This question has came up to my mind and I think it really isn't easy as it sounds. In Newtonian physics we could use the Earth as the basic frame and compare velocities of cars, planes etc. on our planet. But in relativistic physics, things get complicated because, as we are in a non-intertial frame, we may use different coordinate systems and therefore get different relative velocities of things on Earth. So, my questions is, what is the standard or most used way to define velocities wrt to Earth in relativistic physics? Thank you in advance.