abney level: a surveying instrument consisting of a spirit level and a sighting tube; used to measure the angle of inclination of a line from the observer to the target
surveyor's level: surveying instrument consisting basically of a small telescope with an attached spirit level rotating around a vertical axis; for measuring relative heights of land
variation: (astronomy) any perturbation of the mean motion or orbit of a planet or satellite (especially a perturbation of the earth's moon)