6. MEASURING HEIGHT DIFFERENCES - PART 2

6.0 Introduction:contour levelling methods

1. In the previous chapter, you learned how to make several types of simple levels. You also learned how to use them in the field for levelling.  
Contour lines
GR276.GIF (6323 byte)
     
2. These levels can also be used for finding and marking on the ground all points at the same height, such as the points along the centre-line of a future water supply canal. In this case, the height differences between the various points of the line would be made equal to zero. These points make up a contour line. This particular type of level survey is called contouring. There are some simple levels which can be used for contouring. These will be described in the following Sections 6.1-6.5. How to use other levels and slope measuring devices for contouring will be explained in Sections 6.6-6.8.  
GR276a.GIF (4650 byte)
     
3. In this chapter you will also learn how to use the levelling devices already described to set lines of slope with a gradient (see Section 6.9).  
GR276b.GIF (5737 byte)

4. There are several good ways of laying out contour lines. Each of these methods is fully explained in the next sections. Table 8 will also help you choose the method best suited to your needs. Later, in Section 8.3, you will learn how to lay out contours in the field, and in Section 9.4, you will learn how to map the results of your field survey.

GR277.GIF (5448 byte)
 
GR277a.GIF (8125 byte)

TABLE 8
Contour levelling methods

CONTOURING LEVELS

Section1
Method 2
Distance, m
Accuracy
Remarks
People, equipment
6.2*
A-frame
4
Medium
Awkward to transport
1 or 2 people, mason's level
6.3*
A-frame, plumb line
4
Medium to high
Fast to use
1 or 2 people, plumb line
6.4*
H-frame water level
2.5
Medium to high
Awkward but quick
Avoid water loss
2 people
6.5*
Semi-circular water level
100
Medium
Faster for longer distance
Avoid water loss
2 people, target levelling staff

NON-SIGHTING LEVELS (see also Table 7)

6.6*
Straight-edge level
2.5 to 3
Medium to high
Easy transport
Fast
1 person, mason's level
6.6**
Line level
20
Medium
Very easy to transport Quick to operate Useful on rough ground
3 people, mason's level 2 measuring scales
6.6*
Flexible tube water level
10 to 15
High to very high
Awkward to transport Very quick
Avoid water loss
2 people, 2 measuring scales

SIGHTING LEVELS (see also Table 7)

6.7**
Bamboo level
15 to 20
Low to medium
Greatly affected by wind
2 people, 1 levelling staff
6.7*
Hand level
10 to 15
Low
Rough, fast
2 people, 1 levelling staff
6.7***
Surveyor's level
more than 100
Very high
Expensive, delicate
2 people, 1 levelling staff

SLOPE MEASURING DEVICES (see also Table 6)

6.8**
Clinometer, clisimeter
10 to 15
Low to high
2 people, levelling staff

1 *simple **more difficult *** most difficult
2 In italics, equipment you can make yourself

5. In nearly all levelling instruments, horizontality is shown by a spirit level. This is a small level, usually made of an elongated or circular glass tube; the tube is nearly filled with a liquid (usually spirit), leaving enough space to form an air bubble. In the elongated spirit level, a point near the middle of the tube is selected as the zero-point, and clearly marked. Graduations may be added on either side of this point. In the circular spirit level, the zero- point lines up with the centre of the level, and is clearly marked by a small circle. When the air bubble is at the zero-point, the level is horizontal.  
GR279.GIF (7937 byte)

6.1 How to use the mason's level

The mason's level is a simple tool often used during building operations. You have learned how to use the mason's level to set out horizontal lines when you measure short distances on sloping ground (see Section 2.1) and when you determine height differences (see Sections 5.1-5.2).

What is a mason's level?

1. Usually a mason's level consists of a rectangular wooden casing with a small spirit level mounted in one of its narrow faces. The mason's level can also be made of metal. Glass spirit levels are highly breakable, and should be handled very carefully.

2. The casing varies in length. As the length increases, the accuracy improves. The cheapest mason's levels are relatively short, about 25 cm long. They are generally available from hardware stores.

