Measuring contour lines with an A-frame
Contour lines are completely level lines across a slope. When building rainwater bunds or dykes or building contour mounds to control soil erosion, contour lines must be first identified. If people guess, this will result in bunds or mounds which may collapse during heavy rain and fail to prevent the rainwater flowing away.
The A-frame is an easy and well-tested method for measuring contour lines. It can be made at no cost from local materials.
STEP 1 Tie the poles very tightly together to make the shape of a letter A. Hang the stone from the top of the A-frame, making sure the stone hangs below the cross bar.
STEP 2 Hold the A-frame upright and mark exactly where the poles touch the ground. With a pencil, mark where the string crosses the cross bar. Turn the A-frame around, placing the poles in the marked positions. Again mark where the string crosses the cross bar.
STEP 3 Mark the ‘level mark’ on the cross bar – exactly half way between the previous marks. If the first two marks happen to be in the same place, this is the level mark.
STEP 4 Before using the A-frame, collect a number of sticks. Begin at one side of the land where the first contour line is wanted. Hold one pole firmly on the ground. Move the other pole until the string touches the level mark. Place a stick into the soil by each pole. Move the A-frame along the field, alternately moving each pole (pivoting).
STEP 5 This will result in a line of small sticks. Smooth out any sharp bends in the line and use this line as the guide for building bunds and contour mounds.
STEP 6 To find the position of the next contour line, stand with one arm stretched out. Walk backwards down the slope until an identified contour line can be seen in line with your arm. Make the next contour line where you now are now standing.