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.
Demystifying Surveying Equipment: Prisms, Bubbles, and Accurate Measurements
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.
How Prism Constants Affect Measurements
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.
- If the vertical axis aligns with the turning point, the prism constant is 0.
- In most commercially available prisms, the vertical axis is behind the turning point, resulting in a slightly longer measured distance. This requires a negative correction (negative prism constant).
- Ideally, the vertical axis should run through the center of the prism (nodal point). This minimizes the effect of inaccurate prism alignment on measurements. Prism sets designed with this principle are called nodal prism sets.
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 Constant Calibration
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.
The Role of the Bubble Level
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).
Level Sensitivity
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).
Transition to Electronic Compensation
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.