Errors from Incorrect Pointing versus Prism Constant
As you
will see below errors are significant when using 0 Prism Constant as
opposed to a Prism Constant of -40. On this basis alone, for high
accuracy work, the use of a zero constant will notably affect results
which may not be correctly adjusted for when a close is made or if
repetition and averaging is not carried out.
Horizontal and vertical angle accuracy
At close range work, when the center of the prism as sighted through
the telescope serves as the reference point for angle and distance
measurement, accuracy depends on the precise alignment of the center
of the total station, the prism center, and the plumbing & pivot
point. Ideally, the plumbing & pivot point and the prism center
should coincide to reduce error resulting from not perfectly pointing
the prism at the total station.
When the spacer is removed, the -40mm prism constant of the Sokkia
range of prisms provide this ideal reflecting point.
For comparison, the plumbing & pivot point of a zero prism
constant is located far behind the prism center. With a 51(5.6gon)
error in pointing the prism, a typical margin of error in the field,
the line-of-sight error is about 3.7mm, yielding an angle error of as
much as 7.6" (2.35mgon) at 100m. The prisms provide consistency
and improved accuracy for close-range angle measurement by
eliminating the error inherent in not perfectly pointing the prism.
Ideal Reflecting Point Provided by Locating the Plumbing & Pivot Point at the Prism Center
(-40mm prism constant)
Magnitude of Error at 100m with Zero Prism Constant
