The GSM radio signal being at frequency 900/1800MHz has short wavelength enough to reflect on almost any surface. For example it has been proven in practise that the GSM signals tend to follow the countryside roads because the surrounding trees form a "tunnel" for them. Especially when there is snow on those trees (hopefully not in California).

In urban environment when there is a lot of multipath propagation the only usable method is to know the time of travel and quess the route. This ofc ourse requires that the geometry and the surface materials of the environment are known and that the channels of the GSM base stations as well as their positions are also known. Usually this kind of knowledge (the combination) is very much protected IP and is certainly NOT available to everyone.

The field strength method OTOH is useless even on the open landscape as then You should know the antenna properties. They have very seldomly circular radiation pattern. Instead a mast usually has 2, 3 or even 4 TX antennas all pointing to different directions and having kidney formed radiation patterns. The antennas are located in such a manner that the FS would stay relatively constant - at least if You follow common roads. This is because the GSM spesification, unlike UMTS, does not specify transmitter power control for base station.