# 6. HORIZONTAL LINES, SLOPES, CONTOUR LINES AND DIFFERENCES IN ELEVATION

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. 33). This Section will only deal with elementary equipment. Most equipment can be home-made and be used by the farmers themselves after little training.

The various types of equipment and their use described in the sections that follow, are:

- Boning rods: horizontal lines and slopes
- N-frame level: slopes and contour lines
- Flexible tube water level: countour lines and differences in elevation
- Hand level: contour lines and differences in elevation.

## 6.1 Boning Rods

### 6.1.1 Description

Boning rods are T-shaped and made of wood. Their height is normally 100 cm and the cross-lath is 50 cm x 10 cm. The bottom part is sometimes reinforced with metal (see Fig. 34).

Fig. 34 A boning rod

It is important that all boning rods have exactly the same height (100 cm) and while working with the boning rods, the sun should be kept in the back, as it would otherwise be difficult to see them. Usually a total of 3 or 4 boning rods is required.

### 6.1.2 Use of boning rods

Boning rods are used to set out horizontal lines or lines with a constant slope. In particular they are used for setting out canal excavation works, but also for roads and dyke construction.

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.

#### 6.1.2.1 Setting out horizontal lines

Suppose a horizontal line has to be set out between the Bench Marks A and B. Bench marks A and B have the same elevation. The procedure is:

Step 1

Set out a straight line between A and B (see Chapter 2) and place intermediate pegs at regular intervals (see Fig. 35a; pegs C and D).

Step 2

Place boning rods on top of the two Bench Marks and on top of peg C. The observer, looking just over the top of boning rod A tries to bring the tops of the boning rods A, B and C in line.

As can be seen from Fig. 35b, boning rod C and thus peg C is too high; the tops of the boning rods are not in line.

Step 3

Hammer peg C further into the soil. It may be necessary to excavate some of the soil surrounding peg C in order to be able to lower peg C sufficiently.

The top of peg C is at the correct elevation when, looking over the top of boning rod A, the tops of the boning rods A, C and B are in line (see Fig. 35c).

Step 4

Place a boning rod on peg D. When looking over the tops of the boning rods A and B it is not possible to see the top of the boning rod on peg D, as peg D is too low (see Fig. 35d).

Step 5

Replace peg D by a longer peg or pull out peg D and add some soil in the immediate surroundings of D and hammer peg D again into the soil. Repeat this process until the correct elevation of peg D is found (see Fig. 35e).

Step 6

The two Bench Marks A and B and the pegs C and D all have the same elevation. Line ACDB is horizontal (Fig. 35f).

Fig. 35f Setting out a horizontal line, Step 6

#### 6.1.2.2 Setting out slopes

The use of boning rods when setting out a slope is the same as described in 6.1.2.1 only, in this case, the Bench Marks A and B do not have the same elevation. Bench Mark A is either higher or lower than B. 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).

## 6.2 The N-Frame Level

### 6.2.1 Description

This instrument, used to set out contour lines or slopes, consists of a wooden frame (a main lath, 2 legs and 2 cross poles) as shown in Figure 36a. On the main lath, a carpenter level is firmly fixed (e.g. with metal strips).

### 6.2.2 Testing the N-frame level

Before fixing the carpenter level to the frame, the instrument must be tested to make sure that the carpenter level is in the correct position.

The frame is placed on two points which have the same elevation (for example on a horizontal table or on a floor that has been checked previously with the carpenter level). If the bubble of the level tube is not exactly in between the marks, the carpenter level must be adjusted by putting a spacer (e.g. thin piece of board) under one end of the level (see Fig. 36a and b).

Fig. 36a The N-frame level

Fig. 36b Testing the N-frame level

### 6.2.3 Use of the N-frame level

The N-frame level is used to set out contour lines and slopes on the field.

