Basics of Coordinate Metrology
Unit 4:  Geometric Elements - Projections

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As mentioned for the geometrical element circle, projections play an important part in coordinate metrology. Projection is the representation of spatial objects on a plane (pattern). In practice, the main objects to be projected are points, lines and circles:  

The practical use of projection can be explained best taking the circle as an example: Since it is next to impossible to probe a circle by means of points located exactly at the same height, each determined circle diameter would always be larger than the correct diameter.

This is why first the probing points must be projected by the computer to the plane in which the circle is located according to the technical drawing.

 
Projection of a point on the XY plane Projection of a line on the XY plane Projection of a circle on the XY plane