Isometric and oblique projection
Isometric and oblique projection
This domain tests pictorial drawing: turning orthographic views into a single 3D-looking view using isometric or oblique axes, with correct proportions and curves.
Core rules
- Isometric axes are 120° apart: one vertical axis and two receding axes at 30° to the horizontal. True isometric projection uses an isometric scale of 0.816, but an isometric drawing is usually made full length along the axes; state which convention you use.
- Draw the three axes first, then box the object with construction lines. Measure only along the isometric axes, never along slanted object edges. Circles appear as ellipses; use the four-centre method or an isometric template, and keep the major axis perpendicular to the axis of the cylinder.
- Oblique projection keeps the front face true shape. Cavalier oblique uses full depth; cabinet oblique uses half depth. The receding axis is usually 45°, though 30° or 60° is acceptable if stated.
- Do not show hidden edges in a pictorial view unless the question asks for them. Keep outlines thick and construction lines light.
Worked example
Draw a 30 mm cube in isometric. Draw a vertical axis and two axes at 30° to the horizontal. Mark 30 mm on each axis from the near corner. Through each mark draw lines parallel to the other two axes to complete the top rhombus and the two visible side faces. Reason: each visible edge is parallel to one isometric axis, so the box is built from three measured directions.
Traps
Using 45° for isometric axes, measuring along a slanted edge, and drawing circular holes as true circles are frequent errors. In oblique views, do not confuse cavalier (full depth) with cabinet (half depth), and do not draw the front face in perspective.