  We re-derive hydrodynamical equations in General Relativity (GR) in the
comoving reference frame for spherical symmetry and obtain from them the
well-known but not explicitly derived Lagrangean equations in Special
Relativity (SR), that is, for the case of weak gravitational fields. Explicit
formulae are presented which relate General Relativistic independent variables,
Lagrangean mass coordinate and variables in the lab-frame. Conversion of one
set of variables into another requires knowing the solution to the set of
equations. These formulae allow one to translate the solution to the exact set
in General Relativity into the form in which Special Relativistic solutions are
usually obtained. This is applicable for comparison of SR-numerical simulations
of collapses, GRBs and Supernovae explosions with more precise GR-simulations.