 
Mason's level
GR280.GIF (13723 byte)

Using a mason's level to check horizontality

3. When a mason's level is horizontal, the bubble of the spirit level lies exactly at its zero-point.  
GR281.GIF (8903 byte)
     
4. When the air bubble moves away from the zero-point, it shows that the level is no longer horizontal. There is either an uphill or a downhill slope.  
GR281b.GIF (8464 byte)
     
Note: the direction in which the bubble moves always indicates the direction of the highest point on the slope.  
GR281a.GIF (8201 byte)
     

6.2 How to use the A-frame level

A simple device for contouring can be made from three pieces of wood and a mason's level. This device works on the same principle as the straight-edge level (see Section 5.1), but it is easier and faster to use.  
GR282.GIF (10229 byte)

Making your own A-frame level

1 . Get three pieces of soft wood, all at least 2 x 6 cm thick; two of the three pieces should be about 2.80 m long, and the other about 2 m long. The A-frame made of these will be about 1.70 m high by 4 m long -- small enough to handle easily.  
GR283.GIF (6008 byte)
     
2. Attach the two 2.80 long leg-pieces about 30 cm down from their tops by drilling a hole through the centre of each piece and bolting them loosely together. Adjust the legs until they are 4 m apart at the bottom.  
GR283a.GIF (5147 byte)
     
3. Measure up 1.60 m from the bottom of each leg and loosely attach the 2 m cross-piece by drilling and bolting it to the legs. The cross-piece should be about 1 m above the ground.

4. Cut the bottom of the legs level, so that they rest evenly on the ground when the A-frame is upright. To do this, stand the A-frame upright on its legs and place a long straight piece of wood so that it touches both legs at the base. Make a mark along the legs, level with the top of this piece of wood, and cut the legs at the mark.

 
GR284.GIF (5507 byte)
     
   
GR284a.GIF (5245 byte)

Adjusting the A-frame level

5. Place the A-frame upright, resting the legs on two points which are at exactly the same level. Put the mason's level (see Section 6.1) on the middle part of the cross-piece and check to see if it is on the middle part of the cross-piece and check to see if it is horizontal. If it is not, adjust it by moving the cross-piece slightly, or by cutting a little more off one of the legs. When the cross-piece is horizontal, tighten all the bolts on the A-frame.  

Check the level
GR285.GIF (5124 byte)

     
6. To check the horizontality, turn the A-frame around and, with the mason's level, check to see that the cross-piece is still horizontal.  
Then reverse and check again GR285a.GIF (5520 byte)
     
7. Using light string, lash the mason's level securely to the cross-piece at its mid-point.  
GR285b.GIF (6700 byte)

Using the A-frame for contouring  

8. With a marking pin, mark point A where you will begin contouring. Place one leg of the A-frame at this point. Move the other leg uphill or downhill until the mason's level shows a horizontal position. At this point place another marking pin B.  
Adjust the forward leg until the level is horizontal
GR286.GIF (5547 byte)
     
9. Move the A-frame up to the second point B. Find the next horizontal point C and mark it.  
Repeat at each station
GR286a.GIF (3507 byte)
     
10. Repeat this process until you have plotted the length of the contour line AE  
Contour line AE
GR286b.GIF (1999 byte)

6.3 How to use the A-frame and plumb-line level

The A-frame and plumb-line level is a simple device very similar to the A-frame, except that the mason's level is replaced by a plumb-line. The device is used in the same way as the standard A-frame for contouring (see Section 6.2).

Making your A-frame and plumb-line level

1. Construct an A-frame as described above (see Section 6.2, steps 1-4)  
A-frame
GR287.GIF (3150 byte)
     
2. Screw a small hook, or drive a nail, into the frame near its summit.  
GR287a.GIF (11836 byte)
     
3. Attach a plumb-line (see Section 4.8) to the hook or nail. The plumb-line should be long enough for the plumb to reach below the cross-piece of the frame.

