What type of error can be minimized by darkening cross hairs in surveying instruments?

Darkening the crosshairs in surveying instruments is particularly effective in minimizing parallax error, which arises from a misalignment between the observer's eye and the point being measured through the instrument. Parallax occurs when the line of sight is not directly in line with the crosshairs, leading to an apparent displacement of the point. By enhancing the visibility of the crosshairs, the surveyor can more accurately align their perspective directly with the measurement point, thus reducing the likelihood of this type of error.

In contrast, systematic errors are inherent issues in the measuring process that do not change with repeated measurements and are typically associated with the equipment or methodology. Instrument calibration error occurs when the instrument has not been properly calibrated, which can affect all measurements taken with it. Random errors arise from unpredictable fluctuations in measurement, often due to environmental factors or human mistakes, and while improving visibility may help reduce some of these errors, it is particularly effective for minimizing parallax specifically.