Computer-aided draughting

Computer-aided drafting systems can normally provide equivalents for all the usual manual operations. They also have extra capabilities of their own. The most important of these capabilities is probably their ability to use the same drawing many times in different contexts as parts of a larger drawing. At its simplest level, this eliminates the frequent re-drawing of standard building components and fittings. Computer-aided drafting systems can also alter drawings faster than is possible with manual methods. Very few drawings are right first time. Most undergo change and refinement as the design progresses. Manual erasure of ink lines involves laborious removal with an eraser or scalpel. The most basic division of computer-aided drafting programs is into interactive and non-interactive types. With an interactive program, the computer continuously monitors the actions of the user and helps to produce and modify drawings. Another way to categorize computer-aided drafting systems is into those that work in two dimensions and those that work in three. A two-dimensional system is equivalent in its basic philosophy to manual drafting; the user draws by arranging lines and text on a flat surface. However, if a computer-held drawing contains information about the third dimension, it becomes possible to look at other views of the arrangement. The most common way to construct three-dimensional drawing files is to compose them entirely of pre-defined elements rather than individual lines. By providing information on the third dimension of the elements, alternative views of any room can be calculated.