The main errors in GPS are usually due to variation in the propagation of the signals through the atmosphere, and uncertainty in the exact position of the satel

The main errors in GPS are usually due to variation in the propagation of the signals through the atmosphere, and uncertainty in the exact position of the satellites. (Solutions for these exist, but are generally too expensive and/or impractical for a lot of applications. But e.g. realtime GPS on the order of cm is something you can get now, if you pay for a subscription to the right service or operate your own reference base station)