With this adjustment, the axis of the plate levels is perpendicular to the vertical axis of the theodolite. This is important because the vertical axis must remain truly vertical in order to accurately measure vertical and horizontal angles. To test this, the theodolite must be set on a firm ground. Clamp the lower plate and turn the upper plate until the surface of the plate is parallel to both pairs of foot screws. Bring the bubble to the center of its run by turning the foot screw. Now rotate the device 180° around the vertical axis. If the bubble remains centered, the vertical axis of the theodolite is perpendicular to the axis of the plate level.
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