  We present a new scheme of defining invariant observables for general
relativistic systems. The scheme is based on the introduction of an observer
which endowes the construction with a straightforward physical interpretation.
The observables are invariant with respect to spatial diffeomorphisms which
preserve the observer. The limited residual spatial gauge freedom is studied
and fully understood. A full canonical analysis of the observables is
presented: we analyze their variations, Poisson algebra and discuss their
dynamics. Lastly, the observables are used to solve the vector constraint,
which triggers a possible considerable reduction of the degrees of freedom of
general relativistic theories.
