Third-Angle Projection Definition

Third-angle projection is a method used in orthographic projection where the object is placed between the observer and the plane of projection. This results in the views being arranged differently compared to first-angle projection.

Key Characteristics

• Observer-Object-Plane Relationship: In third-angle projection, the sequence is Observer -> Object -> Plane of Projection.
• Plane Position: The plane of projection is located beyond the object, relative to the observer.
• View Arrangement (Typical): The Top view is usually at the bottom, the Front view is in the center, and the Right-side view is on the right (based on ANSI standards). This differs from first-angle projection where the object is between the observer and the plane.

Analysis of Options

• Option 1: Incorrect. Quadrant placement isn't the defining factor.
• Option 2: Correct. This accurately describes the spatial relationship: the observer looks through the object at the plane.
• Option 3: Incorrect. This describes first-angle projection (Observer -> Plane -> Object).
• Option 4: Incorrect. Quadrant placement is not the primary definition.

Therefore, the defining characteristic of third-angle projection among the choices is that the plane of projection lies between the object and the observer.