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This part of the extended material mainly examines the knowledge points of descriptive geometry:

The subject of studying the theory and method of representing shapes on a plane and solving space geometric problems is also the projection theory basis of mechanical drawing. In the application of descriptive geometry, there are several methods, namely, projection method, polyhedron orthographic projection method, projection transformation method, intersection line and intersection line method, and expansion diagram method.

It uses projection method to study the drawing principle of polyhedral orthographic drawing, axonometric drawing, perspective drawing and vertical projection drawing, in which polyhedral orthographic drawing is the main research content. Descriptive geometry also includes projection transformation, intersection line, intersection line and expansion diagram.

In engineering and science and technology, it is often necessary to express the shape of space on a plane. Drawings of houses or buildings must be drawn on paper so that construction can be carried out according to these drawings. But the plane is two-dimensional and the spatial form is three-dimensional. In order to display the three-dimensional shape correctly on the two-dimensional plane, some methods must be specified and adopted, which are the study of descriptive geometry.

In engineering practice, we should not only express the shape of space on the plane, but also apply these figures expressed on the plane to solve the geometric problems of space. It is necessary to design the route of roads or canals, decide where to dig and fill, and calculate earthwork according to the topographic map drawn by the survey results. It is also the study of descriptive geometry to explain the spatial geometry according to the figure of the object on the plane.