Offset angle of the photons hitting the observation level from different emission heights (H_em, measured above sea level) for vertical 100 GeV gamma rays. Points show the average (markers) value of the offset measured along the true main axis of the shower, and the corresponding standard deviation (error bars). Empty markers report the heights that do not contribute considerably to the total image (below 2% of the total). The pure geometrical model described by Eq. (1) is shown as the red dashed curve, while the global fit model (Eq. (2)) is expressed as the solid blue line. The left, middle, and right panels show the results for the impact parameters of 40 m, 130 m, and 220 m, respectively.