To compute the MAC matrix between your test results expressed at sensors, and your numerical results expressed at node finite element coordinates, you first have to match your experimental DOF to your numerical DOF.

The application of the MAC formula requires shapes expressed on the same DOF sets. In general you must project your numerical data in your experimental space.

If your sensors have been placed at nodal locations, it is simple to extract the nodal information that is matched.

If not, you need to interpolate the numerical finite element displacement at your exact sensor location to get a meaningful expression of your numerical deformation on the sensor DOF set. This is a more demanding task that requires the implementation of a finite element code to be done properly.

My society provides a finite element toolbox for MATLAB for structural vibration analysis, that provides the tools to perform complete test correlation (sensor match, MAC computation, ...), identification and with updating capabilities.

If interested ,you can check out the functionalities at

www.sdtools.com

Kind Regards,