We need to build plane models of spaces and on them to be able to solve various spatial problems. If three-dimensional spatial forms are created on the two-dimensional plane is a drawing. The drawing is a certain set of points and lines on the plane. The descriptive geometry is engaged in creation of drawings of spatial forms and relations. What two-dimensional drawings can be models which would display properties of space, spatial forms and the relations?
In an extreme antiquity of people drew and drew on rocks, stones, walls and household goods of the image of things, trees, animals and people. It did it for satisfaction of the requirements, including esthetic. Thus the main requirement to such images consisted in that the image caused the correct visual idea of a form of the represented subject.
Method of descriptive geometry is the graphic method based on projection operation - binary constructive model of space, spatial forms and the relations, i.e. a method plane (binary, two-dimensional) models of spaces.
At the set projection device (S) each point of space will have one and only one central projection (since through two various points it is possible to carry out one and only one straight line). The converse does not make sense as the point of A can be the central projection of any point belonging to direct (AS) (For example the central projections of points of A and D coincide).
The device of the central projection is set if the provision of the plane of projections and the center of projections of S is set. If the device of projection is set, it is always possible to define the provision of the central projection of any point of space to the planes of projections.
We cross the straight line formed by two set points from the straight line formed by projections of the same points. (MK and M'K'). The received point (P belongs to a trace of a secant of the plane on the basis plane. We find the second point (P and we build a straight line (a trace of a secant of the plane).