Task- We did a levelling circuit around Union Street, Harbour Terrace, Forth Street and St David Street. The aim was to level around from point A through to point D with one inverse shot to find the height of A Block. We levelled the route completely once but swapped around and adjusting and moving the staff and tri pod each time. We all had turns at booking, shooting and holding the staff. We adjusted the lenses of the Dumpy Level to suit everyone’s eye so we would be able to eliminate parallax error.
We tried to keep our back site and fore site shots at equal distances although we were not able to on St David Street. By keeping our Back Site and Fore Sites at equal distances, we cancelled out any collimation error. We carried out a two peg test before we began to make sure our instrument was accurate but then continued to eliminate any chance of error by keeping the same distances between shots.
To find the height of block A we used a method of levelling called inverted levelling. We did this by shooting to the staff but holding the staff upside down. We recorded the inverted staff reading in our booking sheets as a negative and then when calculating it we used the same method but taking into account that negative sign. We used the same staff for the whole exercise so we eliminated any errors that may have been caused by defects in the staff.
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We always used the spirit level to insure that the staff was sitting vertically and horizontally straight so we knew the reading that we were reading was accurate. We were levelling in quite cold weather so the pavement was always hard. We always set the tripod up in a stable position so that the Dumpy Level would not move or be bumped when we were shooting to the staff. We continued to check that it was levelled from the spirit bubble on the side.
The biggest problem we came across was the wind. The wind made it very difficult to keep the staff straight and levelled. We overcame this by waiting for the staff to become level for a period of time and taking our reading when the wind was minimal. Another place errors are likely to occur is at change points. We choose our change points carefully making sure that we would be able to hold the staff in the same position and same height for the Back Site and Fore Site shots. We knew the start and finish Datum so had an origin and finish point that we knew.
This way any serious misclosures that we may have faced would have been easily identified in our surveying booking sheet and instead of having to repeat the whole survey, we would easily be able to redo and fix the small error. We continued to check the spirit level of the Dumpy Level and the Staff and keep all Back Sites and Fore Sites as even as possible to keep errors at a minimum.
I ended up being out by 35mm and made adjustments to make my final reduced level add back to zero. I think the only major error that would have caused the 35mm error would have been that of the wind. If I was to do this circuit again I would choose a calm day and hopefully could eliminate all forms of error.