Isometric and Oblique Projections
This domain tests pictorial drawing: isometric and oblique views that show three faces of an object in one picture.
Isometric axes and scale
Isometric axes are 120° apart, drawn at 30° to the horizontal. An isometric drawing sets off true lengths along the three axes. An isometric projection uses the isometric scale, 0.816 (usually 0.82), so a 40 mm edge is drawn 32.6 mm. Lines not parallel to the axes are not isometric lines; locate their endpoints on the axes and join them. Circles appear as ellipses; use the four-centre method or an isometric square.
Oblique projection
The front face is drawn true shape. The receding axis is usually 45°. Cavalier projection uses full scale on the receding axis; cabinet projection uses half scale. Oblique is useful when the front face has circles or irregular outlines.
Worked example
A 40 mm cube in isometric drawing: draw three axes at 30°, set off 40 mm on each, complete the parallelogram faces and vertical edges. In isometric projection, each axis length is 40 × 0.816 = 32.6 mm. Reasoning: the projection shortens all three axes equally; the drawing keeps true lengths for convenience.
Traps
Drawing isometric axes at 45°; using true lengths in an isometric projection; drawing a circle as a circle on an isometric face; making parallel edges converge; confusing cavalier and cabinet scales.