DnD: Pendulum Map

First off, I have to give credit where it's due: This idea was directly inspired after seeing this image.

The Pendulum Map



Description
A large, flat circular depression contains a map. It is surrounded by a number of columns, each with a rune or glyph or marking of some sort. These columns are made in such a way as to hold something - a stone or a rod of some sort.

Additionally, two to four raised platforms surrounding the map, in between the columns. These platforms are also marked in some way. A metal pendulum is suspended directly over the center of the map.

Operation
The map can be presented as either a magical or a mechanical device, but the basic operation remains the same: One or more lodestones are placed on the columns. The pendulum is then released from one of the platforms and allowed to swing. The lodestones will either pull or push the pendulum in such a way as to affect its movement across the map.

A marking device will repeatedly cross over certain areas of the map, while other portions will remain relatively untouched. The areas that are marked will vary depending on which columns the stones are placed on as well as from which platform from which the pendulum is released.

The method by which the map is marked may vary: a magical crystal that leaves fading lines of light, sand, chalk or (more crudely) charcoal, etc.

Recommended Environments
A device such as this is best suited to larger Arcane or Mechanical institutions - academies, libraries, observatories, or possibly guild headquarters. The delicate nature of the map means that not just anyone would be given access to it.

Possible Use
The PC's need the map to find a hidden location. Before they can use it, though, the [lodestone, marking device, appropriate column/platform combination] must be found.