Impact of prism type and prism orientation on total station measurements

The achievable accuracy of total station measurements always depends on the instrument, the atmospheric conditions along the measurement path and the used target. From investigations performed several years ago it is known, that certain configurations and prism types are not suitable for high-accurate applications. E.g. 360° prisms are not recommendable because horizontal and vertical angles and also the slope distances vary depending on the orientation of the prism. In recent years, manufacturers redesigned some of their prisms and new total stations with improved distance measurement capabilities and better automated target aiming systems were released. In our laboratory we tested active and passive prisms of different manufacturers and show that newer EDM technologies can reduce the distance errors on 360° prisms. However, also circular prisms can adversely influence angle and distance measurements. A special situation arises when a circular prism is perfectly aligned to the instrument. In this case front reflections caused by the front surface can cause errors in the range of several millimeters.