Errors from Instrument used

Knowing ones instrument is the first step to identifying possible errors and minimizing it. The consultants used the following combinations of instruments - first chalk to mark datum, nylon string to hold between 2 points on the plane and plumb bob, replaced by a spirit level to check horizontality of the marked datum line. Each set had its own limitation and implication on the errors in marking the datum line and thereon in drafting the profile of the section plane. 

Set one - A Plumb bob, String and Chalk - The plumb bob is probably one of the oldest tools at hand to check horizontal levels. Strings were held tort between two ends between which the level were to be checked with a plumb bob. Unfortunately, it was found that, due the wind, the plumb swung incessantly, leading to errors in reading. The use of this set of tools were abandoned in this case. 

However, if the distance between the points (where levels had to be checked) was lesser than 1 meter and the force of the wind was lower, perhaps the use of plumb to measure horizontality would have been effective. In case, it was in-effective. When verified using a spirit-level, considerable difference in level measuring up to 25 mm maximum was noted.

Set two - Spirit-Level, String and Chalk - A spirit level, at that point of time, was the most convenient replacement to the plumb-bob. Prior to procurement, the accuracy of the tool needs to be checked i.e., if the reference planes (top and side bases) are horizontal or not. In this case, while procurement, the tool was not verified for correctness and after commencement of work, the inclination of its reference planes were realized. 

Moreover, the spirit level gives correct reading within a certain range i.e. when the bubble remains within the 2 lines (Refer to Figure - Range within which reading of spirit level is considered correct). Since the reading remains affected by the judgement of the user, the  correctness of the horizontal level varies from person to person. 

Lesson Learnt

• It is a must to calibrate instrument and identity its errors before commencing work. Incase a large area should be covered, a smaller area must be measured as a sample to identify the extent and impact of errors from the instrument.                                                            As calibration is critical to accuracy, we plan to first locate the correction position of  the  bubble of spirit-level in use at this point to avert any subjectivity in reading. As a result of erroneous marking, there is a discrepancy of 80mm over an area of 25m long by 18m wide (Ground Floor).                                                                                                                                       

• Using a plumb bob is more difficult than it apparently seems, especially while using it for the first time. Hence, we feel that one must practice and verify the horizontality of the level acquired while marking with a plumb bob. Also, one needs to buy a plumb bob where the bob is heavy enough to negate wind action - a problem we faced. It is certain that plumb-bob will swing in windy areas. 
• A soft chalk is an inappropriate tool to mark as it leaves a thick mark as well as it fades.  It was initially chosen as the marks made would be temporary. However, the thickness of the chalk line as well as the mark it left on the building on erasing (incase the same had to be rectified) created confusion. Also, it was consciously decided that string or the measuring tape were to be held from the top portion of the marking (Refer to Figure - Point to Mark and Measure from).