  Type I planetary nebulae (PNe) are defined as those with high He and N
abundances (Peimbert & Torres-Peimbert 1983). These objects present in general
bipolar geometries and have high stellar temperatures (Corradi & Schwarz 1995,
Torres-Peimbert & Peimbert 1997). In this paper we analyse the empirical
methods for abundance determination in order to check if the He and N
overabundances in Type I PNe are a consequence of a geometrical effect, due to
the bipolarity, or the ionization stratification, due to the stellar
temperature. For this, we obtain simulated spherically symmetrical as well as
bipolar nebulae, using a 3D photoionization code. From the projected emission
line intensities for: a) the whole nebula; b) for a slit crossing the nebula;
as well as c) for different positions in the nebula, we applied the formulae
used in the literature to obtain empirical abundances. These empirical
abundances are then compared with the adopted ones. We show that empirical
abundances depend on the particular line of sight covered by the observation
and can simulate an overabundance and/or the presence of abundance gradients of
He and N in planetary nebulae with high stellar temperature. The geometrical
effects are also discussed. Systematic errors in abundance determinations by
empirical methods are higher for the N/H ratio than for N/O. Thus, it seems
better to use the N/O value when discussing N rich objects.
