Around 120 B.C., Greeks developed the science of geometry and were using it for precise land division. Greeks developed the first piece of surveying equipment (Diopter). In a work entitled Diopter, Hero of Alexandria, describes it as a portable instrument, an application of the cogwheel, screw, and water level, for taking terrestrial and astronomical measurements. Because of some similarities, Hero's diopter is usually recognized as the ancestor of the modern theodolite.