Margen
Without the theory of relativity, GPS would build up an error of about ten kilometers a day. Your map works because someone corrected time itself.
GPS satellites carry atomic clocks, and those clocks don't run at the same pace as clocks on Earth. Einstein explained why: twenty thousand kilometers up, gravity is weaker, so time runs a little faster; at the same time, the satellites move so fast that time slows down a bit for them. The net balance: their clocks run about 38 microseconds ahead each day.
It sounds like nothing, but GPS works out your position by timing signals with nanosecond precision. Without correcting that gap, the error would grow by kilometers every day and the system would be useless within hours. That's why the clocks are adjusted before launch. Every time your phone places you on the map, the most abstract physics of the twentieth century is working for you.
Source: American Physical Society