What are the sources of Errors in levelling ? What precautions are necessary to avoid them?
Errors in levelling :
The errors which are occurred at the time of levelling in survey.
These errors may be further divided as instrumental Errors, natural Errors, Error due to earth curvature and personal Errors in Levelling field. Error can be reduced by proper adjustment and establishing standard field procedures and adhering to these but they cannot be eliminated.
The error which mainly due to the instrument we used, like auto level, dumpy level or total station. so we should check the calibrate of it before use. Take a two peg test if required.
The main adjustment of the level makes the line of sight parallel to the axis of the bubble tube, so that it horizontal plane is traced out as the telescope is revolved the error introduced can be eliminated by equalizing the back sight and foresight distances, which is often impractical due to the field conditions. The error may be avoided by checking the bubble after bisecting the target. An experienced surveyor looks at the bubble both before and after reading the staff.
The levelling staff is the main focus point for reading instrument so it should be place very accurately and hold it vertically on a point. To obtain an accurate reading it should be waved front and back and vice versa few time so the surveyor should take the minimum reading from it.
The graduate scale of staff should checked regularly with the help of standard tape because regular use make it variable.
The error which happen due to nature like change in climate (temperature variation, weather, raining etc), longer side distance, due to .
In loose, muddy or swampy ground the instrument may settle into the ground in the interval of time elapsed between a backs light and a foresight observation. To lower this errors, readings must be taken quickly and without walking around the instrument.
Error due to Earth Curvature
The earth surface is not linear, its line of observation is varied place to place so it is necessary to level in a limited length of 275 m. Sights longer than 275 m should be avoided However, for precise work, sights longer than 98 m are not permitted.
The error which are happened with the mistake of any person who are taking reading or manipulating observation are called personal errors. These error mainly happen due to Mistakes in setting up the level, focusing the eyepiece and objective, setting the collimation, and in holding the staff.
Surveyors should be careful in making and observing the adjustments. most common error in the staff reading, fails to extend the staff properly and takes wrong reading.
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