#### 6.2.3.1 Setting out contour lines

Starting from peg (A), a contour line has to be set out. The procedure to follow is:

Step 1

One leg of the instrument is placed close to peg (A). By turning the frame around this leg, a position of the frame is found such that the second leg is on the ground and the bubble of the carpenter level is in between the marks. This means that the spot thus found by the second leg of the frame is at the same elevation as the starting point. Both points belong to the same contour line. A new peg (peg B) is driven in close to the second leg to mark the place (see Fig. 37 a).

Step 2

The N-frame is moved to the newly-placed peg and the procedure is repeated until the end of the field is reached. All the pegs, thus driven in the ground, form a contour line (see Fig. 37b).

Step 3

When the first contour line has been pegged out it might be necessary to make minor adjustments by moving some of the pegs to the left or to the right to find a smooth line. Most of the pegs will remain in the same place. The smooth line thus formed by the pegs represents the first contour line.

Step 4

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. This choice should be based on the required accuracy (a little difference in height means it is more accurate), the general slope of the area and the regularity of the general slope of the area. In practice, the height difference will vary between 10 and 50 cm.

In this example, a height difference of 20 cm was chosen. This means that the ground level near peg A should be 20 cm higher than the ground level near peg A (see Fig. 38). The position of peg A. is found by trial and error, using e.g. the method described in section 3.4 to measure the vertical distance between the ground levels near A and A1. peg (A1) represents the starting point of the second contour line. Now follow the procedure described above to determine the second contour line (see Fig. 38).

Fig. 38 Setting out the second contour line

#### 6.2.3.2 Setting out slopes

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. for setting out furrows or ditches.

Example

Suppose that the slope of a ditch to be set out on the field is 1% (one percent). In order to use the N-frame level to set out slopes, it requires a modification; one leg has to be shortened. In this example, one leg has to be shortened by 2 cm, as the length of the main lath is 2 m and the required slope is 1%. (Note 1% of 2 m = 2 cm). See

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.

Step 1

The shortest leg of the N-frame is placed close to the starting peg (A). By turning the N-frame around this leg, a position is found such that the second leg is on the ground and the bubble of the carpenter level is in between the marks. The spot thus found is 2 cm lower than the starting point and is marked with a new peg (peg B)(see Fig. 40a).

Step 2

The N-frame is moved and the short leg is placed near peg (B). The procedure is repeated until the end of the field is reached. The succession of pegs thus placed form a line with a slope of 1% (see Fig. 40b). This line would be, after correction, the centre line of a ditch with a slope of 1%.

Fig. 40b Setting out a slope, Step 2

## 6.3 The Flexible Tube Water Level

### 6.3.1 Description

The flexible tube water level, used for contour lines and measuring differences in elevation, consists of two staffs with a length of about 2 m and a transparent flexible tube of about 14 m long. The ends of the tube are firmly fixed to the staffs (see Fig. 41).

Sometimes, a 10 m long rope is fixed to the staffs to limit the distance between the staffs. The rope thus helps to prevent damage to the tube.

### 6.3.2 Use of the flexible tube water level

The tube is filled with muddy water so that the water level is about 1 m high in each of the tube ends. It is essential Chat no air bubbles are trapped in the tube. Air bubbles can be removed by tapping the tube with the finger.

Wherever the two staffs are set, the free water surfaces in the tube ends have the same level (see Fig. 42). This is called the "communicating vessel" principle.

Fig. 41 Flexible tube water level

Fig. 42 The "communicating vessel" principle

#### 6.3.2.1 Setting out contour lines

To set out a contour line with a Cube water level, the following procedure is used:

Step 1

The two staffs are placed back to back at the starting point marked with peg (A). After the air bubbles have been removed and the water has come to a rest, a mark is made on both staffs, indicating the water level (see Fig 43a).

Step 2

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 he obtains a position where the water level coincides with the mark (see Fig. 43b).

Fig. 43a Setting out a contour line, Step 1

The point where the staff is then standing is at the same level as the starting point. A second peg (peg B) is placed at this point.

