1. An apparatus for computing position, comprising: a receiver operable to obtain pseudorange measurements at a location; a terrain module operable to contain a regional-terrain model, the regional-terrain model including altitude information associated with a modeled region, wherein the modeled region defines its boundary as a function of a range of altitudes, and wherein the location is within boundary; and a processor configured to obtain from the regional-terrain model defined by the boundary as the function of a range of altitudes, the altitude information, and compute a position of the location as a function of the pseudorange measurements and the altitude information.