Incorrect Bisection of Signals: If the signal arriving at the observed station is not clearly visible, due to vegetation cover or intervening ground, the observer may incorrectly bisect the signal. This introduces an error whose magnitude varies inversely with the length of the gaze. It can be eliminated by looking clearly and always at the bottom of the signal.
Non-verticality of signals: If the signal is not truly vertical, an error is raised. This error is inversely proportional to the length of sight. This error can be eliminated by making the signal truly vertical and bisecting its bottom.
Error due to parallax: This error is introduced if the objective and eyepiece are not properly focused before the station mark is bisected. The error can be corrected by properly focusing the eyepiece and objective before bisecting the station mark.
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