# Roughness Evaluation of Burnished Topography with a Precise Definition of the S-L Surface
## 3 Results
### Selection of Procedure for a High-Frequency Errors Reduction for S-Surface Definition  
The process of a high-frequency measurement noise reduction can be roughly divided into two separate but dependent subprocesses. First, the noise must be properly defined and then a suitable method can be applied for its reduction (removal). From the point that it is extremely difficult to validate what the noise is, but, correspondingly, what are the real data, the eye-view analysis is not suggested, even for experienced metrologists [10]. In the current state of knowledge, there are many even more sophisticated algorithms and procedures for surface roughness evaluation. The existence of all of them is entirely justified, nevertheless, the application of each of them requires mindful users, and even if a precise measurement technique (measuring device) is used, the errors in the ISO 25178 parameters can arise when each of the approaches is implemented erroneously. Regarding this, the precise measurement method also requires an accurate data processing procedure. It was previously presented that in the high-frequency measurement error detection (definition of S-surface) process are valuable methods available in the commercial software such as PSD, ACF or TD approaches [10; 17; 46; 51]. When defining it to the NS [34], these approaches can be suitable for both areal (3D) [58] or profile (2D) [29] roughness analysis. When detecting high-frequency measurement errors, it has been observed that the shape of the centre part of the ACF is different, if the noise occurs [46]. In practice, the maximum (centre) value of ACF increases more rapidly when the high-frequency noise is found against when those errors are not defined. Currently, it has been observed that if the received raw measured data were filtered with a cutoff equal to the three values of the sampling or spacing (depending on the stylus or optical measuring method) and if the NS contained other than noise features (e.g., scratches or valleys), the amplitude of the high-frequency measurement error is too small to significantly influence the values of the ISO 25178 parameters. For this type of investigation, the FFT was applied. Its suitability for high-frequency noise suppression has already improved [29]. From this suggestion, if the sampling interval is 5 \(\upmu\)m (stylus method) and the spacing is 3.27 \(\upmu\)m (optical measurement), the cutoff value should not be smaller than 15 \(\upmu\)m and 10 \(\upmu\)m, respectively. This dependence was already proposed in defining the S-nesting index, which should be set at a 3:1 ratio with the maximum sampling distance [91]. However, the nesting index can be clearly beneficial in many metrological issues [92; 93; 94; 95], where the L- or S-filtering characteristics were applied. According to the NS properties, first, it was observed that it only contained the high-frequency components. This characteristic can be evaluated by the use of a PSD function, nevertheless, the thresholding methods can be a suitable alternative [46; 64]. In fact, proper NS received by the removal of high-frequency measurement errors from the results of the surface topography measurements should only include features in the high-frequency domain. Therefore, the PSD graph should only contain the features with the smallest value, which are indicated as those in the required (high-) frequency domain. Second, the NS in the high-frequency domain, if it does not contain features with other frequencies than in the high-frequency domain, should be isotropic, despite the directionality of the surface topography studied. The occurrence of the non-noise [51; 64] features can considerably reduce the isotropic properties of the surface. For the inspection of the directionality of the NS, the TD graph can be extensively used. In Figure 7, the NS received from filtration of the turned cylindrical surface with additionally burnished deep and wide dimples is presented. Various available commercial software filters were applied.