|Soil and Water Conservation (SWC) Technologies and Agroforestry Systems (International Institute for Rural Reconstruction (IIRR), 1992)|
One does not need to have expensive soil surveying equipment to locate the contour lines of the land. The Aframe is a simple and practical instrument, which can easily be made by the farmer using locally available materials.
The steps in making the A-frame are as follows:
1. Secure the following materials:
· 3 wooden or bamboo poles with a 1.5 inch diameter (2 of which should be 2.1 In long and one about 1.2 m)
· sturdy string for tying or nail
· a rock about the size of a fist or any similar heavy object object
2. Tie or nail the two longer poles at one end, about 10 cm from the end. Make sure they are securely fastened. These will make the legs of the A-frame. Make notches on the points of contact so that the poles will not slip.
3. Spread the legs and brace with the shorter pole to make a figure "A". Tie or nail the crossbar (about 10 cm from each end) to the middle of the legs of the "A". The crossbar will support the legs of the frame and will serve as guide in making the level ground position.
4. Tie one end of the string to the point where the two legs of the A-frame are joined.
5. Tie the other end of the string to the rock or any object for weight. The rock should be heavy enough so that when it is suspended, it will not sway with the wind. The rock should hang about 20 cm below the crossbar.
CALIBRATING THE A-FRAME
1. Locate a reasonably level ground and place the A-frame in an upright position. Mark the spots where the legs (A and B) touch the ground. Then mark the crossbar where the weighted string passes it.
2. Reverse the position of the Aframe's legs such that leg A is exactly on the same spot where leg B was and vice-versa. Again, mark the crossbar where it is crossed by the string.
If the two marks exactly coincide, this means that you have found the midpoint on the crossbar and that the A-frame is standing on level ground.
If the two marks are separate, make another mark at the midpoint between them.
3. To check the accuracy, move one leg around until the string passes the level point of the crossbar. Mark the point where the adjusted leg touches the ground. Reverse the placement of the legs of the Aframe. If the string passes the same point, the level position has been located.
4. Check calibration from time to time.
MARKING THE CONTOUR LINES
1. Cut tall grasses and remove other obstructions so that you can move about easily. Two people will make the work much faster and easier. One will operate the A-frame while the other marks the located contour lines.
2. Begin near the highest point. Drive the first stake at the boundary of the area and position the left leg of the A-frame beside and just above it.
3. Adjust the right leg such that the weighted string passes through the midpoint of the crossbar. (This means you have found the contour.) Mark this point by driving another stake just below the right leg of the A-frame.
4. Move the A-frame to the right by placing the left leg on the spot where the right leg previously was. Adjust the other leg again until the string passes through the midpoint mark. Again, mark this with a stake. Follow this procedure until you reach the other side of the field
5. Repeat steps 24 until you reach the bottom of the hill. The vertical distance between contour lines should be 1.5 m (the actual distance varies with the slope of the hill), which can easily be determined as shown in the figure on this page