Orthographic Projection Questions and Answers

Answer: A

Correct answer: parallel to one another and perpendicular to the plane

Both conditions matter. Parallel projectors mean there is no convergence and hence no foreshortening with distance; perpendicular projectors mean a face parallel to the plane is projected at its true size. Together they are what make the drawing measurable, which is the whole reason engineering uses orthographic projection rather than perspective.

Answer: B

Correct answer: preserves true size for faces parallel to the plane

Perspective converges the projectors on the eye, so distant edges are drawn shorter and nothing can be scaled off the drawing. Orthographic keeps them parallel, so a face parallel to the plane appears at true size and true shape, and a dimension can be measured directly. The drawing looks less natural, and that is the price of being usable.

Answer: C

Correct answer: parallel but inclined to the plane of projection

An oblique projection keeps the projectors parallel, so it shares orthographic’s freedom from convergence, but inclines them to the plane. The consequence is that one face is drawn true while the receding edges are pushed off at an angle, giving a pictorial view from a single drawing - useful for illustration, but the receding dimensions are foreshortened.

Answer: D

Correct answer: reference line or ground line

The horizontal and vertical planes of projection intersect in a line, and when the planes are opened out into one sheet that line remains as the reference or ground line, usually marked XY. It is the datum from which heights are measured in the elevation and depths in the plan, so it links the two views.

Answer: A

The view is projected through the object onto the plane beyond it, so each view appears on the side away from where it is seen. In third angle the plane lies between observer and object, which reverses the arrangement.

Answer: B

Correct answer: between the observer and the object

This is the defining difference between the two systems. In third angle the plane sits between the observer and the object, so the view is projected forward on to it and each view appears on the side of the front view from which it was taken. In first angle the object lies between observer and plane, so each view appears on the opposite side.

Answer: C

Projecting through the object throws the top view down onto the horizontal plane beneath, so the plan sits below the elevation. Third angle places the same view above the front view, which is the practical distinction.

Answer: D

Correct answer: on the left of the front view

In third angle every view is placed on the side from which it was taken, so looking from the left puts the view on the left. First angle reverses this, placing it on the right. This is precisely why the projection symbol must appear in the title block - the same arrangement of views means two different objects under the two systems.

Answer: A

The frustum's two views appear in different relative positions in the two systems, which identifies the convention at a glance. Placing it in the title block is mandatory, since the same drawing means different things under each.

Answer: B

Reading a first angle drawing as third angle, or the reverse, transposes the side views and so reverses the part. Dimensions would be correct and the shape wrong, which is the most expensive kind of error to discover late.