Determining which reference-level pressures, from among a plurality of available reference-level pressures, are used when estimating an altitude of a mobile device. Different systems and methods determine isobars based on reference-level pressures of weather stations, and then use the isobars in different ways to identify particular reference-level pressures for use in estimated an altitude of a mobile device. One approach determines the smallest distance between an initial estimated position of a mobile device and a neighboring isobar, and then uses that distance to identify reference-level pressures. Another approach identifies reference-level pressures between an isobar on which an initial estimated position of a mobile device is location and a neighboring isobar. Yet another approach compares the number of identified reference-level pressures and/or locations of the identified reference-level pressures against threshold conditions before determining which reference-level pressures to use.