3D (Areal) Filtering
The combination of an S-filter and an L-Filter may be used to eliminate a bandwidth of spatial wavelengths from analysis, maintaining the spatial structure with wavelengths less than the S-Filter Cutoff and spatial wavelengths greater than the L-filter Cutoff.
When the spatial wavelengths of the structure to be analyzed consists of spatial structure finer spaced that the S_Filter Cutoff and spatial structure with coarser spaced features than the L-Filter cutoff, the filter is referred to as a Notch Filter.
The figure below demonstrates that with a Notch Filter, features such as the isolated wider peaked regions (circled in the images) and moderate spatial structure lengths between the dominant mountain/valley features are eliminated.
As measured surface without filtering consisting of finer spaced features superimposed on longer wavelength components.
Notch Filter applied, resulting in the longer spatial wavelengths and shorter spatial wavelengths being preserved for subsequent analysis.