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Decision-support groundwater modelling generally requires the use of many parameters. The integrity of both calibration and predictive uncertainty analysis depends on this.

Regularised inversion and linear uncertainty analysis requires that a Jacobian matrix be filled. Unless adjoint sensitivity methods are available (which they generally are not), sensitivities must be calculated using finite parameter differences. This is generally not possible for cell-based parameters. There are too many model cells, and their sensitivities are too small for finite-difference derivatives calculation to work. Hence an intermediate parameterisation device must be sought. This device must support highly parameterised inversion and uncertainty analysis on the one hand, and finite-difference calculation of derivatives on the other hand. Pilot points comprise such a parameterisation device.