As per ASME B46.1 (1995) and ISO 4287 (1997) standards, either the fitted mean line or least squares mean line can be used as a reference mean line ӯ in surface roughness calculations.
Mean line defined as [BS-1134] "A line having the form of the geometrical profile within the limits of mean cutoff, and so placed that within the meter cut-off the sum of the squares of the deviations of the profile from the mean line is minimum".