The GCMAS Standards Council has proposed a protocol for quantifying the accuracy of a motion analysis system (i.e., how accurate the system is at locating markers). This Standard Assessment of Motion System Accuracy protocol (SAMSA) is intended to test video-based systems that employ reflective markers, though it is possible that it could be used to test systems with active markers as well.
SAMSA uses a simple device based on a earlier design used by Jim Richards to compare motion systems . This device consists of a beam fitted with markers and rotated at 60 RPM by a motor. The protocol is designed to test the ability of a motion system to (1) track moving markers; (2) resolve markers using a subset of the cameras; and (3) resolve markers that pass close to one another during a trial. The protocol has been tested in the laboratories of seven Standards Committee members and the results have been used to formulate an accuracy standard in the form of acceptable error thresholds. This testing and the error thresholds were presented at the 2007 GCMAS Meeting in Springfield; please refer to this abstract for further details . A manuscript detailing these findings is currently in preparation.
GCMAS members interested in testing their own labs using this protocol have two options:
- Build your own SAMSA device using the plans available on this website.
- Borrow a SAMSA device that belongs to GCMAS and is available to members on a first-come, first-served basis. There is no cost to members for this service; all that is required is that you take no more than one week to complete your testing and that you pay to ship the device to the next lab in line when your testing is complete. Contact the Standards Committee here.
Some notes on SAMSA:
- Standards Council members tested with the device located in the centers of their volumes, but you may wish to repeat the test with the device in other locations to locate "dead spots" in your volume.
- The error thresholds presented in the abstract are not final. The Standards Committee fully expects that they will evolve over time as a clearer picture emerges as to what is reasonable. We do think they represent a worst -acceptable-case corresponding to about twice the average error measured in our seven labs.
- Post-processing of marker locations is up to you; you should follow the same procedures that ordinarily would for filtering, filling in gaps, etc.
- Further notes on implementation are included with the plans.
Respectfully submitted by the GCMAS Standards Council:
Steve Piazza (Chair)
- Richards JG (1999). The measurement of human motion: A comparison of commercially available systems. Human Movement Science, 18, 589-602.
- Piazza SJ, Chou L-S, Denniston NL, McMulkin ML, Quigley EJ, Richards JG, Schwartz MS (2007). A proposed standard for assessing the marker-location accuracy of video-based motion analysis systems. Proceedings of the 12th Annual GCMAS, Springfield, MA.