An optimum truncation level, N, in a spherical harmonic analysis of the geomagnetic main field at the core mantle boundary is determined by harmonic spline analysis. Specifically, we find that value of N at which the two analyses are closest in a well defined sense and, for that value of N, we examine how "close" the two models are. Depending slightly on the definition of closeness, optimum N is found to be either 10 or 11. For those values the two analyses give remarkably similar results, showing that the conveniences of spherical harmonics can be retained with little penalty.