First-angle and third-angle projection are two different methods of representing a three-dimensional object in two dimensions, particularly in engineering drawings. The main difference between them lies in the placement of the object and the direction of viewing.
First-Angle Projection:
In first-angle projection, the object is conceptually located between the observer and the plane of projection.
The object is imagined to be in the first quadrant (between the observer and the plane) of the coordinate system.
The views are created by projecting lines of sight from the object to the plane of projection.
Third-Angle Projection:
In third-angle projection, the object is imagined to be in the third quadrant (behind the plane) of the coordinate system.
The views are created by projecting lines of sight from the object through the plane of projection to the observer.
To illustrate this further, imagine you have a box, and you want to create orthographic projections (2D representations) of this box using either first-angle or third-angle projection:
In first-angle projection, you would place the box in the first quadrant, between the observer and the projection plane. The views would be created by projecting lines from the box to the projection plane.
In third-angle projection, you would place the box in the third quadrant, behind the projection plane. The views would be created by projecting lines from the box through the projection plane to the observer.
The choice between first-angle and third-angle projection is often a matter of convention and regional standards. Different countries and industries may have preferences for one method over the other. In the United States, for example, third-angle projection is more commonly used, while in Europe, first-angle projection is often preferred. Standardization organizations like ISO (International Organization for Standardization) have guidelines that help ensure consistency in engineering drawings regardless of the projection method used.