The document discusses tacheometry, which uses a theodolite fitted with a stadia diaphragm to measure distances. It describes the key components of tacheometry including the tacheometer, stadia rod, and stadia interval. The document explains that tacheometric distance (D) can be calculated using the formula D=Ks+C, where K is the multiplying constant, s is the stadia interval, and C is the additive constant. It also provides methods for determining the tacheometric constants K and C, including direct measurement and computation from multiple measured distances. Examples of solving tacheometry problems involving distance, elevation, and reduced level calculations are given.