and the object.
This difference results in different locations of the top and side views.
In first angle projection, the top view is below the front view. In third angle projection, the top view is above the front view.
Similarly, in first angle projection, the right-side view is placed on the left. In third angle projection, the right-side view is placed on the right.
Comparison Table
| Feature | First Angle Projection | Third Angle Projection |
|---|---|---|
| Quadrant used | First quadrant | Third quadrant |
| Relative position | Object between observer and plane | Plane between observer and object |
| Top view | Below front view | Above front view |
| Right-side view | Left of front view | Right of front view |
| Left-side view | Right of front view | Left of front view |
| View arrangement | Opposite to viewing direction | Same as viewing direction |
| Common use | India and many other countries | Common in North America and some other regions |
Steps in Preparing Orthographic Projections
The first step is to study the object carefully and select the view that gives maximum information as the front view.
The front view is then drawn according to the object dimensions.
Projectors are extended from the front view to construct the top view.
The side view can be obtained by transferring dimensions between the top and front views. A 45-degree mitre line is frequently used to transfer dimensions between the top view and side view.
All visible edges are generally represented by continuous lines. Hidden details are represented using dashed lines. Centre lines are used to show axes of symmetry, circular features, and centres.
Dimensions and annotations are then added according to engineering drawing standards.
Importance of Orthographic Projection
Orthographic drawings allow engineers and architects to communicate complex three-dimensional forms accurately. They provide precise information about dimensions, edges, holes, surfaces, heights, widths, and depths.
Manufacturers can use these drawings to produce components without physically seeing the original object. Similarly, architects use orthographic projection to prepare plans, elevations, and sections of buildings.
The method is important because it eliminates the visual distortion present in perspective drawings.
It also forms the foundation for computer-aided drafting and three-dimensional modelling. Modern CAD software automatically generates orthographic views from digital models, but the designer must still understand the fundamental principles.
Orthographic Projection in Architecture
In architectural drawing, orthographic principles are extensively used. A building plan is essentially a horizontal orthographic representation, while elevations show vertical faces.
Sections also follow orthographic principles but include a cutting plane to reveal internal spaces and construction details.
Thus, architectural plans, elevations, sections, furniture drawings, structural drawings, and working drawings all depend heavily on orthographic representation.
Advantages of Orthographic Projection
Orthographic projection offers several major advantages. It provides accurate dimensional representation and clearly communicates the geometry of an object.
The system allows multiple views to be coordinated with one another, making it easier to understand complicated components.
It also provides a universal graphical language for architects, engineers, technicians, manufacturers, and contractors.
Most importantly, orthographic drawings can be measured and dimensioned accurately, unlike pictorial or perspective drawings.
Conclusion
Orthographic projection is one of the most important methods of graphical communication in engineering and architecture. It represents three-dimensional objects through two-dimensional views such as the front, top, and side views.
The two principal systems are first angle and third angle projection. In first angle projection, the object is located between the observer and projection plane, resulting in the top view being placed below the front view and side views appearing opposite to their viewing directions.
In third angle projection, the projection plane lies between the observer and the object. Therefore, the top view appears above the front view and the side views are positioned on the same side from which they are observed.
A clear understanding of both systems is essential for interpreting and producing engineering drawings. These techniques provide the foundation for accurate technical communication, manufacturing, building design, CAD modelling, and professional engineering practice.
