I have just started a similar project. I was thinking of few ideas. One of them was to trigger something in the system ( alert/alarm or whatever) if the latitude / longitude seconds differ by 2. For example, if the location is 000.000.2 & 00.00.5 >- let us assume that, the next location which comes from GPS are 000.00.4 & 00.00.5 or where 5 becomes a 7.

Will this approach be a good approach to know when the stationary vehicle is moved? or there is any flaw in this which I am missing!