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Our tutor gave us an OSBM and we had to establish a TBM on site by carrying out a series of ‘flying levels’. The equipment I used was a dumpy level, a tripod stand and a staff. We started by setting our equipment up, we had to make sure the stand was at an appropriate height that suited you and that the dumpy level was made level by adjusting it so the bubble was in the middle. Someone then stood at the point with the staff which we were asked to measure, in our case it was the drains. We made note of the measurement then moved the person with the staff moved to the next point. We then had to adjust the dumpy level to make it level again before we took another measurement. We kept the dumpy level in the same place until the staff got so far away that when we looked through the dumpy level it looked over the staff. This is when we moved the dumpy level; this would be our first change point. It continued until we reached the TBM.
My Collimation method looks like;
When our group carried out a linear survey surrounding a plot of land we first set up our ranging poles, these are cylinder shaped timber poles which are usually 2.5mm in length. We then used a fibreglass tape to measure the distance of each object from each pole. The accuracy of some measurements may be affected by a range of different reasons, for example;
- Slope – If, on a 30m tape, the difference in height at each end is less than 600mm then accuracy can be achieved.
- Sag – If the distance being measured was in an arc then the tape should sag more than 300mm in the centre of a 30m tape.
- Temperature – If it is warmer than 20°c then this may cause the tape to expand or contract if it is cooler. However, if the temperature does not vary from 18°c then accuracy can be achieved.
- Tension – To prevent the tape from sagging it may be pulled, this results in the tape stretching. These can often be avoided.
Errors can also occur whilst booking and measuring. Examples of some errors could be, miscounting, misreading and recording the wrong measurement. If these mistakes do not get noticed early enough then the survey will have to be redone. The best way to avoid these mistakes is to double check that what you have written is the same as the measurement you have just read. However, not all errors are down to the person conducting the survey. Some errors may occur due to poor equipment. For example, using a tape that has expanded due to exposure to a high temperature or a tape that has been permanently stretched whilst trying to prevent it from sagging. You can prevent this by comparing the tape to a standard steel tape. Some errors may just be small. The surveyor could be short sighted therefore his readings are affected. The only way to overcome this error is for the surveyor to repeatedly check their readings.
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