pla·nim·e·ter 
(plə-nĭmĭ-tər, plā-)
(plə-nĭmĭ-tər, plā-)n.
An instrument that measures the area of a plane figure as a mechanically coupled pointer traverses the perimeter of the figure.
pla′ni·metric (plā′nə-mĕtrĭk), pla′ni·metri·cal (-rĭ-kəl) adj.
pla·nime·try n.