  We analyzed ionized gas motion and disk orientation parameters for 15 spiral
galaxies. Their velocity fields were measured with the H-alpha emission line by
using the Fabry-Perot interferometer at the 6m telescope of SAO RAS. Special
attention is paid to the problem of estimating the position angle of the major
axis (PA_0) and the inclination (i) of a disk, which strongly affect the
derived circular rotation velocity. We discuss and compare different methods of
obtaining these parameters from kinematic and photometric observations, taking
into account the presence of regular velocity (brightness) perturbations caused
by spiral density waves. It is shown that the commonly used method of tilted
rings may lead to systematic errors in the estimation of orientation parameters
(and hence of circular velocity) being applied to galaxies with an ordered
spiral structure. Instead we recommend using an assumption of constancy of i
and PA_0 along a radius, to estimate these parameters. For each galaxy of our
sample we present monochromatic H-alpha- and continuum maps, velocity fields of
ionized gas, and the mean rotation curves in the frame of a model of pure
circular gas motion. Significant deviations from circular motion with
amplitudes of several tens of km/s (or higher) are found in almost all
galaxies. The character and possible nature of the non-circular motion are
briefly discussed.
