Home » GIAA Mailbag: Steady Poles and Li-ion Batteries
Tool tips on steady poles and Lithium-ion batteries.
A. Some prism and antenna poles can appear to be easier to hold than others. Unless there is something attached to the pole that makes it harder to hold steady, or the pole lacks rigidity, the actual difference is the sensitivity of the circular vial. All level vials, whether tubular (as in the plate level of a total station or theodolite) or circular (as in prism and antenna poles, and tribrachs) work on the principle of aligning their horizontal axis to be perpendicular to gravity, the force used by surveyors to define the vertical. Level vials have a curved surface of a known radius: circular for a tube vial, spherical for a circular vial. As the radius increases, the sharpness of the curvature decreases, increasing the sensitivity of the vial. Sensitivity is indicated by the angular change in the horizontal axis of the vial per two millimeters of bubble movement. In tubular vials typically used for plate vials of total stations, it is common to see sensitivities of 20 arc-seconds to 40 arc-seconds per 2 mm. Circular vials on tribrachs, to facilitate rapid leveling, generally have sensitivities in the range of 10 arc-minutes per 2 mm. Because they are intended to be handheld during measurements, the vials of prism and antenna poles have a much lower sensitivity; they range from approximately 20 arc-minutes to 60 arc-minutes (one degree) per 2 mm. As the sensitivity of the vial decreases, the pole will appear to be easier to hold steady, as a greater angular change is required for small bubble movements. For example, if the bubble is rated at 20'/2 mm, and the prism is located 6' above the point, when the bubble has moved 2 mm the prism will have moved a little more than 0.03' away from the vertical. But if the sensitivity of the bubble is 60'/2 mm, the antenna phase center will have to move 0.1' away from the vertical before the bubble moves 2 mm. Thus, if the user holds both poles with equal stability, tiny movements of the bubble position observed on the 60' pole will appear to be much larger with the 20' pole, giving the appearance that the 60' pole is steadier.