We present an atlas of position-velocity (PV) diagrams of ionized gas in the inner regions of 23 disk galaxies. The gas kinematics have been obtained from ground-based long-slit spectra along the major axis. We discuss the variety of shapes of the emission lines in the PV diagrams that we find for the sample galaxies. To discuss the different shapes we consider a modelling tecnique which assumes that the gas resides in an infinitesimally thin disk whose mean motion is characterized by circular orbits in the plane of the galaxy. This modelling tecnique that takes into account the instrumental and the seeing effects allows to reproduce the observed PV diagrams. In addition this kind of analysis is useful to select spiral galaxies which are good candidates to present ordered gas rotation around central supermassive black holes (SMBH), to be followed-up by Hubble Space Telescope. For a few cases we will try to determine accurately the mass of the SMBH using forthcoming STIS observations of the gas kinematics.