Heliocentric distance versus Galactic longitude map of the difference between the surface densities for b > 0° and b < 0° relative to the uncertainty of the density of the full pP30 sample. Positive values mean larger densities in the northern bin. Estimations of completeness limits for the b < 0° (b > 0°) subsample are shown as orange circles (blue squares) connected by a dotted (dashed) line that guides the eye.