Each staff is graduated in centimetres and used as a measuring staff. Suppose the difference in elevation between two points A and B has to be measured; A and B are less than 10 m apart.
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When the top of the ranging mark on the pole coincides with the horizontal hairline, the ranging pole is set on a point (B) which is exactly at the same elevation as the starting point (A) (see Fig. Vision will become poor beyond this distance and accuracy cannot be maintained. The value indicated by the horizontal hairline is read and written down by the observer (see Fig. The observer turns around, the bush pole remaining in the same spot, and sights the staff at point B. Continue until the difference in elevation between the last intermediate point and B has been determined.
After moving the instrument to the horizontal position, the value indicated by the horizontal hairline on the staff is read and written down (see Fig.
thin piece of board) under one end of the level (see Fig. This means that the spot thus found by the second leg of the frame is at the same elevation as the starting point. A new peg (peg B) is driven in close to the second leg to mark the place (see Fig. All the pegs, thus driven in the ground, form a contour line (see Fig. The next step is to determine the second contour line.
A choice has to be made on how many centimetres lower (or higher) the next contour line should be.
Surveying or survey levelling is practised to determine the differences in elevation (= vertical distances) between various points in the field, to measure distances (horizontal distances), to set out contour lines etc.
Major surveying works are done by engineers or qualified surveyors using sophisticated equipment such as the levelling instrument (see Fig. This Section will only deal with elementary equipment.In this example, a height difference of 20 cm was chosen. peg (A1) represents the starting point of the second contour line. 38 Setting out the second contour line In addition to the determination of contour lines the N-frame level can be used to set out lines with a uniform slope, which is useful, e.g. Suppose that the slope of a ditch to be set out on the field is 1% (one percent).This means that the ground level near peg A should be 20 cm higher than the ground level near peg A (see Fig. Now follow the procedure described above to determine the second contour line (see Fig. In order to use the N-frame level to set out slopes, it requires a modification; one leg has to be shortened. 39 Modified N-frame level A slope of 1.5% would require one Leg to be 3 cm (1.5% of 2 m) shorter; a slope of 2% would require a 4 cm (2% of 2 m) shorter leg.To be able to set out horizontal lines or lines with a constant slope, the elevation (or height) of two points on the line (preferably the starting and end points) must be known.Suppose a horizontal line has to be set out between the Bench Marks A and B. The procedure is: Place boning rods on top of the two Bench Marks and on top of peg C. 35f Setting out a horizontal line, Step 6 The use of boning rods when setting out a slope is the same as described in 184.108.40.206 only, in this case, the Bench Marks A and B do not have the same elevation. When the difference in elevation and the horizontal distance between A and B are known, the slope can be calculated (see Volume I, Chapter 3 and Volume 2 Chapter 3 and sections 6.3 and 6.4).The difference in elevation between points A and B is calculated by the formula: In this case, the reading on staff B is higher than the reading on staff A; the result of the subtraction is negative which means that point B is below point A.