Marking contour lines

You may want to mark contour lines (level lines running across a slope) to decide where to dig contour canals, plant trees along the contour, and several other methods of controlling soil erosion.

This section describes two ways of doing this: using an A-frame, and a line level. Aside from marking contours, the A-frame and the line-level can be used to mark graded terraces or to measure the slope of a field.

Location

A-frames and line-levels are useful in hilly areas subject to erosion, or on gently sloping fields in which the farmer wishes to build ridges or dig ditches to allow water run-off to sink into the soil.

Advantages

• A-frames and line-levels are cheap and easy to make and use.
• They can be made from readily available materials.

Disadvantages

• A-frames are not practical for marking contours over large distances. Line-levels are better for this.

A-frame

An A-frame consists of three pieces of wood, fixed together in the shape of a capital letter "A". The A-frame is held upright, and a weight on a string hangs down from the top of the "A" to act as a plumb-line. If the A-frame is on perfectly level ground, the string crosses the horizontal bar of the "A" at a certain point. This point is marked during calibration.

To use the A-frame, it is "walked" across the slope, making sure that the two legs are level each time by checking if the string crosses the horizontal bar at the calibrated point. If not, the forward leg is moved until the string shows the frame is level. The position of the legs on the ground are marked with pegs, and then the frame is pivoted around to mark a new point on the slope.

Requirements

• Two straight wooden poles (1.5-2 m long), and one straight pole (1-1.5 m long).
• String, hammer, nails, pencil.
• A round stone.

Making the A-frame

1. Use the poles and nails to make a frame in the shape of an "A" (Picture 1).

2. Tie one end of the string to the top of the "A".

3. Tie the stone to the other end of the string, so it hangs down just below the horizontal crossbar of the "A".

Calibrating the A-frame

4. Stand the A-frame upright on reasonably level ground. Mark on the ground where the two legs stand.

5. Hold the A-frame still, and use the pencil to mark lightly on the crossbar where the string crosses it (Picture 2).

6. Turn the A-frame round, so that each leg stands exactly where the other had stood.

7. Make a second light mark on the crossbar where the string crosses it.

8. The two marks on the crossbar should be fairly close together. Halfway between them shows where the string would cross if the A-frame were standing on exactly level ground. Make a heavy pencil mark or notch the bar with a knife at this point (Picture 3).

Marking the contour

9. Choose a place on the slope to begin. Stand the A-frame up and mark where the first leg stands with a peg or large stone.

10. Keeping the A-frame upright, and without moving the first leg, swing the second leg up or down the slope until the string crosses the crossbar exactly at the heavy pencil mark (Picture 4).

11. Mark where the second leg stands with another peg or stone.

12. Keeping the second leg in the same place, lift the first leg up and pivot it around. Move it up and down the slope until you find the place where the string crosses the crossbar at the heavy pencil mark.

13. Mark where the first leg is now standing with another peg or stone.

14. Continue in this manner to the end of the field.

15. The line of pegs or stones will mark a contour line: they will all be at the same height on the slope. The pegs are usually not in a straight line. If necessary, make a smooth curve by moving them a little up or down.

16. To mark another contour line, move up or down the slope a certain distance—usually about 20 m (20 paces) on a gentle slope, or a drop of 1.5 m on steeper slopes. Repeat the process from Step 9 above onwards.

17. You can then dig ditches, construct terraces or plant trees along the contour lines using the pegs or stones as a guide.

Using an A-frame to mark or “grade” the slope of a field

An A-frame can also be used to “grade” a field or a drainage canal to achieve a certain slope. This can be a useful as it allows a farmer to ensure proper drainage of a field.

Once you have assembled the A-frame following the directions above, you need to calibrate it. However, if the A-frame is to be used to grade the slope, the calibration procedure is different.

1. Decide what percentage slope you want the field or drainage canal to have. For this example, let’s say you want a 6% slope.

2. Cut a small block of wood 12 cm high. The height of the block for other percentage slopes for other percentage slopes will be either more or less than this (Picture 5).

3. Place the block on level ground against a wall so that it does not move. Place a straight, 2-metre-long board or pole on top of the block, with the other end resting on the ground. Place a heavy stone against that end so that the board does not move. Do not stand on the board when marking the A-frame.
4. Place the A-frame on the board, and mark where the string passes on the crossbar of the A-frame. The A-frame is now calibrated; when the string aligns with the mark on the crossbar, this indicates a 6% slope (Picture 6).
5. Use the A-frame to mark out the slope of a field or drainage ditch, following the same procedure as described above for marking a contour. Move one of the A-frame legs up and down the field until the string passes through the 6% mark on the crossbar.

Line-level

A line-level is another tool used to mark a contour or measure the slope of a field, as well as to lay out cut-off drains, retention ditches, terraces and other conservation structures. A line-level is easy to use and effective, and is more accurate than using an A-frame. However, three people are needed—as well as a spirit level, which may be difficult to find in the field.

Making a line-level

1. Cut two straight poles, 1.5 m long. Nail a small wooden base on the bottom of each pole to prevent them from sinking into the soil when you are using them.

2. Tie a string (preferably made of cotton) between the tops of the poles, so there is exactly 10 m between the poles when the string is taut. The loops around the poles should be a little loose so you can move the string up and down if you want.

3. Tie a spirit level to the middle of the string, so that when the string is held taut and exactly level, the bubble of the spirit level is in the middle of its run.

4. Starting near the top of the first pole, make notches down the pole every 5 cm. Mark the top notch with a “0 cm”, the first notch below it with a “5 cm”, the second with a “10 cm”, and so on. Do the same on the second pole at the same heights as on the first one.

Marking a contour

1. Make sure the string is at the “0 cm” notch on both poles.

2. Choose the place where you want to start marking the contour, and mark this place with a peg. Hold one pole upright next to the peg.

3. Making sure the string is taut and both poles are upright, move the other pole up or down the slope, until the bubble of the spirit level shows the string is level (Picture 7).

4. Mark with a peg where the second pole is standing.

5. Repeat the procedure across the slope to mark out the contour line.

Measuring the percentage slope

1. Stand one pole at the top of the slope, and the other directly downslope from it. Be sure that the string is placed at the two notches marked “0 cm” (near the top of the poles).

2. Move the string on the up-slope pole down until the spirit level indicates that the string is level.

3. Record the number of 5 cm notches below the “0” mark. Each 5 cm notch indicates a 0.5% slope. For example, if you have moved the string moved down 10 notches, the slope is 5% (Picture 8).

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