This paper presents an optimum condition to obtain the root-mean-square slope of the profile Δ_q, and an application method of Gaussian low-pass digital filter to eliminate high-frequency noises. The main conclusions are as follows. (1) The range of convolution integral is 0.9 times cut-off wavelength λ_s and the space of data is λ_s/12. (2) It is appropriate that Δ_q is calculated by means of the seven-point method of numerical difference according to Lagrange's interpolation. (3) We make clear the effect of fluctuation of sampling interval on Δ_q. (4) The low-pass filter affects little the roughness parameters in the direction of profile height but largely those in the direction of profile length.