Orthographic projection is a technique used in drafting or engineering drawings to depict a three-dimensional object in two dimensions. Typically, this type of projection will show the top, side, and front views of an object. In cartography, or map-making, an orthographic projection refers to a two-dimensional map with a perspective of infinity. A map using this technique appears similar to a view from space, but land mass shapes and distances are distorted.
In engineering drafting, one way to visualize an orthographic projection is to picture the object inside a cube. When viewed from the front, side or top, the object projects an image onto that face of the cube. Opening the cube so the front image is placed on the bottom right of the page, the left view will be placed to the left and the top image will be located above the left-side view.
An advantage of this type of projection is that dimensions of sides or common edges only have to be shown once. The projection shows the relationship of the common edge dimension from front to side or from side to top views. Once the projection is created, an artist or draftsperson can create dimensional drawings showing the part from different perspectives or angles.
Another option for designers is a sectional projection. A sectional drawing is any graphic that uses an imaginary plane intersecting the part to expose hidden internal parts or dimensions. Sectional projections will often be shown along with front and side views to fully represent an object.
When planning an orthographic projection, a designer can view the object from different directions. The front and top views are common to most projections, but the designer can choose to view the left or right side view. An orthographic projection that views the left side is known as a first-angle projection. The object viewed from the right side is a third-angle projection. Common practice is to use first- or third-angle views, but in theory any of the six sides of an object can be included in the projection.
Cartography is the science of making maps, and orthographic projections are one of several ways to represent land or water areas. On a globe or map, lines of longitude or latitude are known as parallels. These projections preserve the dimensions along parallels, which results in a distorted view with expanded areas near the center and compressed views near the edges or horizon. These differ from stereographic projections, which use an imaginary view from the Earth's north or south pole.