"Backsight and foresight distances should be approximately equal to avoid any errors due to collimation,
refraction or earth curvature.
Ã¢â‚¬Â¢ Distances must not be so great as to not be able to read the graduations accurately.
Ã¢â‚¬Â¢ The points to be observed must be below the level of the instrument, but not lower than the height of the staff. "
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