Fig. 1. (A) A schematic depiction of a planar substratum showing its altitude () and azimuth () angles. N, north. (B) For a limpet on a vertical planar substratum, lines of sight (depicted by the broken arrow) within arc S end at the sky, whereas lines of sight within arc O end at the ocean (which in this hypothetical case extends to infinity in a horizontal plane). Because arc S and arc O each account for half the `visual field' of the limpet, each has a view factor of 0.5. The fraction of the view factor allocated to sky and ocean depends on the altitude angle of the substratum (see text, Eqn 1 and 2).