We have developed a method to estimate the complete magnetization in thin-film longitudinal recording media from magnetic force microscopy (MFM) data. The method uses a medium model described by a Voronoi tessellation of the film plane. The magnetization lies in that plane, has constant magnitude, and is uniform within each convex region, or grain, of the tessellation. The effect of a single grain on the MFM simulation is isolated by considering the difference between the MFM images before and after that grain undergoes a 180° magnetization reversal. Using this difference image, the complete magnetization of the grain and the grain's boundaries are estimated. By isolating each grain in turn, as if a series of incremental applied fields had been applied, the magnetization for the whole pattern is estimated.