Orthographic projection is a common technique used in a number of engineering fields to communicate important information about the layout and design of an object or structure. There are two particularly important orthographic projections used today: First angle projection is more common in European countries, while third angle projection is commonly used in the United States. These two projection styles differ only in the placement of the different elements of the image.
Orthographic Projection Basics
Either major style of orthographic projection involves three different views, or plans, of the three-dimensional object that it is representing in two dimensions. In contrast to more artistic styles of representation, which tend to illustrate objects from oblique angles, these engineering styles illustrate an object from angles perpendicular to their planar sides. The three angles involved in both major orthographic projections are known as the front view, side view and plan view. Plan view is traditionally from the "top" of the object, while side view is usually from the right side.
Individual View Basics
Each of the three distinct "views" involved in a third angle projection are drawn as if from 90-degree angles from one another. Another way of visualizing this is to imagine a glass cube around the object being drawn, and then tracing the image of the object onto each respective face of that cube. Important details of an object that cannot be seen from a particular angle, such as indentations or notches on far sides, are indicated by dashed lines.
Particular View Details
Plan view is drawn as if looking down at the top of the object, and is placed at the top of the drawing; the front view is then drawn directly beneath it, with corresponding edges of the views collinear. Side view is then placed to the side of the front view, on the same side as the drawing is modelling -- usually the right, but it can be the left. The top and bottom of the object in the side view are collinear with the top and bottom in the front view.
Comparison to First Angle Projection
The other common style of orthographic projection is known as first angle projection; this approach is more common in Europe. Both orthographic projection styles use front, side and plan views arranged with their common edges collinear to corresponding edges in the other views; additionally, front and side views are in the same row on the page in both. However, in first angle projection, plan view is placed below the row of the other views, instead of above it as in third angle projection.
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