Note: the taller the frame is, the more sensitive the level will be to differences in height. The dimensions given in Section 6.2 provide a good average accuracy, usually better than 32 cm over 100 m.

 
GR287b.GIF (4260 byte)

Adjusting your A-frame and plumb-line level 

4. Place the A-frame upright with its legs resting on two points which are at exactly the same level.

5. When the plumb-line comes to rest, lightly mark the position of its string on the top side of the cross-piece of the A-frame.

 
GR288.GIF (4336 byte)
     
6. Place the A-frame the other way around, so that its legs are reversed on the same horizontal points. When the plumb-line comes to a stop, lightly mark the position of the string on the cross-piece.  
GR288a.GIF (4785 byte)
     
7. Make a permanent mark on the front side of the cross-piece at the precise mid-point between the two marks. This shows where the legs of the A-frame are exactly level.

Note: to improve measurements in windy weather, slow the movement of the plumb-line letting it rub slightly against the cross-piece of the A-frame.

 
Cross-piece
GR288b.GIF (8680 byte)

6.4 How to use the H-frame water level

The H-frame water level is a simple device made of a light wooden frame and some clear plastic piping, which is partly filled with water. Like the flexible-tube water level (see Section 5.3), it is based on the principle that, under atmospheric pressure, the free surfaces of interconnected water columns will reach equal heights, which follow a horizontal line.

GR289.GIF (7289 byte)
 
GR289a.GIF (6491 byte)

Making your H-frame water level

1. Get two 5 x 5 cm thick pieces of soft wood 1 m long, and one 5 x 5 cm piece 2.5 m long. Join the three pieces of wood together to form an "H" shape, using strong nails or bolts. The horizontal piece of the frame should be about 20 cm above ground level. The two upright legs should make 90 angles with the horizontal piece. Check this.  
GR290.GIF (6189 byte)
     
2. Get 3.90 m of clear, non-reinforced plastic tubing with an inside diameter of about 1.2 cm. Using soft wire or string, secure it to the upper face of the horizontal piece and to the inside faces of the two vertical pieces. Tie or bind the plastic tube tightly to the wooden pieces, but be careful not to pinch the tube  
GR290a.GIF (4140 byte)
     
Note: if you do not have enough clear plastic tubing, use about 1.90 m of dark rubber or plastic piping or metal water piping, and two 1 m lengths of clear plastic tubing. Connect one length of clear tubing to each end of the dark piping with a hose clip. Then tie the dark piping to the horizontal piece of the H-frame, and the clear lengths of tube to the two vertical pieces.  
GR290b.GIF (6037 byte)

3. Pour water into the tubing until the level reaches about halfway up each vertical section, making sure to get rid of any air bubbles. Put a cork stopper in each tube-end to prevent water losses during transportation.

GR291.GIF (7955 byte)
 
GR291_a.GIF (11223 byte)

Adjusting your H-frame water level

4. With the help of an assistant, place the H-frame upright, with its legs resting on two points which are at exactly the same level.

5. Remove the two stoppers from the tube ends and look at the water level in each tube from the side. You and your assistant should then make a light mark on each vertical leg, level with the water level in the tubes.

 
Mark at the water level 
GR292.GIF (8245 byte)
     

6. Turn the H-frame around and place its legs, reversed, on the same points.

7. Again, lightly mark the water level on each vertical leg.

 
Then turn the frame and mark again
GR292a.GIF (8426 byte)
     

8. Make a permanent mark on each leg at the precise mid-point between the two previous marks. When the water is at this level in the tubes, it indicates horizontality.

9. Replace the stoppersfor transportation.

Note: it is best to check this adjustment before each contouring survey. If any water has been spilled from the tubing, you should adjust the device by adding water as necessary.  

