Precise measurements of secondary yields in hadron-nucleus collisions in the few GeV/c region are relevant to several areas of particle physics, particularly experimental neutrino physics. In particular, measurements made at HARP can have a direct impact on the detailed understanding of the neutrino fluxes of several accelerator-based neutrino experiments, including the K2K experiment in Japan and MiniBooNE and SciBooNE at Fermilab. HARP took data with these exact beam energies and target materials using both thin and thick targets. Strange particle production in the forward direction must be well known when trying to determine the ve background in a vµ beam, since Ke3 decays of K± and K0L constitute an irreducible background in the search of the vµ -> ve oscillations. This applies particularly to experiments such as MiniBooNE where G4 simulations showed that kaon decays contribute to more than 40% of the total intrinsic ve background of the beam. A complete analysis method has been developed to extract the total positive kaon differential production cross-sections from the 12.9GeV/c protons on aluminum target data set.