When the profiles are curved, the procedure is more complicated. As in the case of the cone shape, there is naturally some discrepancy between the height of the elevation and the profile, the actual dimension of which is affected by perspective (as also in plan curvature).
To obtain the actual height of the area the profile must be measured vertically with some flexible material, such as thin lead wire, which will readily embrace the curvature.
If a division of a third or a sixth is required the diameter or half diameter can be taken, but the segment of the area should be set off on a fresh centre line quite independent of the elevational drawing.
In order to obtain the true shape of the segment the elevation should be divided by horizontal lines drawn at the points of marked change in curvature, and these can be lettered or numbered for identification. The distance between each of these lines should be measured and set off on the new centre line, and then these can be used for drawing the parallels through.
The various diameters can be determined from the corresponding lines on the elevation. Lines drawn through the points thus obtained will give the required segment or area on the flat.
[Illustration: No. 227. Method of obtaining a Segment of one-fifth of Vase.]
If other divisions than those deducible from three or six are required, it will be necessary to draw also the plan curves from which the division can be obtained. Assuming that the elevation has been vertically divided as before, and the plan to be circular, a circle should be struck which is to represent the largest diameter and its circumference divided into the required number of parts. Lines are then drawn through to the centre. On the same centre other circles are struck with radii equal to the remaining horizontals, and each identified with the corresponding number or letter. The heights are obtained as before, and the diameters of the variations in the curvature can be ascertained by measuring round each of the plan curves in succession.
In the case of the plan being other than circular, the same rules apply, but the different plans would have to be drawn in each individual example.
Owing to the effect of perspective on rounded shapes, it is undesirable to employ the human figure, unless in bold relief, and then only on straight or slightly curved profiles.