Abstract: In a sensing system having relatively movable fixture and sample elements in which measured data is corrupted in proportion to the departure of the fixture from its ideal position, as from mechanical intolerances, etc., a reference disc and reference sensor are disclosed. The reference sensor and disc are provided for measuring the non-repeatable departure from ideal so that the measured data can be compensated thereby in real time for the error thereby improving the accuracy of the measured data. This system is operable in combination with a repeatable error elimination system insuring complete error removal.