  We present a static, model-independent, experimental determination of the
spin-spin correlation length \xi in a quantum spin system. This is achieved in
doped Haldane (i.e., S=1 Heisenberg antiferromagnetic) chain
Y(2)BaNi(1-x)Mg(x)O(5) by Y NMR imaging of the staggered magnetization induced
around the Mg-impurities (i.e., chain boundaries) by magnetic field. The
magnitude of this magnetization is found to decay exponentially with \xi equal
to the theoretical prediction for an infinite S=1 chain, and the staggered
magnetic moment at the edge site showing the Curie behavior of an effective
S=1/2 spin.