 
GR292b.GIF (12557 byte)

Using your H-frame water level for contouring

10. Place the rear leg of the H-frame at the starting point A.

11. Remove the stoppers frorn the tube ends.

 
Remove the stoppers
GR293.GIF (8534 byte)
     
12. Move the forward leg uphill or downhill until the top water level reaches the permanent mark you have made on the leg.  
Adjust the forward leg
GR293a.GIF (3761 byte)
     
   
Read the water level
GR293_a.GIF (11796 byte)

13. Mark the position of the forward leg at B with a peg and replace the stoppers in the tube ends.  
GR294.GIF (6115 byte)
     

14. Move the frame forward, place the rear leg at the marked point B, and repeat the previous procedure. Continue in this way until you reach the end of the contour line AE.

Note: it is easier to work with an assistant, who can move the forward leg until he or she finds the horizontal level. Then you can check that the water level on the rear leg also lies opposite the permanent mark.

 
Contour line AE
GR294_a.GIF (1912 byte)

6.5 How to use the semi-circular water level

The semi-circular water level is a simple device based on the same principle as the H-frame water level. Its main advantage is that you can use it on longer distances without moving it. You need only several small pieces of wood and a short piece of clear plastic tubing to make it, but it is a little more difficult to build than the H-frame.

GR295.GIF (4235 byte)

Making your semi-circular water level

1. Get a 1 x 10 cm piece of wood 60 cm long and drill a hole through each end of it from the 10 x 60 cm face. These holes should be just wide enough to hold the plastic tube (see step 5).

2. Drill a small hole in the centre of the piece of wood.

3. Prepare two wooden discs with a diameter of 10 cm, and drill small holes in their centres.

 
GR296.GIF (6847 byte)
     
4. Nail or screw one of these discs under the centre of the piece of wood, aligning the hole in the centre of this piece with the hole in the disc. Do not block the hole.  
GR296a.GIF (3725 byte)
     
5. Get a piece of clear plastic tube about 80 cm long and 1 to 1.5 cm in diameter. Pass the ends of the tube from below through the holes in the ends of the piece of wood so that the tube forms a semi-circle on the side where the disc is. The two ends of the tube should extend above the piece of wood by about 10 cm. Keep the tubing in place by putting a hose clip just at the point where the tubing passes through the hole in the board. Tighten the clip so that the tube does not slip, but be careful not to pinch the tube. The hose clip will keep the tube in place, since it is bigger than the hole.  
GR296b.GIF (7093 byte)

6. Now make the supporting leg. Get a pole 5 cm in diameter and 1.40 m long. Find the centre point of one end. Then take the second disc you prepared, and loosely nail it to the pole so that its centre hole is over the centre of the pole.

7. Attach the semi-circular level you made in step 5 to the supporting leg. Use a strong screw, and align the central holes of the wooden discs carefully. Do not tighten the screw too much. You must be able to turn the semi-circular level around. The flexible tube will be off to one side of the pole.

8. Place the device upright on its support and fill the plastic tube with water. The level of the water should reach about 4 to 5 cm from each end of the tube. Place a stopper in each end of the tube to prevent water loss during transportation.

 
GR297.GIF (8721 byte)
GR297a.GIF (8020 byte)

Using your semi-circular water level for contouring

9. At the starting point A of the contour you need to level, ask your assistant to place a levelling staff in a vertical position. Since you are contouring with a sighting level which does not include a telescope, you should use a target levelling staff.

You can easily make one. Get a straight wooden stick, a piece of bamboo, or a maize stalk 2 m long. Get another pole or stick 50 cm long, and attach it to the first one with string, to form a cross. The location of the point where you attach the 50 cm pole, called the target, depends on the contour you are levelling.

GR298.GIF (4850 byte)
 
GR298a.GIF (8113 byte)
     
   
GR298b.GIF (3020 byte)

10. To station the semi-circular water level, drive its support vertically into the ground at a central point from which you will be able to survey about 100 m of the contour line. Remove the stoppers from the ends of the plastic tube.  
GR299.GIF (2783 byte)
     
   
GR299a.GIF (3135 byte)
     
11. Standing about 1 m behind the semi-circular water level, rotate its upper part and sight along a line which joins the two water-surface levels in the plastic tube to the levelling staff. Signal to your assistant to adjust the target of the levelling staff up or down until it is exactly on the sighting line. Then, ask your assistant to tie the target firmly at that height.  
Adjust height of target
GR299b.GIF (7488 byte)

12. Your assistant will then mark the starting point A with a stake, and walk about 10 m away, where he will place the levelling staff in a vertical position.

