Thermal inertia, which represents the resistance to change in temperature of the upper few centimeters of the surface, provides information to help understand the surficial geology and recent processes that are potentially still active today. It cannot be directly measured on Mars and is therefore usually modelled. We present a new analytical method based on Apparent Thermal Inertia (ATI), a thermal inertia proxy. Calculating ATI requires readily available input data: temperature, incidence angle, visible dust opacity, and a digital elevation model. Because of the high spatial resolution, the method can be used on sloping terrains, which makes possible thermal mapping using THEMIS in nearly any area of Mars. Comparison with results obtained by other approaches using modeled data shows similarity in flat areas and illustrates the significant influence of slope and aspect on albedo and diurnal temperature differences.