Step 3

The procedure is repeated, starting from peg (B), to find the third point (peg C) of the contour line.

Care should be taken to avoid spilling water whenever the staffs are moved. For this purpose, the ends of the tube can be closed with plugs during transport. It is essential to remove the plugs during the measurements, otherwise the communicating vessels principle is not applicable anymore and measurements will be wrong.

#### 6.3.2.2 Measuring differences in elevation

For the measurement of differences in elevation between two points in the field, the tube water level is adapted. Each staff is graduated in centimetres and used as a measuring staff. The zero point usually coincides with the foot of the staff (see Fig. 44).

A. Measuring the difference in elevation between two close points

Suppose the difference in elevation between two points A and B has to be measured; A and B are less than 10 m apart.

The first staff is set on point A and the second staff on point B (see Fig. 45). After the water level in both stand tubes comes to a rest, a reading is made on both staffs. The difference in elevation between points A and B is calculated by the formula:

 Difference in elevation between A and B = reading on staff A - reading on staff B

Fig 44 Graduation of a staff

Fig. 45 Determination of difference in elevation between two close points

In our example (see Fig. 45):

Measured
reading on staff A: 0.50 m
reading on staff B: 1.50 m

Difference in elevation between A and B =
reading A - reading B = 0.50 - 1.50 = -1.00 m

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.

If the reading on staff B is lower than the reading on staff A, the result of the subtraction is positive which means that point B is above point A.

B. Measuring the difference in elevation between two distant points

Suppose the difference in elevation between two points A and B has to be measured, and A and B are more than 10 metres apart.

The flexible tube of the instrument is too short to take only one measurement. Several steps are needed.

Step 1

In between points A and B, pegs are placed at intervals slightly less than 10 metres (see pegs C, D and E in Fig. 46a).

Step 2

The back staff is set near peg A, and the front staff near peg C (see Fig. 46b).

A reading is made on both staffs and the results written down in a book. The back reading in one column, the front reading in another column.

 Between pegs Back Reading (m) Front Reading (m) A and C 0.75 1.25

Step 3

Both men move. The back staff is set near peg C and the front staff is set near peg D. Again, readings are made and entered in the book (see Fig. 46c).

The procedure is repeated until the front staff is set near peg B and the back staff is set near the last intermediate peg (E in our example). The last readings are made and written down in the book.

EXAMPLE:

 Between pegs Back Reading (m) Front Reading (m) A and C 0.75 1.25 C and D 0.52 1.48 D and E 1.23 0.77 E and B 0.41 1.59 Total 2.91 5.09

Step 4

The difference in elevation between point A and point B is given by the formula:

 Difference in elevation = sum of the back readings - sum of the front readings

In our example:

Measured
sum back readings = 2.91 m
sum front readings = 5.09 m

difference in elevation between A and B
= 2.91 m - 5.09 m = - 2.18 m

The negative result means that point B is below point A. A positive result would indicate that point B is above point A.

## 6.4 The Hand Level

### 6.4.1 Description

The hand level consists of a 10-12 cm long tube with an eye piece at one end and two hair lines (one horizontal and the other vertical) at the other end. Attached to the tube is a small carpenter level (see Fig. 47).

Fig. 47 A hand level

When the operator looks through the eye-piece, the mirror inside the tube, reflects (on the right hand side) the position of the bubble of the carpenter level. The instrument is made in such a way that when the bubble is in sight on the horizontal hair line, the instrument is horizontal and the line of sight is horizontal (see Fig. 48).

Fig. 48 Use of the hand level

For greater stability the instrument can be supported by a forked bush pole, with a metal plate attached to the bottom. This assures that the instrument is always at the same height above the ground surface.

### 6.4.2 Use of the hand level

The hand level can be used to set out contour lines and to measure the difference in elevation between two points.

