Measurement method and apparatus

1. An error correction method for a coordinate positioning apparatus, the method comprising the steps of:

(i) taking a first data set comprising one or more first data values derived from measurement of a first object by a first coordinate positioning apparatus, each first data value describing a position on the surface of the first object;

(ii) taking a second data set comprising one or more second data values derived from measurement of the first object by a second coordinate positioning apparatus, each second data value describing a position on the surface of the first object;

(iii) calculating an error map comprising one or more error values, each error value describing a positional difference between the surface of the first object as described by the first data set and the surface as described by the second data set,

the surface normal of the first object being known at each position described by each first data value and each error value being calculated by determining the positional difference substantially in the direction of the known surface normal; and

(iv) using the second coordinate positioning apparatus to perform a processing or measurement operation on the first object, or an object nominally identical to the first object, in which positions on the surface thereof are corrected using the error map calculated in step (iii).

2. A method according to claim 1 , wherein:

step (iv) comprises the steps of taking a series of further objects that are nominally identical to the first object and using the second coordinate positioning apparatus to measure each of the series of further objects, and the error map is used to correct the measurement of each of the further objects.

3. A method according to claim 1 , wherein:

step (iv) comprises the step of performing a processing operation to alter the surface shape of the first object or the object nominally identical to the first object, and

the error map is used to adjust the surface shape produced by the processing operation.

4. A method according to claim 3 , wherein the processing operation comprises adding material to, or removing material from, the first object or the object nominally identical to the first object.

5. A method according to claim 1 , further comprising the step of generating the one or more first data values and/or the one or more second data values using an interpolation process that ensures each second data value lies on the known surface normal of a first data value,

wherein each error value calculated in step (iii) comprises the positional difference between a first data value and its associated second data value along the known surface normal.

6. A method according to claim 1 , comprising, for each of a plurality of first data values, the steps of:

defining a plane that includes a selected first data value and is orthogonal to the known surface normal thereof;

projecting at least some of the second data values along the known surface normal of the selected first data value and determining the position at which the second data value projections intersect the defined plane;

establishing which one of the second data value projections intersects the defined plane at an intersection point that is closest to the selected first data value; and

calculating the error value from the positional difference between the closest intersection point and the associated second data value from which it was projected.

7. A method according to claim 1 , wherein the second coordinate positioning apparatus comprises a measurement probe.

8. A method according to claim 1 , wherein the first coordinate positioning apparatus comprises a calibrated coordinate measurement machine, and the first data set is derived from measurement of the first object by the first coordinate positioning apparatus.

9. A method according to claim 8 , further comprising the step of calculating the surface normal of the first object at each position described by the first data values using the points measured on the surface of the first object by the first coordinate positioning apparatus.

10. A method according to claim 8 , wherein:

the first object has a known geometric form, and

the method further comprises the step of using the first coordinate positioning apparatus to perform a datuming operation to establish the orientation of the first object such that the surface normal of the first object is known at each position described by the first data values.

11. A method according to claim 1 , wherein the second coordinate positioning apparatus comprises a parallel coordinate positioning apparatus.

12. A method according to claim 1 , wherein:

step (ii) comprises using an interpolation process to calculate the one or more second data values from the positions of a plurality of points on the surface of the first object as measured by the second coordinate positioning apparatus, and

each second data value calculated in step (ii) is arranged to lie on the surface normal of a first data value.

13. A method according to claim 1 , further comprising the steps of:

taking a third data set comprising one or more third data values, each third data value describing a position on the surface of the first object; and

calculating a second error map comprising one or more error values, each error value describing a positional difference between a second data value and a third data value, the positional difference being determined substantially in the direction of the known surface normal associated with the first data value.

14. A method according to claim 1 , wherein the second coordinate positioning apparatus has a lower accuracy than the first coordinate positioning apparatus.

15. An error correction method for a coordinate positioning apparatus, the method comprising the steps of:

(i) taking a first data set comprising one or more first data values derived from design data associated with a first object, each first data value describing a position on the surface of the first object;

(ii) taking a second data set comprising one or more second data values derived from measurement of the first object by a second coordinate positioning apparatus, each second data value describing a position on the surface of the first object;

iii) calculating an error map comprising one or more error values, each error value describing a positional difference between the surface of the first object as described by the first data set and the surface as described by the second data set, the surface normal of the first object being known at each position described by each first data value, and each error value being calculated by determining the positional difference substantially in the direction of the known surface normal; and

(iv) using the second coordinate positioning apparatus to perform a processing or measurement operation on the first object, or an object nominally identical to the first object, in which positions on the surface thereof are corrected using the error map calculated in step (iii).

16. An error correction method for a coordinate positioning apparatus, the method comprising the steps of:

(i) taking a first data set comprising one or more first data values, each first data value describing a position on the surface of a first object;

(ii) taking a second data set comprising one or more second data values derived from measurement of the first object by a coordinate positioning apparatus, each second data value describing a position on the surface of the first object;

(iii) calculating an error map comprising one or more error values, each error value describing a positional difference between the surface of the first object as described by the first data set and the surface as described by the second data set, the surface normal of the first object being known at each position described by each first data value, and each error value being calculated by determining the positional difference substantially in the direction of the known surface normal; and

(iv) using the coordinate positioning apparatus to perform a processing or measurement operation on at least one further object that is nominally identical to the first object, the positions on the surface of each further object being corrected using the error map calculated in step (iii).

17. A method according to claim 16 , wherein:

the at least one further object comprises a series of further objects, and

step (iv) comprises using the coordinate positioning apparatus to measure each of the series of further objects, the error map being used to correct the measurement of each of the series of further objects.

18. A method according to claim 16 , further comprising the step of generating the one or more first data values and/or the one or more second data values using an interpolation process that ensures each second data value lies on the known surface normal of a first data value,

wherein each error value calculated in step (iii) comprises the positional difference between a first data value and its associated second data value along the known surface normal.

19. A method according to claim 16 , comprising, for each of a plurality of first data values, the steps of:

defining a plane that includes a selected first data value and is orthogonal to the known surface normal thereof;

projecting at least some of the second data values along the known surface normal of the selected first data value and determining the position at which the second data value projections intersect the defined plane;

establishing which one of the second data value projections intersects the defined plane at an intersection point that is closest to the selected first data value; and

calculating the error value from the positional difference between the closest intersection point and the associated second data value from which it was projected.

20. A method according to claim 16 , wherein the coordinate positioning apparatus comprises a parallel coordinate positioning apparatus.