Relative errors for the gravitational potential (top panel) and gravitational acceleration (bottom panel) for the oblate ellipsoid as a function of the number of nested grids. The black and red lines present the maximum and mean errors, respectively. The dashed lines correspond to the convolution method, while the solid and dash-dotted lines show the results for the OiCG method with the tolerance ϵ set equal to 10⁻⁶ and 10⁻⁸, respectively.