Projections of Planes Questions and Answers

Answer: B

When a plane is perpendicular to a reference plane, it is seen edge-on from that plane, so its projection there is a straight line. The projection would be a point only for a line (or an edge) perpendicular to the plane, not for a whole plane, and the projection of a flat plane is never a curve. 'Apparent line' is not the standard name, which is why the keyed answer is the plain straight line. In descriptive geometry, a plane is shown as a line in a view where it is edge-on, which happens when it is perpendicular to the plane of projection.

Answer: A

Correct answer: horizontal plane of projection

The traces are where the plane, produced if necessary, cuts the planes of projection: the horizontal trace where it meets the horizontal plane and the vertical trace where it meets the vertical plane. Between them the two traces fix the plane completely, which is why a plane is specified by its traces rather than by any point in it.

Answer: B

Correct answer: on the reference line

The reference line is the intersection of the two planes of projection, so any point on it lies in both. A plane cutting both of them must contain the point where all three meet, and that point lies on the reference line - which is why the traces, produced if necessary, always meet there and never anywhere else.

Answer: C

Correct answer: inclined to the reference line

A plane perpendicular to the horizontal plane cuts it in a definite line, and the direction of that line is the plane’s inclination to the vertical plane. So the horizontal trace is inclined to the reference line at exactly that angle, which is why the angle can be measured directly from the trace.

Answer: D

Correct answer: parallel to the horizontal plane

A plane parallel to the horizontal plane never meets it, so it has no horizontal trace at all; it meets the vertical plane in a line parallel to the reference line, which is its only trace. The absence of one trace is itself the identification, and the mirror case - a single trace in the plan - means parallel to the vertical plane.

Answer: A

Correct answer: both principal planes

A profile plane stands perpendicular to the horizontal and the vertical planes at once, so both its traces are perpendicular to the reference line and a figure lying in it appears as a straight line in both principal views. Its true shape needs a side view, which is precisely why the profile view exists.

Answer: B

Correct answer: both principal planes

An oblique plane is inclined to both principal planes, so a figure in it is foreshortened in both the plan and the elevation and its true shape appears in neither. That is what makes two successive auxiliary projections necessary - the first to bring the figure to appear as a line, the second to view it face on.

Answer: C

Correct answer: in its true shape

A figure parallel to a plane of projection is projected onto it without foreshortening, so the plan of a horizontal figure is its true shape at full size. Every dimension can be measured directly from it, which is the whole reason for choosing to place a face parallel to a plane whenever the drawing allows.

Answer: D

A true-shape plan requires the figure to be parallel to the horizontal plane, and such a figure is seen edge on from the front. Its elevation therefore reduces to a line whose length is the figure's extent across the view.

Answer: A

Correct answer: a straight line inclined to the reference line

Being perpendicular to the horizontal plane, the figure collapses into a line when viewed from above; and the direction of that line is its inclination to the vertical plane, so it is inclined to the reference line by that angle. The elevation will show it foreshortened rather than as a line.