The angular diameter of minor planets compared with best observation conditions of about one arcsec (seeing) at ground-based observatories, though situated at high sea levels with best climate conditions, usually is too small to be resolved for surface studies or diameter determinations with direct photographic or similar imaging methods. Nevertheless the rough geometry and/or small scale structures on the asteroid's surface can be studied with light-curve observations using high precision photoelectric photometry and the fact that the rotation of an asteroid during a spin period is now determined for slightly more than 200 minor planets. On only a few selected asteroids (63 Ausonia, 88 Thisbe, 92 Undina, 110 Lydia, 118 Peitho, 128 Nemesis, 139 Juewa, 337 Devosa and 599 Luisa) do we show, from details detected in the light-curves, how observations of this type were carried out successfully. From the small scale features we get the rough linear extensions on the asteroid surface from differences in magnitude and time. Such observations will be more useful and important in future with respect to an optimum selection of objects for a possible direct asteroid spacecraft mission.