Resolved Question

It's the north south line that runs through the Royal Observatory at Greenwich - 0 degrees of longitude. It runs through Greenwich because we had the most powerful navy in the world at the time and called the shots. The French tried to insist that their own Rosa Ligna meridian ran through Paris was the one everyone should use, but it never really caught on. There is a brass line and a few brass markers on the French meridian in Paris, (depicted in Dan Brown's Da Vinci Code). You don't measure it, as such, but measure your position relative to it. These days normally by a GPS fix from 3 or more satellites.