  We present a new approach to the analysis of entanglement in smooth bipartite
continuous-variable states. One or both parties perform projective filterings
via preliminary measurements to determine whether the system is located in some
region of space; we study the entanglement remaining after filtering. For small
regions, a two-mode system can be approximated by a pair of qubits and its
entanglement fully characterized, even for mixed states. Our approach may be
extended to any smooth bipartite pure state or two-mode mixed state, leading to
natural definitions of concurrence and negativity densities. For Gaussian
states both these quantities are constant throughout configuration space.
