1. A grid arrangement used in solving the acoustic wave equation. See Claerbout (1976, Fig. 10-1). 2. A potential-field continuation method for converting level-flown data to drape-flown and vice-versa. Level-flown data are continued to a series of levels that span the drape range, giving a set like stacked 3D chessboards; interpolation is carried out vertically to give drape profiles.