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SPHEREPACK requires gridded data on either a Gaussian grid or an equal angle lat/lon “cap” grid which includes points at the poles. It relies on a combination of associated Legendre transforms and FFTs. For efficiency the grid should have dimensions N x 2N (Gaussian), or (N+1) x 2N (equal angle cap) with N the product of powers of small primes. The most common lat/lon data is an equal angle offset grid, which avoids pole points (see. CAM-FV Grid Overview ). Data on this offset grid would have to first be interpolated to a cap grid before computing spherical harmonic transforms.
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