#### 6.4.2.1 Setting out contour lines

Step 1

The forked pole is set on the starting point. The hand level is placed on the crotch of the forked pole and tilted slowly until the bubble is seen at the horizontal hair line (see Fig. 49a).

Fig. 49a Setting out a contour line, Step 1

Step 2

A ranging pole is brought close to the hand level, and placed on the ground at the same elevation as the starting point (see Fig. 49b).

Fig. 49b Setting out a contour line, Step 2

Step 3

Looking through the hand level, the elevation of the horizontal hairline is marked on the ranging pole. This can be done by tying a piece of coloured cloth around the pole, or with some coloured tape. The top side of the cloth or tape should coincide with the horizontal hairline of the instrument (see Fig. 49c).

Fig. 49c Setting out a contour line, Step 3

Step 4

The ranging pole is placed about 10 to 15 metres away from the instrument in the general direction of the contour line. The assistant moves with the ranging pole slowly up and down the slope. The observer sights the pole and follows it by rotating the instrument, holding the bubble on the horizontal hairline. 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. 49d). This point is marked with a peg.

Step 5

The same process is repeated, this time starting from peg (B), to find the next point (C) of the contour line.

REMARK: The hand level can only be used with accuracy up to a distance of about 15 m. Vision will become poor beyond this distance and accuracy cannot be maintained.

#### 6.4.2.2 Measuring differences in elevation

A. Measuring the difference in elevation between two close points

The difference in elevation between two close points (A and B) can be measured with a hand level and a graduated staff. The procedure to follow is:

Step 1

The observer takes a position about half way between the two points A and B, that are less than 25 m apart, Over this distance the hand level can be used with reasonable accuracy (see Fig. 50a).

Step 2

The assistant places the staff at point A. The observer sights the staff at point A and moves the instrument to the horizontal position. The value indicated by the horizontal hairline is read and written down by the observer (see Fig. 50b). This reading is called a back reading.

Step 3

The assistant walks to point B and places the staff on point B. The observer turns around, the bush pole remaining in the same spot, and sights the staff at point B. 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. 50c). This reading is called a front reading. forward reading 1.38 m

Step 4

The difference in elevation between point A and point B can be calculated with the formula:

 Difference in elevation between A and B = reading on A - reading on B = back reading - front reading

In our example:

Measured
reading on A (back reading): 1.62 m
reading on B (front reading): 1.38 m

Difference in elevation = 1.62 - 1.38 = + 0.24 m

The result is positive, point B is above point A. A negative result would mean that point B is below point A.

B. Measuring the difference in elevation between two distant points When points A and B are further apart than 25 m, the procedure to follow is:

Step 1

Place pegs at points A and B and at intervals of 25 m or less in between points A and B. See example Fig. 51.

Step 2

The observer takes up a position between A and C and measures the difference in elevation between point A (near peg A) and point C (near point C) as described in the previous section.

Step 3

The observer takes up a position between point C and point D. The assistant turns the staff at point C in the direction of point D. The staff should stay in the same position and not be lifted.

Step 4

Measure the difference in elevation between points C and D as described in the previous section. Continue until the difference in elevation between the last intermediate point and B has been determined.

Step 5

The difference in elevation between point A and point B is the sum of the differences in elevation between point A, all intermediate points and point B.

Note: The difference in elevation between point A and point B can be found with the formula:

 Difference in elevation between A and B = sum of back readings - sum of front readings

EXAMPLE (see Fig. 51):

 Between points: Back Reading (m) Front Reading (m) Difference in Elevation (m) A and C 0.65 1.40 - 0.75 C and D 0.20 1.25 - 1.05 D and E 1.80 0.50 + 1.30 E and F 1.75 0.95 + 0.80 F and B 1.37 1.24 + 0.13 Total 5.77 5.34 + 0.43

Difference in elevation between A and B = Sum of back readings - sum of front readings
= 5.77 - 5.34 = + 0.43 m

The difference in elevation is positive, which means that point B is above point A.