Basics of Coordinate Metrology
Unit 4:  Geometric Elements - Circle

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The next standard geometric element is the circle. A circle is a plane curve generated by one point moving at a constant distance R from a fixed point M. A circle has the following characteristics:
Extension: Radius R
Its direction is given by the normal vector

A circle is determined by at least three (probing) points (on the circle line). These points are used to calculate the center M (x, y, z). The distance from the center to each of the (probing) points is called radius R.

In practice the diameter (Ø) D is usually used instead of the radius R. It is twice the value of the radius (D = 2 R).

Tip: For a circle to be measured correctly, all probing points must be on a common plane. This plane must be precisely the plane in which the circle is located according to the technical drawing. Since in practice it is near to impossible to probe three or more points in space at exactly the same height, it is recommended to project the probing points into the circle plane (Digression: Projections).  

       

Element:
Circle
Display:

Parameters:
center M (x, y, z), 
radius R,
normal vector N (x, y, z)
Required minimum number of probing points:
3 probing points
Example using a part:

(Bore hole in sheet metal)