The mathematical definition of silhouettes within NPAR is based on the assumption that they are the eye-centred projections of occluding contours on an imaging plane. However, occluding contours are insufficient for sketching the silhouettes of hills as drawn by cartographers. The research reported in this paper is based on the proposition that silhouettes are mental visualisations of the outlines of objects that arise from knowledge and experience of the visual world. This paper does not seek to provide an alternative definition of silhouettes. Instead, it tests the proposition that missing silhouette elements should be drawn on those visible surfaces brought into view through adoption of a higher viewpoint.