13. Rotate the upper part of the water level until you can sight again at the cross on the staff. Signal to your assistant to move the levelling staff uphill or downhill until the fixed target lines up with the sighting line. He or she will then mark this point B with a stake.

 
GR300.GIF (3769 byte)
     
Swivel the level to sight at point B
GR300a.GIF (2217 byte)
 
Move the staff until the target is in the line of sight
GR300b.GIF (5457 byte)

14. You may keep on levelling points on the same contour AG from one central station X for about 100 m. To continue the same contour line, leave the target levelling staff at point G, and move the level to a new central station Y. Adjust the height of the target and go on levelling contour GZ from station Y.

Adjust target without moving staff
GR301.GIF (8648 byte)
 
Continue levelling
GR301a.GIF (6149 byte)

Note: you may also want to find contours with a fixed height difference, for example, every 0.20 m. To do this, you will keep working from the same station, but change the height of the target on the levelling staff. When you reach point G, have your assistant lower the target by 20 cm. He then walks up the hill along line XGH until the target is level with your line of sight, marking point H on the next contour. Continue the second contour line HN by finding point I on line XFI, point J on line EXJ, and so on. If the distance is short enough for you to see clearly, you may lower the target again to set a third contour line from the same station.

Lower the target
GR302.GIF (3067 byte)
 
GR302a.GIF (2942 byte)
     
Find point H
GR302b.GIF (3231 byte)
 
Continue the contour
GR302c.GIF (5173 byte)

6.6 How to contour with non-sighting levels

1 . In Sections 5.1 to 5.3, you learned how to use non-sighting levels to measure differences in height. These devices can also be used for contouring.

Using the straight-edge level for contouring

In Section 5.1, you learned about the straight-edge level. For contouring, use it in the following way (steps 2 to 7).  
Make the level horizontal
GR303.GIF (3727 byte)
     
2. Mark the point A where you will begin contouring with a stake. Place one end of the straight-edge at this point, and move the other end uphill or downhill until the mason's level shows horizontality. Mark this point B with another stake.  
Mark point B
GR303a.GIF (3323 byte)
     

3. Move the straight-edge up to point B. Find the next horizontal point C as shown above and mark it with a stake.

4. Continue this process until you have marked the length of the contour line.

 
Continue the contour line
GR303b.GIF (4106 byte)

5. Mark the route of the contour line you have found by leaving a stake about every 10 m. If the contour curves, you may need to use more stakes.  
Mark the contour with stakes
GR304.GIF (3728 byte)
     
6. If the surface of the ground is somewhat rough (i.e. covered with lumps of earth, stones or grass), it may help to use two bricks or wooden blocks of the same height to support the ends of the straight-edge while you are levelling.  
GR304a.GIF (2968 byte)
     
7. If the surface of the ground is very rough or covered with dense grass, you can use two stakes under the ends of the straight-edge to lift it above ground level. Be sure that both stakes are the same length, and that you drive them into the ground to the same level. This way you can transfer the horizontal you find, which lines up with the top level of the stakes, to ground level without error.  
Use bricks...
   GR304b.GIF (4668 byte)
     
   
...or stakes to lift the level above obstacles
GR304c.GIF (6962 byte)

Using the line level for contouring

In Section 5.2, you learned how to make a line level. The line level is very efficient for contouring because it allows you to progress quickly, even on rough grass, and it is reasonably accurate (the maximum error is less than 6 cm per 20 m distance). Remember that you need three people to use the line level.  
GR305.GIF (2170 byte)
     

8. The rear person places one end-staff on the marked starting point A and keeps the cord on the 1 m graduation, for example. The front person, also keeping the cord on the same graduation, moves the second end- staff up or down the slope until the centre person signals that the mason's level is horizontal. The front person then marks the point B where the staff touches the ground.

