As waves propagate from deep to shallow water, various effects of the shallowing depth of water spatially modify spectral characteristics of the sea surface. In such spatially inhomogeneous regimes, spectral computations based on a finite field size are subject to errors due to an inherent spatial smoothing in addition to resolution errors of a more conventional nature. The conflicting requirements of resolution and smoothing errors for an accurate analysis prescribe an optimal field size for which the combined magnitude of such errors is minimal. The optimal field size, the associated errors in resolution and smoothing, and the conditions for limiting accuracy are derived from computations in a shallow water wave field in which refraction and shoaling constitute the predominant inhomogeneity effects. Bottom friction, percolation, and reflection are neglected, and calculations are based on a first-order approximate shallow water wave theory.