For an S-F surface, if not otherwise specified, the evaluation area shall be a square.NOTE 1 An example of an F-operation with a nesting index is a spline filter. The total least squares fit of the nominalform is an example of an F-operation without a predefined nesting index.NOTE 2 The value of the F-operation nesting index is typically chosen to be five times the scale of the coarseststructure of interest