The inherent smoothing in bending stiffness measurement of wood boards as occurs in the machine grading of lumber can mask the effect of knots and other local characteristics affecting structural value. Improved estimates of local (pointwise) stiffness will be useful in decisions about further processing and use of a tested board. Measured compliance is reciprocally related to measured stiffness and is the convolution of local compliance and a "span function". Span function is specific to the bending span configuration used and can change during measurement of a board. A general procedure for computing span function, which heretofore has been known only for simple bending spans, is disclosed. A Kalman filter uses this and other available information to optimally estimate local compliance from an observed relationship between local compliance and state variables of a state-space model. Method for linear algebraic determination of local compliance also depends on span functions and is disclosed.