planimeter /plə-nimˈi-tər/ noun

An instrument for measuring the area of a plane figure

planimetric /plān- or plan-i-metˈrik/ or planimetˈrical adjective

planimˈetry noun

The mensuration of plane surfaces

planisphere /planˈis-fēr/ noun

1. A sphere projected on a plane
2. A map of the celestial sphere, which can be adjusted so as to show the area visible at any time

planispherˈic adjective