The correct analysis of spinwave resonance spectra in ferromagnetic films must properly account for the conditions which determine the rf magnetization at the film surfaces. Over the past twenty years, the treatment of the surface conditions has evolved from the Kittel conditions of complete pinning, through a partial pinning condition to the inclusion of anisotropy energies acting at the film surface. Recently, it has been shown that to properly account for the angle dependence of the spinwave spectra in homogenious garnet films, the surface energies must be expanded in the Neél expression having the form: Ks(φ) =Σ l b l P l (cosφ) where φ is the angle between the surface normal and the direction of the magnetization and the P l (cosφ) are the Legendre polynomials. Using a new method of analysis based on the pinning parameter model, the coefficients b l have been determined for each surface by using detailed measurements of the angle dependence and intensity ratios of the surface modes and the body modes of the spinwave spectra of YIG films on GdGaG substrates. A spin Hamiltonian is presented which includes microscopic interactions that predict surface energies consistent with the angular, frequency and temperature dependence of the data.