The property of the corner cube is such that the beam from the prism to the EDM is returned along the same exact path parallel to the incoming beam. There is some error in this return path called angular beam deviation. So, the distance that the user wants is the distance from the center of the instrument (vertical axis) to the vertical axis (plumb line) of the prism. However, the path of the beam includes the distance the beam must travel through the prism (distance a+b+c in Diagram A) and must be corrected for this “extra” distance and the effect on the speed of light when the beam travels through the glass instead of air.
Search Knowledge Base Articles
Setting the Prism Constant Correctly
Setting the correct prism constant is a simple operation but sometimes leads to some confusion and the cause of systematic errors in survey measurements. The measurements from an EDM using infra-red beams to a corner cube (commonly called a prism) uses a beam emitted from the EDM to the prism and then the beam is returned from the prism to the EDM.
Did you find this article useful?
Related Articles
-
What is a Prism Constant?
A prism constant is a given property of the particular prism model that defines the relation of the distance measurement to the mechanical reference...
-
What to consider when buying surveying and construction Tripods
When purchasing a Construction or Surveying Grade Tripod there are several different options to choose from, the material, the head, the clamps, and...