Increasing accuracy in the conversion of h′(f) to N(h) profiles is obtained by increasing the order of the polynomial used to interpolate between measured points. Linear and parabolic lamination techniques correspond to first- and second-order interpolation. Fourth-order interpolation (as in the 5-term overlapping polynomial method) is about optimum. In comparing different methods, it is essential that fixed boundary conditions be employed; when this is done an adjacent polynomial technique is much less accurate than overlapping polynomials. All methods (including least-squares procedures) are equally sensitive to errors in the virtual height data. Possible procedures for reducing the errors caused by underlying and valley ionization are critically reviewed. It is concluded that, in general, only a single parameter is necessary or desirable to describe these regions. This parameter can be determined from ordinary and extraordinary ray measurements, at any latitude.
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