9. The rear person walks to this marked point B while the other two people walk ahead until the cords are well stretched. The entire procedure is repeated and another point C of the contour line is marked.

 
GR305a.GIF (11430 byte)
     
10. This process is continued until you have marked the length of the contour line.  
GR305b.GIF (2648 byte)

Using the flexible-tube water level for contouring  

In Section 5.3, you learned how to make the flexible-tube water level. You can contour quite quickly with this device even on rough ground, and it will give very good accuracy (the maximum error is about 1 cm per 10 m distance). You should be very careful not to lose water during the procedure. You will need an assistant for this method.  
GR306.GIF (4929 byte)
     
11. Bring the two stand pipes together at the starting point A of the contour line, remove the stoppers, and mark the height of the water levels on each measuring scale. These heights should be the same.  
Put both stand pipes at point A and remove the stoppers
GR306a.GIF (5014 byte)
     
12. Replace the stoppers in the tube ends.  
Mark the water level on both scales
GR306b.GIF (16311 byte)
     
13. Place your measuring scale at the starting point A of the contour line. Have your assistant walk ahead until the end of the hose is reached. Both of you remove the stoppers, and your assistant moves the scale up or down the slope until the water level is at the marked height. Check that the water level is at the marked height at your end, too. When it is, signal to your assistant to mark the location B of that scale with a stake. Replace both the stoppers.  
When the water level is at the mark on the standpipe,
you have found point B

GR307.GIF (5212 byte)
     
14. Both of you then move forward until you are standing at the point B where your assistant was standing, as marked with the stake. Have your assistant walk ahead until the end of the hose is reached. Repeat the procedure in step 13, and continue in the same way to the end of the contour line.  

Begin the next measurement at point B

6.7 How to contour with sighting levels

1. In Sections 5.6 to 5.9, you learned how to use sighting levels to measure differences in heights. These devices can also be used for contouring.

Using the bamboo sighting level for contouring

2. You learned how to make and adjust a bamboo sighting level in Section 5.6. You and an assistant can use this level for contouring, as follows.

GR308.GIF (6442 byte)

3. Place the bamboo sighting level next to a levelling staff and read the height on the scale by sighting through the tube.

4. Mark this height on the scale. You can use paint, or tie a piece of cord or a coloured rag at that height. You can also use the target levelling staff described in Section 6.5, and attach the target at that height.

Mark your sighting level on a pole
GR308a.GIF (6032 byte)
 
GR308b.GIF (3833 byte)
     
5. Place the bamboo level in a vertical position at A, the beginning of the contour line you want to plot.  
Send your assistant ahead
GR309.GIF (3745 byte)
     
6. Have your assistant, holding the levelling staff, walk 15 to 20 m ahead along an approximate horizontal line and place the staff vertically. Have him move it up or down the slope until you signal that the mark is lined up with the sighting line seen through the bamboo tube.  
Have him move until you sight the mark
GR309a.GIF (2581 byte)
     

7. You will have to turn the bamboo level from left to right to see the mark on the measuring scale. Check frequently to make sure that the bamboo tube remains horizontal.

8. When you signal that you have sighted the mark, your assistant should mark the position B of the levelling staff with a stake.

 
You may have to turn the level to see him
GR309b.GIF (2604 byte)
     

9. Now move up to this stake B and place the bamboo level on that point in a vertical position.

10. Have your assistant walk another 15 to 20 m ahead with the levelling staff, and repeat steps 7-9.

11. Repeat this process until you have marked the entire length of the contour line.

 
Proceed from point B
GR309c.GIF (2573 byte)

Using the hand level for contouring

12. You can survey a contour line quickly by using a hand level (see Section 5.7), although this will not give the most accurate results. The method you use with the hand level is the same as that just described for the bamboo level, except that you should make the mark on the levelling staff at the height of the sighting line. The sighting line's height will either be at your eye-level, or at the height of the pole supporting the hand level (which is used to improve accuracy). The distance from one point to the next should not exceed 15 m.  
Use the hand level alone...
GR310.GIF (3784 byte)
     
