To evaluate and compare tapping mode and contact mode AFM measurements of surface roughness, images of quartz and mica were acquired by both methods and the height distributions and variance correlation functions analyzed. Significant deviation from the expected Gaussian profiles for the height distributions were observed for contact mode images of quartz but not for tapping mode images. Additionally, variance correlation functions were found to be highly scan size dependent for contact mode images and scan size invariant for tapping mode images. One possible explanation for the observed differences is that the scan speed limit is exceeded in contact mode for linear scan velocities as low as 0.5 mu m/s.