  We review and compare the reconstruction methods of the inclusive deep
inelastic scattering variables used at HERA. We introduce a new prescription,
the $\Sigma$ method, which allows to measure the structure function of the
proton $F_2(x,Q^2)$ in a large kinematic domain, and in particular in the low
x-low$Q^2$ region, with small systematic errors and small radiative
corrections. A detailed comparison between the $\Sigma$ method and the other
methods is shown. Extensions of the $\Sigma$ method are presented. The effect
of QED radiation on the kinematic reconstruction and on the structure function
measurement is discussed.
