A new numeric method for extrapolation of rectangular grid numerical models of geophysical fields is presented, which is based on numerical minimization of fourth derivatives with Dirichlet boundary condition. An extrapolated model does not represent a field and is targeted on its use in further data processing. The method is compared to other well-known extrapolation methods used in different GIS, most notably GIS INTEGRO. Pros and cons of the method are reviewed. The method is illustrated on the example data.