GR310b.GIF (3930 byte)
 
...or with a support staff
GR310a.GIF (3096 byte)

Using the surveyor's level or the theodolite for contouring

13. You can very quickly and accurately determine contour lines with a surveyor's level or a theodolite and a precisely graduated levelling staff (see Section 5.8).  
Use the theodolite with a graduated levelling staff
GR311.GIF (2920 byte)
     
14. Since the range of the telescope on either of these devices is several hundred metres, you can reduce the number of stations. As you did with the semi-circular water level (see Section 6.5), you can survey several points from a single station. In open country, it is possible to use this method over long distances. In areas with forests, you might need to measure over shorter distances and to clear sighting lines.  
You can survey several points from a single station
GR311_a.GIF (2147 byte)

6.8 How to contour with slope-measuring devices

1 . In Sections 4.1 to 4.6, you learned how to use various types of clinometers to measure slopes. These devices can also be used for contouring because a contour line is defined as a line along which the slope gradient equals zero, see Section 8.3.

2. When you contour with slope measuring devices, it is best to use a target levelling staff, such as the cross- shaped one described in Section 6.5. If you use such a staff, the target should be tightly fixed at eye level.

Sight at a target levelling staff...
GR312.GIF (2718 byte)
 
... or at your assistant
GR312a.GIF (2586 byte)

Note: if you do not have a levelling staff, you can use the height of your assistant as a reference level* instead.

 

GR312b.GIF (5875 byte)


3. Your assistant, holding the levelling staff vertically, should stand about 10 to 15 m away from the starting point A of the contour line you want to plot. From this starting point, use the clinometer to sight at the levelling staff. Signal to your assistant to move the staff up or down the slope, until its target lines up with the zero-graduation of your clinometer. Have your assistant mark this ground point B and repeat the same procedure from it  

Have him move until you sight the mark
GR313.GIF (2791 byte)

     
Note: if you are using a clisimeter, remember that you should use the left scale and make the sight of the levelling staff line up with its zero line (see Section 4.5).  
Proceed from point B
GR313b.GIF (3364 byte)

6.9 How to set graded lines of slope

1. Graded lines of slope are often used in fish-farms to assist gravity* in moving water. Water-supply canals and pipelines, as well as drainage canals, are built with a graded slope. Fish ponds should be built with an adequate bottom slope so that you will be able to drain them completely. Knowing how to set graded lines of slope is therefore very important when you are building a fish-farm.

2. You can set graded lines of slope in several ways, using three series of methods with the devices described in Chapters 4, 5 and 6.

 
GR314.GIF (5646 byte)

Using slope-measuring devices for setting lines of slope  

3. You can use any one of the slope measuring devices described in Sections 4.1 to 4.6 to set graded lines of slope. The clisimeter, in particular (see Section 4.5) , is commonly used for this purpose, but any other clinometer can be used instead.

4. It is best to use a target levelling staff such as the one described in Section 6.5; its target should be tightly attached at your eye level. Remember that you can use the height of your assistant as a reference level instead.

 
Use the target levelling staff
GR315.GIF (3815 byte)
     
5. From the starting point A of the line of slope, sight the target levelling staff; the graduation of the clinometer should correspond to the slope you have chosen. Signal to your assistant to move the levelling staff up or down the slope, until the sighting line of the clinometer lines up with the reference mark on the levelling staff. Mark the ground point B with a stake and repeat the procedure from that point.  
Sight with the clinometer at the desired graduation
GR315a.GIF (5630 byte)

Using sighting levels for setting lines of slope

6. You learned about various sighting levels in Sections 5.4 to 5.9 and in Section 6.5. These devices may be used to set lines of slope but, with the exception of the surveyor's level and the theodolite, their limited accuracy makes it difficult to lay slopes with gradients less than 1 percent. For smaller gradients, it is best to use non- sighting levels (see from step 12, below).  
Measure the horizontal distance...  GR316.GIF (2994 byte)
     

