A-Frame Level

Part of Surveying

How to build and use an A-frame level for determining elevation differences and establishing level lines without instruments.

Why This Matters

Establishing level is one of the most fundamental tasks in construction, agriculture, and infrastructure. Drainage channels must slope in the right direction. Foundation trenches must be level or water will pool inside buildings. Irrigation canals must descend at controlled gradients to flow without eroding. Terraced fields must be level across their width.

The A-frame level accomplishes all of these tasks with two straight sticks, a crossbar, and a plumb bob made from string and a stone. It requires no glass, no precision machined parts, and no knowledge of optics. It can be built in an hour from materials available anywhere with wood. Used correctly, it produces results accurate to within a few millimeters over distances of several meters — more than adequate for construction and agricultural work.

This is the same instrument (under various names) used to level the Pyramids, build Roman aqueducts, and establish the precise canal grades that made Dutch land reclamation possible. Its simplicity is not a limitation — it is its strength.

How the A-Frame Level Works

The A-frame level consists of two equal-length legs joined at the top (forming the “A”), a crossbar connecting the two legs at a consistent point, and a plumb bob hanging from the apex. When both feet of the instrument are at exactly the same elevation, the plumb bob hangs exactly over a mark at the center of the crossbar. Any difference in elevation between the two feet causes the plumb bob to swing off-center in the direction of the lower foot.

The principle is pure physics: gravity always points straight down. A plumb bob (any weight on a string) always aligns with gravity. When the frame is symmetric and the legs equal, the plumb bob crosses the center of the crossbar only when both feet are at the same elevation.

Building an A-Frame Level

Materials

• Two straight, smooth sticks or boards, equal length: 1,200–1,500mm long, about 30–40mm wide and 20–25mm thick. These are the legs.
• One straight crossbar: about 600–800mm long, same section as the legs.
• One length of fine string: about 600mm.
• One small weight: a stone, a lead sinker, a bolt — anything dense enough to hang the string taut.
• Fasteners: wooden pegs, nails, or lashing cord.

Construction

Step 1: Join the legs at the top Cross the two legs near one end, overlapping by about 150mm. The crossing point is the apex of the A. Fasten the legs together at this crossing with a wooden bolt through both (drill a hole through both legs and insert a snug-fitting wooden peg), or bind firmly with cord or leather lashing. The joint must be stable — no wobble.

Step 2: Attach the crossbar While the legs lie flat on a surface, adjust the spread of the lower ends to a comfortable working width — approximately 1,000–1,200mm between the feet. Mark where the crossbar will be attached (typically about 1/3 to 1/2 of the way up from the feet). Attach the crossbar to both legs at exactly the same height above the feet on each leg. Measure this carefully — even a 5mm difference in crossbar height on the two legs will introduce a systematic error.

Step 3: Mark the center of the crossbar With the crossbar attached, measure the exact center of the crossbar between the two attachment points and mark it clearly with a scratched line, a notch, or ink.

Step 4: Attach the plumb bob Tie one end of the string to the apex joint. Tie the weight to the other end. The string should be long enough that the weight hangs below the crossbar by about 50–100mm when the instrument is held upright.

Step 5: Calibrate Calibration is essential — it finds and corrects any construction asymmetry. Set both feet on a flat surface. Note where the plumb bob hangs relative to the crossbar center mark. Then flip the A-frame so the leg that was on the left is now on the right. If the plumb bob now points to the same mark: the frame is perfectly calibrated. If it points to a different position, the true level mark is halfway between the two plumb bob positions. Mark this true center position on the crossbar permanently.

Using the A-Frame Level

Basic Leveling

To determine if two points are at the same elevation:

1. Place one foot of the A-frame at point A and the other at point B.
2. Wait for the plumb bob to settle.
3. Check where the string crosses the crossbar.
4. If it crosses the calibration mark: A and B are at the same elevation.
5. If it crosses to the left of the mark: the left foot is lower.
6. If it crosses to the right of the mark: the right foot is lower.

For excavation or grading, move the lower foot up (pile soil, add a shim) or dig down the higher foot until the plumb bob centers. Both points are then at equal elevation.

Establishing a Level Line Over Distance

A-frame leveling works in steps. Each step covers a distance equal to the leg spread (about 1 meter). To level over 10 meters, take approximately 10 steps.

Leapfrog method:

1. Place one foot at the start point (mark with a stake), the other foot ahead along the desired line.
2. Adjust the forward foot until the plumb bob centers on the calibration mark.
3. Drive a stake at the forward foot position at the exact elevation of that foot.
4. Move the rear foot ahead of the forward foot.
5. Adjust and mark the next stake.
6. Continue until the end point is reached.

All stakes driven in this manner are at the same elevation. A string stretched between them marks a level line.

Measuring Elevation Differences

To measure how much higher or lower one point is than another, count the shimming needed:

1. Place the A-frame with one foot at point A and the other at point B.
2. Determine which foot is lower.
3. Raise the lower foot with shimming material until the plumb bob centers.
4. Measure the height of the shim. This is the elevation difference between A and B.

For differences too large to measure with a single shim, establish an intermediate point on the slope and measure in two segments.

Establishing a Drainage Gradient

For irrigation channels, drainage ditches, or roadways, you need a controlled slope — not level, but a specific grade.

To create a 1% grade (1cm fall per 1m horizontal) over 10 meters:

1. Set the A-frame level calibration mark in the normal way.
2. For each 1m step, intentionally shim the forward foot 1cm higher than the calibrated level position before driving the stake.
3. Each stake is 1cm higher than the last — so the line of stakes descends from forward to backward at 1cm per meter.
4. Water flowing along a channel following this stake line will flow from back to front.

Adjust the shim amount per step to achieve different grades. A 0.5% grade uses 5mm per step; a 2% grade uses 20mm per step.

Accuracy and Limitations

The A-frame level is accurate to approximately 2–5mm over a 1m leg spread, depending on construction quality and the steadiness of the plumb bob. Over a 10m line established in 10 steps, cumulative error may reach 20–50mm.

For most agricultural and construction purposes, this accuracy is entirely adequate. A 50mm error over 10 meters is a 0.5% error — imperceptible in grading, adequate for canal flow, and sufficient for foundation leveling.

Greater accuracy requires either more careful construction (longer, truer legs; finer calibration mark; very light plumb bob that settles quickly), or the use of a water level (which achieves higher precision over longer distances).

Care and Maintenance

The A-frame must remain dry and protected from warping. Wet wood swells unevenly and changes the geometry. Store out of weather when not in use.

Recalibrate regularly — before each significant project. Recalibration catches changes caused by humidity, joint loosening, or accidental damage. It takes two minutes and ensures reliable results.

If the frame is damaged (a leg warped, a joint loosened), rebuild it rather than attempting to compensate by adjusting the calibration mark. An asymmetric or distorted frame cannot be reliably recalibrated.