  The purpose of this paper is to investigate RBF approximation with highly
nonuniform centers. Recently, DeVore and Ron have developed a notion of the
local density of a set of centers -- a notion that permits precise pointwise
error estimates for surface spline approximation. We give an equivalent,
alternative characterization of local density, one that allows effective
placement of centers at different resolutions. We compare, also, the pointwise
results of DeVore--Ron to previously works of Wu and Schaback and of Duchon.
