What is a reference circle?

The output from the gauge can be represented as a polar profile or graph and although this gives a convenient pictorial representation, deriving actual numbers from it can be time consuming and subjective. We therefore need some means of processing the information to give us accurate and repeatable answers. 
As we are trying to assess departures from true circularity and require a reference from which to measure, it makes sense to try to fit a circle to our profile and relate all our calculations to it. 

There are a number of ways of assessing out of roundness using a number of types of reference circle. All reference circles are used to establish the centre of the component. Roundness is then established as the radial deviations from the component centre.