High resolution two-dimensional long slit spectra of NGC 7009, NGC 3242, IC 2448 and TC 1 in the Hα(6565 Å), OIII(5007 Å) and NII(6584 Å) emission lines have been used as input to determine radial velocity gradients in PNe by fitting calculated high resolution spatially resolved spectral structures of single emission lines. Contributions to the Doppler structure of emission lines taken into account are the global expansion of the PN shell and thermal line broadening due to temperatures of some 104 K. This model is based on two parametrized empirically chosen spherical symmetric functions:

1. 1. A empirical two parameter density function ρ(r).

2. 2. A linear radial velocity component function ν(r).