Enlarging, reducing and scaling

Asked for one-point perspective, candidates draw a 30 degree isometric, or mix the two by tilting edges to 30 degrees instead of keeping the front face horizontal and vertical. In perspective the front face is true and every receding edge converges to one VP; in isometric edges stay parallel and never meet. Fix the horizon and VP first, then project all edges to it.

Positioning features along a scale line (e.g. a 1:20 line) or a scaled line, candidates measure and guess, so spacings come out wrong -- and the nets-per-sheet count too. Two fixes: divide each real distance by the scale (on a 1:20 line a 5 m spacing is drawn as 250 mm), and construct even divisions with the inclined-line method.

On scale-model work (e.g. an architectural concept model) candidates stay in the lower band when they do not show it is a model, do not state the scale, or apply it inconsistently -- reducing plan dimensions but leaving heights near full size, out of proportion. State a sensible scale (such as 1:50), apply it to length, width and height, and evaluate the model.

The ray and grid methods suit different shapes; the wrong one loses accuracy. The ray (radial) method needs clear corners to project rays through, so it suits cornered outlines but struggles with curves. The grid (squares) method copies square by square, so it handles irregular or curved outlines like a logo that rays cannot. Rays for corners, grid for irregular.

Enlarging by a linear scale factor of 2 does not just double area -- area has two dimensions, so it scales by the factor squared: doubling every length makes the area four times larger (tripling, nine times), halving quarters it. This matters for the nets-per-sheet count -- at half scale about four times as many fit, not twice. Scaling area linearly gives a wrong count.

In an estimated one-point perspective, candidates project some corners to the VP but not all, or stop the receding edges short, so the far face is the wrong size or off the projectors. Every corner and receding edge must run to the single VP, and the back face sit exactly on those projectors. Draw the true front face, then take each corner to the VP before the back face.

A testable error: reading a scale ratio backwards. A scale is drawing size : real size, so 1:20 means the drawing is one-twentieth full size (a reduction), not twenty times bigger. The tells: an enlargement has the larger number first (2:1, 5:1) and suits tiny objects; a reduction has the 1 first (1:20, 1:50) and suits large objects; 1:1 is full size.