Metrology
Getting Precise About Accuracy
The quality of being reproducible vs. the quality of nearness to the true value
Published: Monday, May 14, 2012 - 12:09
Any data captured in the scanning process is not perfect. Data accuracy depends on the accuracy of the scanning equipment as well as the conditions under which the measurements are made. To properly report measured data, the error associated with the measurement should be taken into account.
Experimental error should not be thought of as a mistake. The difference between the “true” value of a distance vs. the measured value is the “error” of the measurement. A measurement is deemed accurate if the error is small (a relative term).
Accuracy
Accuracy is an indication of the range of the error that is inherent in the measurement. As an example, if you measure a 10 mm gauge block with a scanner or micrometer and get 10.80 mm, then the measurement method can be considered inaccurate since the gauge block is generally accepted as the standard. Should the micrometer or scanner measure 10.02 mm, then the device could be said to be accurate because the error is relatively small and in reasonable agreement with the artifact.
Precision
Precision is a measurement of the repeatability, or consistency, of a measurement. It is possible to have a very precise measurement without scatter (or noise) that is repeatable and would be considered precise (repeatable); however, it can be inaccurate because of an instrument error. For example, the scanner may be out of calibration and produce inaccurate but consistent (precise) results. It is not possible to have an accurate instrument unless it is also precise.
Resolution
Resolution would be the number of points that can be measured on a surface. For example, a coordinate measuring machine (CMM) would be considered by most to be precise and also accurate; however, the typical “resolution” of the CMM system would result in a very low resolution, comparatively. A 3-D imaging device that is capturing a high number of points per second, or points per unit of area, would yield a much higher resolution. The higher the resolution, the more points are put onto a surface. In reality there is no truly planar surface and no parallelism, both hypothetical mathematical terms. Using the gauge block as an example again, we can state that within an acceptable accuracy a surface is considered planar and two surfaces may be parallel.
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