  Using Cartan's equivalence method for point transformations we obtain from
first principles the conformal geometry associated with third order ODEs and a
special class of PDEs in two dimensions. We explicitly construct the null
tetrads of a family of Lorentzian metrics, the conformal group in three and
four dimensions and the so called normal metric connection. A special feature
of this connection is that the non vanishing components of its torsion depend
on one relative invariant, the (generalized) W\"unschmann Invariant. We show
that the above mentioned construction naturally contains the Null Surface
Formulation of General Relativity.
