How to Survey Levels Across a Wide River Channel
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When surveying for construction on a wide river or wide channels in a braided course or estuary, it becomes impossible to read clearly the numbers on a levelling staff. I devised this method to record accurately the levels on works on the River Jamuna (Bhramaputra) in Bangladesh in 1983. We needed two levelling instruments with their tripods, two levelling staffs, two pairs of survey engineers, two radios, and at least one boat. We also needed something special: two target accessories that each looked like a large slice of Battenberg cake!
A square metal plate about 8 inches (200 millimetres) wide was painted in four quarters with the top two black and white, the bottom two white and black. The pattern needed to be accurately balanced visually. The plate was firmly fixed to a small mount that could be fitted onto any section of the levelling staff, slid into place up or down, and clamped precisley at a chosen position using a rear strap with wing-nuts.
Each team set up its instrument on a caisson or other solid station in the river a distance of 4,000 feet (1.22 kilometres) apart. One team viewed the other team's levelling staff and radioed them to raise or lower their movable plate until the cross-hairs in the levelling instrument sight registered with the centre of the painted plate at the join of the black and white squares on the target. The viewer's eye could not read the measurements on the staff itself, but the eye was very sensitive to any inequality in the painted areas above or below the horizontal cross-hair line. The target team clamped the target in the agreed position and recorded the value on the levelling staff corresponding to centre of the plate. With the level reading in that direction complete, the roles were reversed, and the teams obtained the level reading in the opposite direction. Since the two readings were taken under the same environmental conditions, they were compatible, and the same adjustment factors could be applied to both. The result of surveying a series of 10 such wide gaps between structures, making a total line exceeding 40,000 feet (12.2 kilometres), was that the survey tied in accurately at end stations, and each structure was levelled and relevelled satisfactorily within the specified surveying requirements of the contract.
Date of last edit: 03 April 2015