Isometric is a simple and mechanical way of producing a pictorial representation. Orthographic projections of one kind or another make the most efficient type of drawing from which to work, but the final appearance of the completed piece is often somewhat hidden. This method gives a closer approximation to the visual appearance (333:3). For normal work the sides are inclined at an angle of 30° using the 30° set-square, but the angles can be varied within wide limits, using, for instance, 20° for the front elevation, and 40° or 50° for the side elevation— actual experience alone is the only true guide to the most satisfactory angle of representation for any particular view. Isometric scales proportionate to the true scale were formerly used, but the practice has now been abandoned, and the drawings are made to the true scale.
The method of drawing curves or irregular shapes is shown in 333:5 and 333:6. A plan of the curve is first drawn and included in a square. Ordinates (ordinate: half the chord of a conic section in relation to the diameter which bisects it) are then drawn in 333:5, and the square with its ordinates is then drawn in isometric projection 333:6, and the critical distances for the curve plotted off. In the example given the circle becomes an ellipse.
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