|Soil Conservation Techniques for Hillside Farms (Peace Corps, 1986, 96 p.)|
In many areas, sophisticated surveying levels are not available to farmers interested in designing soil conservation structures. Even where they are available, it is often more practical for the farmer to build a cheap, simple, effective level for use in surveying contour lines. Although less accurate than more sophisticated levels, the two levels presented here' when properly constructed and used are sufficiently accurate for the work an small hillside farms requiring measurements of 0% or 1% slope described in this guide.
Fig. 39. A-Frame level for surveying contour lines.
The materials required are 3 straight boards or sticks, 3 nails or screws, a thin string, and a screw-capped glass bottle or uniform shaped rock. A small line level is very convenient and makes use much easier on windy days.
Important points to consider in building the A-frame level:
-The symmetry of the level is important (2 legs should be same length and crossbar should be positioned identically on the legs so that it is parallel to the ground.
-The dimensions of the level are not important, but if constructed much larger than the one pictured, they should be assembled with screws so that they can be disassembled for transportation. Measuring an exact distance (i.e. 2m ) between the feet makes calibrating the 1% contour position easier.
-The plumb bob must be attached so that it does not deflect the string to either side. If a screwcap bottle is used, it should be hung by a hole made exactly in the center of the cap. If a rock is used, it is important that a very uniformly shaped rock be chosen.
The level should be calibrated every day before use, as warping of the wood can greatly change the results. a. Calibration of 0%
1. The level should be positioned with both feet on firm surfaces but with one end obviously higher than the other.
2. The level is gently rocked, allowing the string with the plumb bob to gently strike the cross bar,
3. When the plumb bob stops swaying side to side and the string strikes the cross bar at the same point repeatedly (510 times), mark this position in pencil on the cross bar.
4. Reverse the position of the level so that the other foot is now at the higher point. Care must be taken to position the feet of the level in exactly the same points as before.
5. Repeat steps 2 and 3, obtaining a second mark on the other side of the center of the cross bar.
6. The 0% position of the level is exactly in between the two marks obtained in this trial. m is position can be marked by measuring with a ruler or paper (1/2 the distance between the 2 marks). Now when the feet of the level are even the string will strike the cross bar at the 0% position which is used to survey contour lines for barriers, terraces or ditches which are to be used for retention, rather than diversion of water.
7. Once calibrated, a small carpenter's or line level can be fastened to the cross bar to facilitate use on windy days.
b. Calibration of 1%
1. Position the level so that the feet are an the same level and
the string strikes the cross bar at the 0% position. me feet should be an firm
2. Raise one foot by the distance required to position the level at a 1% slope. For example, if the distance between the feet is 2m, then a 2 cm. tall object or 2 cm. tall stack of coins should be placed under one foot.
(2cm/200 cm = .01 = 1%)
3. Rock the level gently, now the string strikes the 1% slope position Mark this position an the crossbar.
4. Since this type of contour line will be used to construct structures to divert water, an arrow should be placed pointing toward the rawer foot to indicate the direction of water flow
5. As in the previous calibration, if desired a small level can be fastened to the cross bar.
3. Use of the A-Frame level:
The A-Frame level is used to survey contour lines by placing stakes at the position of the feet when the level gives the desired reading. Stakes should all be placed an the same side of the level, all upslope or all downslope, in order to avoid errors. When not being used the level should be stored in a dry, shady place.
Fig. 40. Simple line level for surveying contour lines.
The materials required are 2 straight boards or sticks, a string of desired length and a line level.
The level should be calibrated every day before using as bending of the hooks on the line level or warping or chipping of the sticks can greatly change the results. a. Calibration of 0%
1. Slots are cut in each stick at the same distance from one end.
2. The string is tied firmly to each stick so that it cannot slip out of the slots.
3. Hook the line level on the string and find a place on firm ground which gives a level reading.
4. Reverse the direction of the line level on the string while maintaining the position of the sticks. If the reading changes, the hocks of the line level must he adjusted slightly by bending them.
5. Repeat steps 3 and 4 until the line level gives identical readings upon reversal.
b. Calibration of 1%
1. Repeat the steps as in the calibration of 0%. However this
time the slots on the sticks should be placed so that a 1% drop occurs over the
distance of the string. (example: if the string measures 2m then the slot on one
stick should be 2 cm higher than on the other).
2. Remember that the stick which has the slot located higher up actually represents the lower ground surface when the reading of the string is level. Remember to mark the sticks so that no confusion as to the direction of water flow will arise when surveying contour lines.
This type of level is easiest to use with. a least 3 people, two holding the sticks and the third reading the line level and placing slakes. When not in use the line level should be protected so that the glass vial and hooks are not damaged.