Three views of one object must agree on width, height and depth. Checking takes three comparisons, and any mismatch points to an error in one of the views.
This lesson is part of orthographic views. It complements distinguishing visible and hidden features.
What are the three comparisons?
| Compare | Shared size | How to check |
|---|---|---|
| Front and top | Width | Carry vertical projection lines |
| Front and side | Height | Carry horizontal projection lines |
| Top and side | Depth | Match the depth figure on each |
For each pair, also check that every feature (a step, a hole, a notch) sits at the same position in both views. A feature in one view must have a matching trace in the others.
Worked example: an error hunt
An original set of views shows a plate 80 mm wide, 45 mm deep and 25 mm high, with a rectangular hole cut straight down through it. The question states the hole is 20 mm wide (left to right) and 15 mm deep (front to back), with its left edge 30 mm from the left side.
The set as drawn:
- Front view: rectangle 80 × 25, with two dashed vertical lines at 30 and 50 from the left edge, running the full height.
- Top view: rectangle 80 × 45, with a hole outline from x = 30 to x = 55, and from 15 to 30 measured from the back edge.
- Side view: rectangle 45 × 25, with two dashed vertical lines at 15 and 30 from its left edge, running the full height.
Run the comparisons:
- Front and top, width: both 80, consistent.
- Front and side, height: both 25, consistent.
- Top and side, depth: both 45, and the hole spans 15 to 30 in both, consistent.
- Hole width: the front view gives 30 to 50 (20 mm), but the top view gives 30 to 55 (25 mm). These disagree.
The question states a width of 20, so the top view is wrong and its right edge moves from 55 to 50.
Why do projection lines help?
Draw a thin vertical line from the hole’s right edge in the top view up to the front view. It lands at x = 55, but the front view’s dashed line is at 50. The misalignment shows the error without any calculation.
The mistake: checking each view alone
A student checks the front view against the question and finds it correct. The top view is also compared with the question, but only the overall size is read, so the hole’s right edge passes.
| Check | Wrong | Right |
|---|---|---|
| Method | Each view on its own | Pairs of views together |
| Hole width | Not compared | 20 in front, 25 in top |
| Result | Error passes | Mismatch found |
Views can each satisfy the overall size and still disagree on smaller features, so always compare features across views.
Check yourself
A front view shows a block 70 wide and 30 high with a step: the left 40 mm is 30 high and the right 30 mm is 15 high. The top view shows a rectangle 70 wide and 35 deep with a vertical line at x = 35. The side view is a rectangle 35 wide and 30 high with a dashed line at height 15. Is there an error?
Answer
Yes. The front view puts the step at x = 40, but the top view puts the line at x = 35. They disagree by 5 mm.
The other comparisons pass: widths are both 70, heights are both 30, and depths are both 35. The dashed line at height 15 in the side view matches the step height.
The fix is to move the top view line to x = 40, or to correct the front view, depending on the stated dimensions.
What to study next
Test all four lessons in the orthographic views practice set. Common slips across views are also collected in reviewing common scale, projection and labelling mistakes.
If you want a teacher to plant an error in a drawing for you to find, see online one-to-one Engineering Drawing tuition.