(a) Explain the term error of closure (b) Highlight any four sources of error of closure in closed traverse survey (c) Outline two meaures that could be tak...

Answer Details

(a) Meaning of error of closure

In a closed traverse the survey lines are run round an area and should end exactly at the starting point. The error of closure is the small gap or difference between the true starting point and the point at which the survey actually closes when it is plotted, that is, the amount by which the survey fails to close back onto its origin. It shows the total accumulated error of the survey.

(b) Four sources of error of closure in a closed traverse survey

• Instrumental errors: a faulty or wrongly graduated chain/tape, a badly adjusted compass or theodolite, or a bent ranging pole.
• Errors in measuring angles/bearings: wrong reading of the compass or theodolite, local magnetic attraction affecting the needle, or failing to take the back bearing.
• Errors in measuring distances: inaccurate chaining, sag or stretch in the tape, sloping ground not corrected to the horizontal, or miscounting the chains/arrows.
• Personal (human) and plotting errors: careless booking, mistakes in recording figures, and errors made when plotting the survey on paper (wrong scale, poor drawing).

(c) Two measures to avoid an error of closure

• Check and adjust the instruments before use and take readings carefully, repeating and averaging measurements of both distances and angles.
• Apply proper field checks such as measuring back bearings, using check lines/tie lines, and keeping away from sources of magnetic disturbance; any small residual error is then distributed by adjustment (for example the Bowditch rule) when plotting.