This paper presents a 50-degree spherical harmonic model of the magnetic field of Mars derived from both low- and high-altitude magnetic data through covariance analysis. Three sets of models are first calculated by using the high-altitude data alone, which include harmonics of degree 55, 60, and 65. Each set consists of four models, depending on whether the north-south and west-east components data are used in a model in addition to the radial component. The models correlate strongly over harmonics of degree up to about 50, except the model that is calculated by using the radial component data alone, which shows appreciable differences from the others. The high-altitude models are then correlated with the corresponding models that were previously derived from the low-altitude data. The correlation between the corresponding models is again very high over harmonics of degree 4--50, confirming the reliability of the spherical harmonic coefficients over these harmonic degrees. The slightly lower correlation, ~0.7, over harmonics of degree lower than 4 implies some contribution from quasi-steady external field. A final 50-degree model is determined from the covarying harmonics of the low- and high-altitude models that use all three components of the magnetic data. The corresponding radial and tangential components of the magnetic anomalies are presented at the surface of Mars, along with their error limits.