The increasing availability of smartphones equipped with barometric sensors enables their use as barometric altime- ters. However, an accurate measurement requires updated infor- mation about the temperature and the mean sea level pressure at the location of the measurement. With this aim, this work proposes, analyzes, and compares spatial interpolation methods applied to atmospheric pressure and temperature reports from nearby reference weather stations in order to derive reference values to be used by altimeters. As a proof-of-concept, we implemented an Android Altimeter application. Its validation was performed by a set field experiments. Preliminary results show the potential of the proposed methods for the accuracy of smartphone-based barometric altimeters.