Principles of Orthographic Projection

Orthographic Projection is a method of producing dimensioned working drawings or blueprints of 3-D Objects using a series of related 2-D views of the object to communicate the object's length, width and depth.

The views are produced by using the fundamental concept of Orthographic Projection - the location of the Spectator (the viewer). The Spectator is always located at an infinite distance from the object and planes of reference - this means that the lines of sight (projection lines) remain parallel and will project on to a Plane which is perpendicular to the projection lines.

The primary views used are called the Elevation, Plan and End Elevation and are produced by projecting an image of the object as viewed by a spectator standing at infinity on to the Planes of Reference which are then folded flat to produce a 2-D drawing. Drawings can be used using one of two methods - First Angle Projection (Used in Europe, Asia & Africa) or Third Angle Projection (Used in the USA). First Angle Projection invloves the projection of the image of the object on to a Plane positioned behind the object while Third Angle projects the image on to a plane located between the object and the spectator. The method of projection alters the layout of the drawing as displayed in the image below. Drawings created using First Angle Projection will have the Plan View located below the elevation and End Elevation positioned on the side opposite the viewing direction (i.e. when viewing the left of the object the view is positioned on the right of the Elevation) while the opposite is the case for Third Angle Projection.

Producing an Orthographic Drawing

When beginning a drawing, you must first select an Elevation. An elevation should show the greatest amount of detail of an object and is often considered to show the "front" of the object. Determine the layout of the views on the sheet and create rectangles which the related views will fit into - this makes it easier to lay the drawing out on the sheet and prevents frustrating mistakes down the road!

Conventions of Orthographic Projection

There are a number of rules and conventions which must be adhered to when producing Orthographic Drawings:

Heights of objects will remain the same between Elevations including End Elevations and Auxiliary Elevations.
Widths of Objects will remain the same between the main Elevation, Plan and Auxiliary Plans.
Lines & Sufaces parallel to the Vertical Plane will appear as true lengths/shapes in the Elevation.
Lines & Sufaces parallel to the Horizontal Plane will appear as true lengths/shapes in the Plan.
45° Lines or Arcs should be used to transfer widths between the plan and End Elevation.
Construction lines should be drawn lightly using a H Pencil.
Finished lines should be drawn heavily using a B Pencil.