  Interpolation by various types of splines is the standard procedure in many
applications. In this paper we shall discuss harmonic spline "interpolation"
(on the lines of a grid) as an alternative to polynomial spline interpolation
(at vertices of a grid). We will discuss some advantages and drawbacks of this
approach and present the asymptotics of the $L_p$-error for adaptive
approximation by harmonic splines.
