A projection is the reproduction of a workpiece feature on another feature, such as projecting a circle or line onto a plane, or a point onto a line.

Projecting one part feature onto another can be compared with the creation of the traditional "flat" map of the world (Mercator projection). The flat map is made by projecting a globe of the world (sphere), onto a cylinder (Figure 15).

In metrology, projections allow you to measure more accurately how mating parts will eventually fit together. In automotive cylinder measurements (e.g., engine blocks), by projecting a cylinder into the plane of the head face, you can accurately determine how the pistons will fit into the cylinder and how it will meet with the combustion chamber in the head.

A minimum number of three points is necessary to measure the diameter of a circle and, if those points are not at the same distance from the top of the bore, the measured diameter will be shown to be elliptical (Figure 16). To overcome this misrepresentation, the measurement data is projected into a plane that is perpendicular to the centerline of the cylinder. The result is an accurate determination of the real size of this workpiece feature.