This Content Originally Published by a land surveyor to Land Surveyors United Network
Very tricky... a 10 metre pole? You would have to check that the pole is straight and plumb, before use.
It could depend on the current as this might cause any pole to flex and give fauly bubble reading. Also depends on accuracy required. I would suspect less than 20mm would be ok...
Hi, it's depending on the required accuracy.
When it needs to be within a few cm than place steel walls, and pump all the water out so you will be working in a dry environment.
Accuracy within a few dm, use a crane with GPS steering AND a motion controller (INS/Gyro). And Marine divers under water to steer the process.
For the steering and placing/sinking of underwater structures like tunnels, accoustic beacons can be used. Expensive and a lot of planning/calculating in advance.
Checking already placed blocks can be done using a well cailbrated multibeam system (top segment equipment 1degree accuracy or better).
So it depends on budget and required accuracy.
I did a similar job for setting pump station intakes and screens in 8.5m water.
Used trimble total station sighting to targets on 8.5m pole. The base of the pole was positioned by hard hat divers underwater and a surface diver to hole the pole. We had voice communication with the underwater diver.
Had to contend with surface wind and currents, so took the mean of 6 to 9 shots at each location. I found that 9 shots is best and 4 is too few, your number depends on how cranky the divers get as too many shots will be slowing down the operation. So if your initial error radius is +/-0.3m you will get +/- 0.1m result. = Residual/SQRT(N)
The residuals are large and if you try to present the raw results, you will get lines that are not orthogonal (Engineers expect to see orthogonal shapes and will reject your data)
So to improve the outcome, I created the orthogonal shapes of each object in my CAD system, imported the mean observed positions and finally moved the shapes to best fit over the observed points. The final located points will be the corners of the orthogonal shapes.
While this is not exact science, it did the trick.
hello Abdul kabeer Kalladi ...you are working on quay wall project. i just finished my marina club quay wall project. and i also had same blocks same depth. 4 layers of blocks. in this for prism u have to make a frame for holding prism rod.... and to make bubble tilt water diver will help u. and u have to check only 1 corner for alignment. i placed it in a curve. there was 5000 approx blocks in a curve round and zig zag shape. so its very easy by making frame for prism pole.which will hold ur prism from bottom and placed by water diver exactly on corner. hope it will help u a lot.