base plane
The ground plane provides the physical support and the visual base for building forms. The floor plane supports our activities within buildings. A simple field of space may be defined by a horizontal plane laying as a figure on a contrasting background. For a horizontal plane to be seen as a figure, there must be a perceptible change in color or texture between its surface and the plan upon which it lines. Elevating a portion of the base plane will create a field of space within a larger spatial context. The change in level along the edge of the elevated plane defines the boundaries of its field and interrupts the flow of space across its surface. (Ching, 1979)