The shearing behavior at particulate-continuum interfaces is influenced strongly by the continuum surface roughness. To date this surface has been characterized with absolute roughness parameters that represent various aspects of the spatial distribution of surface features. However, for particulate-continuum interfaces, the effect of continuum surface roughness on interface behavior is itself dependent on the particulate material characteristics. This interaction suggests that the surface roughness be characterized in relation to the contacting particulate material. Filtering algorithms such as low-pass and high-pass filters are frequently used to isolate components of a surface profile such as waviness and roughness. However, these filters consider peaks and valleys of a surface profile equally and hence implicitly assume each feature has the same effect on the interface behavior. An extensive analysis that traces the centroids of different diameter particles as they traverse across surfaces is presented herein. The resulting observations include: (1) peaks and valleys on a surface affect particulate-continuum interactions differently; (2) conventional surface roughness parameters do not account adequately for the characteristics of the contacting particulate material; and (3) conventional filters incorrectly isolate surface features of interest in particulate continuum interfaces. A framework that allows for these issues to be considered in evaluating the relative roughness of surfaces is presented.