London - Saigon - Hanoi
Greenwich meridianThe prime meridian is used to measure the meridian latitudes. Go through London and from there, you realize that cities like Hanoi and Saigon they are just one step, hence London, Saigon, Hanoi.
Hanoi, Saigon within walking distance. London to Hanoi. London to Saigon
London to Madrid
This is the exact location of the Greenwich Meridian: