|True Meridian, Charleston, SC|
True MeridianThe true meridian is the plane that passes through true north poles and true south poles at the spot of the observer. The difference between true meridian and magnetic meridian is that the true meridian is fixed while the magnetic meridian is formed through the movement of the needle.[unreliable source] True bearing is the horizontal angle between true meridian and a line.
Henry D. Thoreau classified this true meridian versus the magnetic meridian in order to have a more qualitative, intuitive, and abstract function. He used the true meridian since his compass varied by a few degrees. There were some variations. When he noted the sight line for the True Meridian from his family's house to the depot, he could check the declination of his compass before and after surveying throughout the day. He noted this variation down.
Doing an image search it appears there are many of these markers set up in the early to mid 1800's and they continue to be a curiosity.