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spherical harmonics


I want to analyze data on a sphere, representing them by spherical
harmonic coefficients.
This is somehow like a Fourier transform, but based on Legendre
polynoms, which are
defined on the surface of a sphere.
Unlike wavelets, this transform is global.
My data are given in the form [longitude, latitude, data_value], where
longitude, latitude
and data_value are vectors of the same length.
Data points are randomly scattered over the sphere with a resolution of
approx. 100km
on the Earth's surface.

Does somebody know, how to do this transform with idl?