Orthographic projection (or orthogonal projection) is a means of representing a three-dimensional 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. It is further divided into multiview orthographic projections and axonometric projections. A lens providing an orthographic projection is known as an (object-space) telecentric lens.
The term orthographic is also sometimes reserved specifically for depictions of objects where the axis or plane of the object is also parallel with the projection plane, as in multiview orthographic projections.
Types of Dimensioning
Parallel dimensioning consists of several dimensions originating from one projection
•Superimposed Running Dimensions
Superimposed running dimensioning simplifies parallel dimensions in order to reduce the space used on a drawing. The common origin for the dimension lines is indicated by a small circle at the intersection of the first dimension and the projection line.
Combined Dimensions: A combined dimension uses both chain and parallel dimensioning.
Dimensioning of circles
(a) shows two common methods of dimensioning a circle. One method dimensions the circle between two lines projected from two diametrically opposite points. The second method dimensions the circle internally.
(b) is used when the circle is too small for the dimension to be easily read if it was placed inside the circle.
All radial dimensions are proceeded by the capital R
(a) shows a radius dimensioned with the centre of the radius located on the drawing.
(b) shows how to dimension radii which do not need their centres locating.