  This paper - the first of a short series dedicated to the long-stan ding
astronomical problem of de-projecting the bi-dimensional apparent morpholog y
of a three-dimensional mass of gas - focuses on the density distribution in
real Planetary Nebulae (and all types of expanding nebulae). We introduce some
basic theoretical notions, discuss the observational methodology and develope
the accurate procedure for the determination of the matter radial profile
within the sharp portion of nebula in the plane of the sky identified by the
zero-velocity-pixel-column (zvpc) of high-resolution spectral images. Moreover,
a series of evolutive snapshots is presented, combining illustrative examples
of model- and true-Planetary Nebulae. Last, the general and specific
applications of the method (and some caveats) are discussed.

Astrophysics