7. Before using the sighting level, calculate the difference in height (H metres) between two consecutive points according to their horizontal distance (D metres) in order to set the desired slope gradient (S percent) as:

H = (S 100) x D

 
... and the slope...
GR316a.GIF (2489 byte)
     
Example
  • You decide to read levels at 10 m intervals, horizontal distance;
  • The slope you need to set equals 1 % or 1 m per 100 m;
  • The necessary height difference H over a 10 m horizontal distance equals: (1 100) x 10 m = 0.10 m.
 
... to calculate the height difference
GR316b.GIF (2825 byte)

8. On the highest point A of the slope to beset, station your levelling device and measure the height of its sighting line (H') above the ground. Add this value to H (calculated in step 7) to obtain the height to be read (R) at the next point on the levelling staff as:

R = H + H'

 
Set your target levelling staff to show
the proper height difference

GR317.GIF (6478 byte)
     

9. Measure a horizontal distance of 10 m from the starting point, following the contour line as closely as possible. Place a levelling staff vertically at that point.

Note: for this part of the procedure, you can use:

 
Place the levelling staff 10 m from the level
GR317a.GIF (4293 byte)
     
10. Sight with the levelling device at the levelling staff. Signal to your assistant to move the staff up or down the slope until the sighting line lines up with the mark R on the staff. At this point B, have your assistant drive a marking stake into the ground. Point B will be 10 cm lower than point A.  
Move the levelling staff until the target
comes into the line of sight

GR318.GIF (4920 byte)
     
11. Station the levelling device on this marked point B while your assistant walks ahead another 10 m with the levelling staff. Repeat the procedure.  
Proceed from point B
GR318a.GIF (3552 byte)

Using non-sighting levels for setting lines of slope

12. Non-sighting levels are much more accurate than simple sighting levels for setting lines of slope with gradients smaller than 1 percent. Generally, non-sighting levels can be used to set lines of slope with gradients as small as 0.3 percent. The flexible-tube water level is even reliable for slopes as small as 0.1 percent.

13. In Sections 6.2 to 6.4 and Section 6.6, you learned how to use various non-sighting levels for contouring, that is, setting lines of slope with a zero gradient.

14. To set lines of slope with a different gradient (S%), you can use the same procedure described for contouring; the only difference is that you have to keep the forward end of the levelling device above the ground at the height H (calculated as shown above in step 7) for a fixed horizontal distance D metres, as in:

H = (S 100) x D


Distance D varies according to the kind of levelling device you use.
GR319.GIF (12130 byte)


Note: it is best to level going downhill, as suggested above. If you must level going uphill, you should make the rear end of the levelling device higher by H metres.  
GR320.GIF (4113 byte)
     
15. The best way to do this is to prepare a piece of wood with a thickness equal to H. While levelling, place (or, better, nail) this piece of wood under the forward end of the level if you are levelling downhill.  
GR320a.GIF (4109 byte)
     

Example

If S = 0.5 percent and you are levelling downhill:

  • using an A-frame level for setting the line of slope: D = 4m; H = 2 cm
  • using an H-frame level: D = 2.5 m; H = 1.25 cm
  • using a straight-edge level: D = 3 m; H = 1.5 cm
  • using a flexible-tube water level: D = 10 m; H = 5 cm
 
GR320b.GIF (3164 byte)
     
   
The size of the wooden block depends
on the slope and type of level

GR320c.GIF (3309 byte)

16. When using the line level, you can add the height H to the cord height which will be maintained by the front person, instead of placing a piece of wood under the forward measuring staff.  
GR321.GIF (2534 byte)
     

Example

If S = 0.5 percent and you are levelling downhill with a line level:
D = 20 m and H= 10 cm.
Keep the forward end
of the cord at a height 10 cm higher than the rear end of the cord.

 
GR321a.GIF (5037 byte)
     
   
GR321b.GIF (3107 byte)