A.1 Use of the line level for surveying
A.2 Use of the water tube level for surveying
Introduction
Laying out a contour
Laying out a graded contour
Measuring the slope of the land
Important points to remember
1. The line level is a simple surveying instrument which can be used to lay out contours and gradients, and also to measure the slope of land. It is simple to operate and is easier to transport than other similar surveying tools such as the A-frame. It is especially quick and very accurate when used properly. However a line level does require three people to operate it.
2. A line level consists of two poles, between which a length of string is suspended. A spirit level is hung on the string. The level is the type used by builders, but has small hooks at either end.
3. The poles should be of even height (about 1.5 m) and the string (about 2 mm in diameter) and precisely 8 metres in length. A notch is made in each pole at exactly the same height (say 1.4 m above ground level) and the ends of the string tied around these notches.
4. The centre of the string (4 m from each end) is marked and the level itself is suspended there.
5. The poles are held apart by operators with the string extended and the spirit level positioned exactly in the middle of the string. When the bubble in the level is between the two marks this means that the poles are positioned on level points on the land - in other words on the contour. The poles must be held vertically.
6. To lay out a contour across a slope, the team begin at the edge of the field. The operator holding the pole at the field's edge (operator A) remains stationary while the operator holding the other pole (operator B) moves up and down the slope until the third operator is satisfied that the bubble is centred. Points A and B are then marked (with stones or pegs). Operator A then moves to B and operator B moves onwards and the process is repeated. This continues until the contour line reaches the far end of the field.
7. Care should be taken that small obstacles, such as minor high spots, or rills, are avoided by skipping forward a pace or two. This avoids sharp irregularities in the contour.
8. When the contour has been laid out, the curves can be smoothed by eye according to the guidelines given for stone or earth bunds.
Figure A.1 Use of the line level
9. A graded contour deviates slightly from the true contour and is normally used to align a channel, such as a diversion ditch, or to stake out a graded earth bund.
10. In order to lay out a graded contour, further notches must be made on one of the poles. These notches are made below the original notch at intervals of 2 cm.
11. The usual gradient for a structure such as a diversion ditch is 0.25%. The string of the near side operator (A) should be affixed to the second notch down his pole (2 cm below the original) and the far operator (B) retains his string at the original notch. When the bubble in the level is between the two marks, this now implies that A is 2 cm above B, which is equivalent to a 0.25% slope over the distance of 8 metres. For a slope of 0.5%, Operator A fixes his string to the third notch down his pole (4 cm below the top notch) and, when Operator B finds a position where the level reads dead centre, he is at a ground level 4 cm below that of Operator A. Over a distance of 8 metres the slope is then 0.5%.
12. The operation now proceeds as before, operator A moving forward to the spot occupied by B, and B moving onwards - slightly downslope. Once again minor irregularities should be avoided and the curve smoothed.
13. If a diversion ditch must follow a precise field boundary it can be excavated so that the bottom of the ditch is given a suitable gradient. Surveying will therefore take place during excavation.
Figure A.2. Measuring the slope with a line level
14. It is simple to use the line level to measure the slope of the land. Operator A stands exactly upslope of Operator B and adjusts the string to the notch which gives a level reading. For example if this notch is the 3rd (i.e. 4 cm below the top notch) the gradient is 0.5%, if the notch is the eleventh (i.e. 20 cm below the top notch) the gradient is 2.5%, etc.
15. Up to 21 notches should be marked on pole A and the following table shows the percentage slope indicated by each.
|
Notch on Pole A |
% slope |
|
Top |
0 (level) |
|
2nd (2 cm below top) |
0.25 |
|
3rd (4 cm below top) |
0.50 |
|
4th (6 cm below top) |
0.75 |
|
5th (8 cm below top) |
1.00 |
|
7th (12 cm below top) |
1.50 |
|
11th (20 cm below top) |
2.50 |
|
21st (40 cm below top) |
5.00 |
16. Always check the spirit level - by placing it on a horizontal surface and noting the position of the bubble which should be between the two marks.
17. Check the centre point of the string each day and its length also,
18. Remember that when laying out a gradient that operator (A) is upslope.
19. Make sure poles are held vertically.
20. Avoid placing the poles in depressions or on top of minor high spots in the field.
Introduction
Laying out a contour
Important Points to Remember
1. The water tube level is straightforward to use, and farmers can be quickly taught to layout contours. The concept itself - of matching up levels of water - is especially easy to understand. Advantages of the water tube level are that it can be operated by only two people and is more sensitive than the line level on very low slopes. It is, however, slightly less portable than the line level, and is not so simple to use for determining slopes or laying out graded contours.
2. The components of the water tube level are as follows:
- A length of transparent plastic tubing, 6-10 mm inside diameter and about 14 metres long.- Two poles or staves of about 2 metres in length.
- Four rubber straps (easily made from a bicycle inner tube) to attach the plastic tubing to the poles.
- One to two litres of water. Muddy water is preferable as it is more easily visible in the tube.
3. The plastic tubing is firmly attached to the poles or staves using the rubber straps, or other securing devices. The ends of the tube should be about 10 cm from the top of each staff and the bottom fixing point about 20 cm from the bottom of each staff.
4. The tube is then filled with muddy water until the water level is about halfway up each staff. It is essential that no air bubbles are trapped in the tube and, if necessary, they can be removed by tapping with the finger. Wherever the two staves are set, the free water surfaces in each tube will be at the same level.
Figure A.3. Setting out a contour line
5. The two staves are placed back to back at the starting point, marked with a peg (A). After any air bubbles have been removed and the water has come to rest, a mark is made on both staves, indicating the water level.
6. The lead man takes one staff and drags the tube in what seems to be the direction of the contour line. When the tube is almost stretched, the lead man moves slowly up and down the slope until his staff is at a position where the water level in the tube coincides with the mark. The staff is then at a position where the ground level's the same as at peg A. A second peg (B) is placed at this point. The back man now moves from peg A to the other side of peg B where the lead man remains stationary. It is now the back man's turn to find the correct spot which is marked by peg C. This procedure continues until the end of the field.
Plate 20 Use of water tube level in India
7. The operators then measure, or pace, the horizontal distance required between the contours and begin to lay out the second contour.
8. The contour may then be "smoothed" by eye, according to the design specifications.
9. Work should be carried out during the coolest time of the day because heat causes the plastic tube to stretch and this affects the water levels, which may have to be re-marked.
10. It is important to avoid spillage of water, or the water levels will need re-marking. Water is usually spilled during movement of the staves and this can be avoided by closing the ends of the tube with plugs during movement. It is, however, essential to remove the plugs while making measurements.
11. The poles or staves should always be held vertically.
12. Minor depressions or isolated high spots in the field should be avoided.
