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A method of calibrating a contact probe having a contact element includes measuring with the contact probe a first geometric property of a calibrated artifact and a second geometric property of the or a further calibrated artifact. The first and second geometric properties are such that a deviation

A method of calibrating a contact probe having a contact element includes measuring with the contact probe a first geometric property of a calibrated artifact and a second geometric property of the or a further calibrated artifact. The first and second geometric properties are such that a deviation between a measured value and the expected value, resulting from a difference between an effective diameter of the contact element and an assumed diameter used for determining the measured value, has the opposite sign for each of the first and second geometric properties. The method further includes identifying a difference in the effective diameter of the contact element from the assumed diameter including comparing deviations of the measured value to the expected value for each of the first and second geometric properties to determine whether there is a difference in the deviations.

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1. A method of calibrating a contact probe having a contact element, the method comprising: measuring with the contact probe a first geometric property of a calibrated artefact and a second geometric property of the or a further calibrated artefact, the first and second geometric properties being su

1. A method of calibrating a contact probe having a contact element, the method comprising: measuring with the contact probe a first geometric property of a calibrated artefact and a second geometric property of the or a further calibrated artefact, the first and second geometric properties being such that a deviation between a measured value and the expected value, resulting from a difference between an effective diameter of the contact element and an assumed diameter used for determining the measured value, has the opposite sign for each of the first and second geometric properties;identifying a difference in the effective diameter of the contact element from the assumed diameter comprising comparing deviations of the measured value to the expected value for each of the first and second geometric properties to determine whether there is a difference in the deviations; anddetermining an effective diameter of the contact probe from the measurements of the first and second geometric property. 2. A method according to claim 1, wherein the first and second geometric properties are such that a distance traveled by the contact probe in measuring the first and second geometric properties is substantially the same. 3. A method according to claim 1, wherein measuring of the first geometric property comprises measuring a first distance on the calibrated artefact by contacting a first plurality of points on the calibrated artefact, wherein the first distance is determined by adding the assumed diameter to a distance determined from the centre of the contact element when in contact with the first plurality of points, and measuring of the second geometric property comprises measuring a second distance on the calibrated artefact or the further calibrated artefact by contacting a second plurality of points on the or the further calibrated artefact, wherein the second distance is determined by taking away the assumed diameter from a distance determined from the centre of the contact element when in contact with the second plurality of points. 4. A method according to claim 1, wherein the first and second geometric properties comprise first and second surfaces have a cross-section of substantially the same shape. 5. A method according to claim 4, wherein the first surface is a convex surface and the second surface is a concave surface. 6. A method according to claim 1, wherein the contact probe is attached to a quill of a coordinate positioning machine, measurement of the first and second geometric properties carried out by moving arms of the coordinate positioning machine. 7. A method according to claim 6, wherein a distance traveled by the coordinate positioning machine and/or rotary probe head in measuring the first and second geometric properties with the contact probe is substantially the same for both the first and second geometric properties. 8. A method according to claim 1, wherein the contact probe is attached to a rotary probe head, measurement of the first and second geometric properties carried out by rotation of the rotary probe head about an axis. 9. A method according to claim 8, wherein the rotary probe head is mounted on a quill of a coordinate positioning machine, one measurement of the first and second geometric properties carried out by rotation of the rotary probe head with arms of the coordinate positioning machine fixed in position and a further measurement of the first and second geometric properties carried out by moving arms of the coordinate positioning machine with the rotary probe head fixed in position. 10. A method according to claim 1, wherein the first and second geometric properties are geometric properties of different artefacts. 11. The method according to claim 10, further comprising locating the different artefacts at approximately the same position in a coordinate positioning machine. 12. A method according to claim 10, wherein the different artefacts are a female artefact and a male artefact. 13. A method according to claim 1, further comprising measuring a first plurality of geometric properties of one or more calibrated artefacts and measuring a second plurality of geometric properties of the one or more calibrated artefacts or a further one or more calibrated artefacts, wherein a deviation between a measured value and an expected value of the geometric property resulting from a difference between an effective diameter of the contact element and an assumed diameter used for determining the measured value has the opposite sign for the first plurality of geometric properties to the second plurality of geometric properties. 14. A coordinate positioning machine comprising a contact probe;an articulating device for moving the contact probe; andone or more processors for controlling movement of the articulating device to move the contact probe and for receiving signals from the contact probe,wherein the one or more processors are arranged to control the articulating device and process the signals from the contact probe to calibrate the contact probe in accordance with claim 1. 15. A method of calibrating a contact probe having a contact element, the method comprising: measuring with the contact probe a first geometric property of a calibrated artefact and a second geometric property of the or a further calibrated artefact, the first and second geometric properties being such that a deviation between a measured value and the expected value, resulting from a difference between an effective diameter of the contact element and an assumed diameter used for determining the measured value, has the opposite sign for each of the first and second geometric properties; andidentifying a difference in the effective diameter of the contact element from the assumed diameter comprising comparing deviations of the measured value to the expected value for each of the first and second geometric properties to determine whether there is a difference in the deviations,wherein the expected value is a known dimension for the geometric property and/or known relative location of first and second surfaces. 16. A method of calibrating a contact probe having a contact element having an assumed diameter, the method comprising: measuring with the contact probe a plurality of points on a surface of a male artefact and a plurality of points on a surface defining an aperture in a corresponding female artifact;identifying a difference in a diameter of the contact element from the assumed diameter by comparison of the measured points for each of the male and female artefacts; anddetermining an effective diameter of the contact probe from the measurements of the plurality of points.