  Covariant phase space methods are applied to the analysis of a causal diamond
in 2+1-dimensional pure Einstein gravity. It is found that the reduced phase
space is parametrized by a family of charges with a dual geometrical
interpretation: they are geometric observables on the corner of the diamond,
and they generate diffeomorphisms. The Poisson brackets among them close into
an algebra. Knowledge of the corner charges therefore permits reconstruction of
the diamond geometry, which realizes a form of local holography. The results
are contrasted with the literature, and the path to a quantum description of
spacetime geometry is discussed.
