Orthographic projection is a means of representing a threedimensional object in two dimensions. It is a form of parallel projection, where all the projection lines are orthogonal to the projection plane, resulting in every plane of the scene appearing in affine transformation on the viewing surface. A lens providing an orthographic projection is known as an (object space) telecentric lens.
There are two types of drawing in orthographic , First Angle and Third Angle. They differ only in the position of the plan, front and side views. First angle orthographic projection used mainly in Europe. Third angle which is used in USA.
First Angle Projection

Third Angle Projection

Object is imagined in 1st quadrant

Object is imagined in 3rd quadrant

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Object is located between observer and plane of projection

Object is located between observer and object

the plane of projection assumed to be non transparent

the plane of projection assumed to be transparent
