Balancing distances to backsights and foresights helps to eliminate which types of errors?

Balancing distances to backsights and foresights is a key practice in surveying that primarily addresses collimation errors. Collimation error occurs when a surveying instrument's line of sight is not perfectly horizontal, which can lead to systematic errors in vertical measurements. By balancing the distances—using equal or nearly equal lengths for backsights and foresights—a surveyor can cancel out the impact of the instrument's alignment errors and thereby improve the accuracy of the measurements.

While rod plumb errors can be mitigated to some extent by maintaining awareness of instrument leveling, they are not directly addressed by merely balancing distances. Curvature of the earth errors, which pertain to the physical curvature of the Earth's surface over long distances, are also not directly mitigated by this balancing act. Instead, these errors require corrections based on calculations that account for the Earth's curvature. Thus, although rod plumb errors and curvature of the earth errors are important considerations in surveying, balancing distances is particularly effective for controlling collimation error.