Higher order spline interpolation of the spherical harmonic coefficients.

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Some of the magnetic models (e.g.,CHAOS-6-Core) require higher order spline interpolation of the spherical harmonic coefficients.

The magnetic model uses always linear interpolation of the coefficients which leads to small deviation from the right model values.

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When compared with a reference implementation of the CHAOS-6-Core x7 model for randomly selected locations it is apparent that the linear interpolation causes deviation < ±0.04nT:

When using the 6th order piecewise polynomial interpolation (as defined in the SHC file) the deviation drops practically to zero: