For two isolated clumps a two-dimensional Gaussian was fitted to the column density profile (left panels). The residual (right panels) shows different behavior, for the upper row the noise level is very similar to an emission-free region in the surrounding, the lower row exhibits remaining features increasing the measured noise value. The residual RMS can be used as an estimator to the amount of substructure within a clump. Contour lines in the left panels start at 3σ_rms and have a spacing of 3σ_rms. Contour lines in the right panels are spaced with 1σ_rms, solid contours mark positive values, dashed contours are negative, the thick solid lines are zero flux.