Produce isometric and oblique pictorial drawings to show objects in three dimensions, and explain the conventions and uses of each

What this dot point is asking

SEAB wants you to produce isometric and oblique pictorial drawings that show an object in three dimensions, and to explain the conventions (the angles and scaling) and uses of each. Pictorial drawings present the overall look of a design in one view. You should know how each projection is set out, the angles involved, and when to choose isometric or oblique.

The answer

What pictorial drawing is for

Pictorial drawings show an object in three dimensions in a single view, so the viewer sees its overall shape and how it looks as a real object at a glance. This makes them ideal for presenting ideas and for helping non-technical people understand a design quickly. They give the realistic impression that flat orthographic views cannot, which is why designers use pictorial drawings to communicate the look of an idea, alongside orthographic views for the making detail.

Isometric drawing

In isometric drawing, all three dimensions are shown to the same scale along three axes:

• Vertical edges are drawn vertical.
• The two sets of horizontal edges are drawn at 30 degrees to the horizontal.

No face is shown flat or true; instead the object appears tilted so three faces are visible, all drawn to scale. Isometric gives an even, undistorted three-dimensional view and is widely used for general pictorial drawings of products. Circles on isometric faces become ellipses, which take practice to draw.

Oblique drawing

In oblique drawing, one face is drawn flat and true, exactly as a normal front view, and the depth is drawn going back at an angle, usually 45 degrees. To avoid the object looking too deep, the depth is often drawn at half scale; this version is called cabinet oblique. Because the front face is true and flat, oblique is easy to draw and especially good when the front face is complicated or circular.

Choosing isometric or oblique

The choice depends on the object:

• Use isometric for a balanced, realistic 3D view of most objects, where no single face dominates.
• Use oblique when the front face is complex or circular (a clock, a wheel, a dial), because that face is drawn true and flat, which is far easier than the ellipse it would become in isometric.

Both are pictorial; the difference is whether one face is shown true (oblique) or the object is tilted with all dimensions to scale (isometric).

Building a pictorial drawing

Both projections are built up using crating: start with a light three-dimensional box in the correct proportions along the relevant axes, then add the detail inside it. Construction lines and crating keep the drawing accurate and well-proportioned, just as in freehand sketching.

Examples in context

Example 1. An isometric drawing of a wooden toy block set. A set of plain blocks has no complex face, so isometric suits it: each block is drawn with vertical edges vertical and horizontals at 30 degrees, all to scale, giving a balanced, realistic 3D view of the whole set. The even isometric view shows the blocks' proportions and arrangement clearly without any face needing to be true.

Example 2. An oblique drawing of a clock. A clock's defining feature is its round face. Oblique drawing keeps that face flat and true, so it is drawn as a real circle with the numbers in place, then the body's depth is projected back at 45 degrees at half scale. Drawing the same clock in isometric would force the round face into an ellipse, harder to draw and less clear, which is exactly why oblique is chosen for circular-fronted objects.

Try this

• Cue. State the angle of the horizontal edges in isometric drawing and the depth angle in oblique drawing. Answer: 30 degrees for isometric horizontals; 45 degrees for oblique depth.

• Cue. Explain when oblique is a better choice than isometric. Answer: when the front face is complex or circular, because oblique draws that face flat and true (a real circle), which is easier than the ellipse it becomes in isometric.

• Cue. What is cabinet oblique? Answer: an oblique drawing in which the depth is drawn at half scale (rather than full scale) so the object does not look too deep, while the front face stays true and full size.