A simple datum for measurement of the Abbott curve of a profile and its first derivative

The methods used in some recent work to predict boundary friction from surface slopes require that the variation of rms slope with bearing area be calculated: an Abbott curve of the first derivative of the profile. It is shown that this curve can be distorted if a conventional filter is used to set the datum, in the same way as the Abbott curve itself. Better results can be obtained for both the standard Abbott curve and the Abbott curve of slopes if an envelope through the peaks of the profile is used as the datum, rather than a digital filter.