Your position is definitely effected by your immediate surroundings. Trees or overhead vegetation will block the signals from reaching your unit all together and adjacent buildings, truck traffic, canyon walls will cause multi-pathing signals that basically degredate the overall position. This is amplified even more if the obstacles are blocking the southern sky or walls are north of your position since satellites are nearer to the southern horizon in the northern hemisphere.
The technical answer is much longer winded.
The technical answer is much longer winded.