In isometric projection used in engineering graphics, all linear distances measured along the principal isometric axes are shown at a reduced scale compared to true size. Approximately what percentage of the true length is used to represent edges in a standard isometric projection?

Introduction / Context:
Isometric projection is a common pictorial method in technical drawing that shows three principal axes equally inclined to the plane of projection. Because of the equal foreshortening along the three axes, the apparent lengths are reduced compared to true lengths. This question checks your understanding of the standard percentage scaling used in isometric projection.

Given Data / Assumptions:

• A standard isometric projection is used (not isometric drawing at true size).
• All three axes are equally foreshortened.
• The query asks for the approximate percentage of true size shown along isometric axes.

Concept / Approach:

In isometric projection, the edges are inclined so that equal angles are made with the plane of projection. The resulting foreshortening factor is approximately 0.816 of the true length. In teaching practice and many drafting references, this is rounded and communicated as roughly 80% of true size, hence dimensions measured directly from an isometric projection appear at about four-fifths of the real length unless an isometric scaling is applied deliberately.

Step-by-Step Solution:

Verification / Alternative check:

Many drafting texts provide an isometric scale or tables showing that a 100 mm true length would be drawn at roughly 81–82 mm in projection, often rounded to 80 mm for quick estimation. This confirms that 80 percent is the best answer among the choices provided.

Why Other Options Are Wrong:

120 percent: would indicate enlargement, which does not occur in isometric projection.

50 percent and 20 percent: these would represent extreme reductions not associated with the standard isometric foreshortening.

Common Pitfalls:

Confusing isometric projection with an isometric drawing or true-length pictorial where designers sometimes choose to draw edges at full size for convenience; this question is about the projection convention itself.

Final Answer:

80 percent