Isometric -- 30 degree axes, true lengths
Isometric shows three faces of an object in one view. Vertical edges stay vertical, and the two horizontal edge sets recede at 30 degrees, so the three axes sit at 120 degrees. Plot true lengths along each axis with a 30/60 set square -- the syllabus says the isometric scale is NOT required, so you do not shrink receding lengths by the 0.816 factor. Crate first: box the object at the right size, then cut features from the crate.
Planometric -- true plan at 45/45 or 60/30
Planometric (plan oblique) is built from the plan kept as a TRUE shape and TRUE size. Rotate the plan so its edges sit at 45 and 45 or 60 and 30 degrees, then project the vertical heights straight up from the plan corners. Because the plan is undistorted, plan angles and distances are correct and can be measured off the drawing -- which is why planometric suits room layouts, kitchens and interiors. The 60/30 set-up emphasises one wall more than 45/45.
Circles: isometric ellipse vs planometric plan circle
Curves win or lose marks. In isometric, a circle on any face is foreshortened into an ellipse -- never a compass circle. Construct it by the four-centre method (arcs from the corners of the enclosing isometric square) or by crating (grid the circle, redraw the grid in isometric, transfer each point). In planometric the plan is true, so a plan circle is a real compass circle. But a circle on a vertical face is tilted, so it too is an ellipse.
Drawn from real examiner reports.
Confusing isometric with perspective
A headline error is muddling the two systems. In isometric the receding edges are parallel, set at 30 degrees. In one-point perspective they converge to a vanishing point and the front face is true and flat. Candidates asked for perspective frequently drew a 30 degree isometric, or tilted the front and back edges to 30 degrees rather than horizontal. Read the named method before drawing a line.
s23 P51 A2
Planometric heights and depths wrong
Planometric is under-practised, so candidates get the plan roughly right but project the heights inaccurately -- the lower section too tall or short, and recesses, indents and steps missed or flattened. Since the plan is true, project every vertical straight up from the correct plan corner to the measured height. Work to scale for both plan distances and heights.
s23 P51 B4a
Ellipses sketched freehand or under-plotted
Ellipses are repeatedly the weakest construction. Common faults: sketching freehand, plotting too few points, getting the axis orientation wrong, and using a trammel not fixed to the paper. Fixes: the four-centre method inside the enclosing isometric square, or concentric circles with 8+ plotted points. The minor axis lies along the axis going away from you -- get it right first.
s23 P53 A1b; w23 P52/P53 B4b
Isometric circle drawn as a true circle
Dropping a compass point to draw a circle on an isometric view. You cannot: every isometric face is tilted to 30 degrees, so the circle is foreshortened into an ellipse and must be constructed -- four-centre method or plotted ordinates. A true compass circle only survives on a true-shape face, i.e. only the planometric plan. The tell: if the face is tilted, the circle is an ellipse.
Right shape drawn at the wrong size
The base competence every sitting is to work to the stated scale and exact dimensions, yet candidates draw the right shape at the wrong size or height. Crate first -- lightly box the whole object at the exact scaled size before adding detail -- then project features from the crate rather than judging them by eye. A neat pictorial at the wrong size is not a usable drawing.
s23/w23 P51-53 A1
Shrinking lengths by the isometric scale
A subtler slip: applying the isometric scale (the 0.816 reduction of receding lengths) when the syllabus states it is NOT required at IGCSE. Candidates who half-remember it draw the receding edges too short, distorting the object. Plot full (true) lengths along all three axes, straight from the crate. Save any 0.816 reduction for work where an isometric scale is asked for.
Planometric axes at 30 degrees
Assuming every pictorial method uses the isometric 30 degree angle. It does not. Isometric uses 30 degrees and no face is true. Planometric uses 45 and 45 or 60 and 30 degrees, built on a TRUE plan -- that angle set keeps the plan undistorted. Picking 30 degrees for planometric destroys its advantage, a true measurable floor plan. Remember: 30 iso, 45/45 or 60/30 planometric.
Read the named method, set the axes
Identify the named method first -- isometric, planometric or perspective -- and set the board up: a 30 degree set square for isometric, a 45 or 60/30 set-up for planometric, a vanishing point for perspective. The wrong set-up loses the whole question, so confirm the method first.
Crate first, project, don't guess
Crate first: lightly box the whole object at the exact stated scale, because a common loss is the right shape at the wrong size. Then project -- carry verticals and depths up from the crate corners rather than guessing. Light construction lines left visible show the method.
Construct every curve, never sketch
Construct every curve, never sketch: an isometric circle is an ellipse (four-centre or plotted ordinates), a planometric plan circle is a true compass circle, and a free-form curve is crated and plotted point by point. Use enough points, correctly oriented.
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