Tips on centering errors for dual-axis compensators and sun effects on tubular vials.
Q: I have a total station with a dual-axis compensator. As I understand it, it corrects for errors in the vertical and horizontal angles due to tilt of the vertical axis. But what about centering errors? Does the total station account for the fact that the vertical line through the instrument is not truly vertical, and thus not through the point on which the optical plummet is aimed?
A: You are absolutely correct to be concerned. As long as the magnitude and impact of the error is unknown, you will not be confident that the work you do is of sufficient quality. First, you should evaluate the limits of the compensator's range. This will give you some idea of the maximum tilt you might possibly work under. Second, determine the maximum height of your instrument setup. The distance is determined from the survey point to the center of the horizontal circle of your instrument. Third, perform a simple calculation to determine the maximum centering error (see Figure 1). If the maximum tilt range of the compensator is 10 arc minutes and the maximum height is 4.5 feet, then the maximum error will be: