In levelling (surveying), a level instrument establishes a horizontal line of sight so that staff readings taken at points of known and unknown elevation can be related through the equation Ra + ha = Rb + hb; when the instrument's line of sight is not parallel to the axis of its bubble tube, a systematic angular error is introduced that invalidates this equation. This non-parallelism error, along with the related error from earth curvature and atmospheric refraction, is eliminated either through an observational strategy — balancing back sight and foresight distances so equal, offsetting errors cancel — or through permanent adjustment, a physical recalibration of the instrument itself verified by the two-peg test. The sensitivity of a level's bubble tube, defined as the angle of tilt required to displace the bubble a fixed amount, governs how precisely the instrument can be leveled and is determined by the tube's radius, internal friction, and the viscosity of its fluid.
In levelling (surveying), a level instrument establishes a horizontal line of sight so that staff readings taken at points of known and unknown elevation can be related through the equation Ra + ha =…