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How well do we know the physical/chemical properties of PNe? 1D (CLOUDY) and3D (MOCASSIN) photoionisation codes are used in this contribution to model thePNe K 4-47 and NGC 7009 as an attempt to question whether or not the high Te(higher than 21,000K) of the K 4-47's core and the N overabundance of the outerknots in NGC 7009 are real.  These are very basic parameters, obtained for Galactic PNe, e.g. nearbyobjects, even though with large uncertainties. Based on the comparison of themodelling with, mainly, optical images and long-slit spectroscopic data, it issuggested here that K 4-47 high Te can be explained if its core is composed ofa very dense and small inner region --that matches the radio measurements-- anda lower density outer core --matching the optical observations. This approachcan account for the strong auroral emission lines [OIII]4363A and [NII]5755Aobserved, and so for the high temperatures. This teaches us that the assumptionof a homogeneous distribution of the gas is completely wrong for the core ofsuch PN.  In the case of NGC 7009 a simple 3D model that reproduces the observedgeometry of this nebula is constructed. The aim of this modelling was toexplore the possibility that the enhanced [NII] emission observed in the outerknots may be due to ionisation effects instead to a local N overabundance. Hereit is discussed the model that can best reproduce the observations employing ahomogeneous set of abundances throughout the nebula, not only for nitrogen butalso for all the other elements considered.Comment: 4 pages; talk presented in the international conference Planetary  Nebulae as Astronomical Tools; June 28 - July 02, 2005; Gdansk, Poland; Eds:  R. Szczerba, G. Stasinska, and S. K. Gorny; AIP Conference Proceeding

Do We Really Know How to Derive the Basic PNe Parameters?