Hi, if you mean a "strain-free" point, you don't measure one as such. When the displacements are input into the FE model, equilibrium over the plane ensures that there is stress balance. So just measure both surfaces, smooth and average, and input into the FE model. 

There is lots of helpful information at:

http://www.lanl.gov/contour/

Thanks Mr. Fitzpatrick, but I mean, for measuring surface, I should define a point on the part as a reference point for CMM machine or machine base on the measured surface data automatically calculate reference surface?

I think this will depend on the software for the machine: you should consult the manufacturer's guide.

Thanks Mr. Fitzpatrick.