Fig. 6. Definition of track angle, track distance and crab side, the three parameters used to describe the dummy trajectory relative to the burrow and the crab's home vector. `Crab side' measures whether the dummy track intercepts the line defined by the crab's home vector on the crab side of the burrow (crab side = 1) or on the opposite side (crab side = 0). The variable is needed because there are always two paths that have a particular combination of track angle and track distance.