Prism constants are corrections applied to measured distances in surveying. They account for the physical design characteristics of the prism or reflector used with a total station.
All manufacturers, such as Nikon, Pentax, Sokkia, Topcon, Trimble, Geodimeter, Leica, Zeiss, etc., define the prism constant as a correction, that is, a correction of the measured distance based on the physical and design characteristics of the prism or reflector in the instrument.
The distance between the vertical axis of the prism holder and the theoretical turning point of the measuring beam determines the magnitude of the prism constant. This point, located behind the glass in the prism, is where the beam reflects.
The magnitude of the prism constant is determined by the distance between the vertical axis of the prism holder and target point and the theoretical turning point of the measuring beam, which is behind the glass. If the vertical axis is situated right at the point, then the Prism Constant equals 0. In other commercially available prisms, the vertical axis is always in front of the point. The measured distance will then be too long, and the corresponding correction (prism constant) will be negative. If the vertical axis runs through the center of the prism (commonly referred to as the nodal point), the prism has the best mounting from the point of view of error correction. In this case, inaccurate prism alignment in the total station will have a minimal effect on measurements of angles and distances.
Prism sets that follow these design principles are known as nodal prism sets. The Prism constants are set during Calibration in our Survey Service Centre when the instruments come for service and calibration.
In surveying and construction, a total station is a highly precise instrument that combines the functions of a theodolite (angle measurement) and an electronic distance meter (EDM). One of the key components of a total station is the bubble, which is used to ensure that the instrument is leveled properly.
A spirit level, bubble level, or simply a level is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). Two basic designs exist: tubular (or linear) and bull’s eye (or circular).
The sensitivity of a level or vial is defined as the change of angle or gradient required to move the bubble by a set distance (usually 2mm). If the vial has graduated divisions, then the sensitivity refers to the angle or gradient change required to move the bubble by one of these divisions (often spaced at 2mm).
In this era of survey, technology has advanced to a stage where electronics have overtaken the manual procedures of survey to a large extent. Hence, the major manufacturers of survey equipment developed an Electronic compensator that digitally gives the Tilt readings. This compensates for Collimation and Horizontal.