The disclosed embodiments use 3D city models and shadow mapping to improve altitude fixes in urban environments. In an embodiment, a method comprises: generating a set of three-dimensional (3D) candidate positions in a geographic area of interest; predicting global navigation satellite system (GNSS) signal visibility at selected ones of the 3D candidate positions; receiving GNSS signals at a current location of the mobile device; determining observed satellite visibility based on the received GNSS signals; comparing the predicted satellite visibility with the observed satellite visibility; determining a position fix based on a result of the comparing; determining an indoor environment where the mobile device is located based at least on an altitude component of the position fix; obtaining structural data for the identified indoor environment; and determining a floor lower bound for the current location of the mobile device based on the altitude component and the structural data.