Appreciable suppression of field emission (FE) from metallic surfaces has been achieved by the use of improved surface cleaning techniques. In order to understand the effects of surface preparation on field emission, systematic measurements were performed on five single crystal and three large grain samples of high purity (RRR>300) niobium by means of atomic force microscope, x-ray diffraction, scanning electron microscope (SEM), and dc field emission scanning microscope. The samples were treated with buffered chemical polishing (BCP), half of those for 30 μm and others for 100 μm removal of surface layer, followed by a final high pressure water rinsing. These samples provided the emission at minimum surface fields of 150 MV/m and those with longer BCP treatment showed the onset of field emission at slightly higher fields. A low temperature (∼150°C) heat treatment in a high vacuum (10^{-6} mbar) chamber for 14 hours, on a selected large grain Nb sample, gives the evidence for the grain boundary assisted FE at very high fields of 250 and 300 MV/m. Intrinsic field emission measurements on the present Nb surfaces revealed anisotropic values of work function for different orientations. Finally, an interesting correlation between sizes of all investigated emitters derived from SEM images with respect to their respective onset fields has been found, which might facilitate the quality control of superconducting radio-frequency cavities for linear accelerators.