Plumb Bob

Part of Surveying

How to make and use a plumb bob to establish true vertical and transfer points directly downward.

Why This Matters

Gravity acts uniformly and reliably everywhere on Earth: it always points directly toward the center of the Earth, defining a direction we call “plumb” or “true vertical.” A plumb bob uses this fact to establish vertical lines with perfect accuracy. Unlike a level, which can drift or require calibration, a plumb bob is self-calibrating — gravity never lies.

The plumb bob is one of the oldest precision instruments in existence, found in Egyptian construction sites dating to at least 2500 BCE. Ancient pyramid builders used plumb bobs to ensure their stone courses were vertical; medieval cathedral builders used them to plumb columns and walls; miners used them to extend survey points down shafts. The tool has not changed in four thousand years because it cannot be improved: a weight on a string, allowed to hang freely, is already perfect.

In surveying, the plumb bob serves two main functions: centering an instrument exactly over a ground mark, and transferring a point directly downward from a height (or upward from a depth) without lateral deviation. Both functions are essential for accurate surveying, and both require nothing more than a weight, a string, and patience.

Construction

A plumb bob consists of a heavy weight with a pointed lower tip, suspended from a string attached at its exact center of gravity.

Weight requirements:

• Heavy enough that air currents have minimal effect: 100-500 grams is the typical range
• Symmetrical around its vertical axis, so the center of gravity is truly in the center
• A pointed tip for precise location of the plumb point

Making the weight: The simplest plumb bob is a fist-sized stone with a roughly pointed bottom, tied with string around its middle. This works but is not precise — a knobby stone has an indeterminate center of mass.

For a better plumb bob, shape the weight into a cone or cylinder. Cast lead is ideal: melt lead (or pewter, or other low-melting metal) into a cone-shaped mold with the pointed end down. Bronze or copper works. A bone or hardwood weight can be carved if metal is not available, though wood’s lower density makes it more susceptible to air movement.

The critical detail: The string must attach at the exact top center of the weight. If it attaches off-center, the plumb bob will hang at a slight angle rather than pointing straight down. To find the exact center: trace around the top of the weight on paper, find the centroid of the shape (the intersection of diagonal lines through the outline), and mark it. Drill a small hole or make a notch at this point for the string attachment.

String: Use the thinnest strong string available. Thin string reduces air resistance and reaches equilibrium faster. For precision work with a heavier bob, a length of fine wire is better than string. The string must not twist under the weight — test by suspending the bob and watching for rotation.

Using a Plumb Bob for Instrument Centering

Every instrument on a tripod must be centered exactly over the survey station mark on the ground. The plumb bob is the tool for this.

Procedure:

1. Set up the tripod approximately over the mark.
2. Attach the plumb bob string to the center of the tripod head (most tripods have a hook for this purpose; improvise one if not).
3. Adjust the tripod leg lengths to bring the plumb bob close to vertical over the mark.
4. Let the bob hang freely — do not touch it.
5. Wait 15-30 seconds for oscillation to dampen. In wind, shield the bob with your body.
6. Observe where the pointed tip falls relative to the ground mark.
7. Slide the tripod head (or adjust leg positions) until the tip hangs directly over the mark.
8. Re-level the instrument (leveling may shift the centering slightly; recheck and iterate until both leveling and centering are satisfied simultaneously).

Accuracy: A plumb bob centered by eye can be placed within 2-3 mm of the true mark center. Better accuracy requires a more precise centering tool (optical plumb or target with fine crosshair), but 3 mm is sufficient for all but the most precise surveying.

Using a Plumb Bob for Vertical Transfer

Transferring a point downward: When you know the position of a mark at height (on a wall, the top of a pole, or a ceiling) and need to find the corresponding point on the floor directly below it, suspend a plumb bob from the upper mark. The bob’s point indicates the floor position.

Applications:

• Transferring column positions from plan to ground
• Establishing the base of a wall directly below its top
• Finding the ground point below a suspended instrument

Transferring a point upward: Harder, because you cannot hang a weight upward. Instead, set two plumb bobs several meters apart, both hanging freely, with their points on the floor aligned with the known direction. The line between the upper string attachment points is directly above and parallel to the floor line. Any point on the upper line is directly above the corresponding point on the floor line.

This technique is used in mine surveying to extend a surface survey line down a mine shaft, and to transfer bearing from surface to underground workings.

Establishing Vertical in Construction

Plumbing a wall or column: Hold a plumb bob on a string against the face of the wall or column. The string should touch the upper part of the face. If the wall is plumb, the string will be parallel to the face along its entire length (or touching it). If the wall leans, there will be a gap between string and wall at some point. Measure the gap and the distance over which it appears to determine the degree and direction of lean.

For a more sensitive check, hold the string at the top of the wall and measure the gap at the base. Even a gap of 2 cm over a 3-meter height is a significant lean (0.67%) that should be corrected before the work is complete.

Plumbing tall structures: For columns or walls over 3 meters, use two plumb bobs at right angles to each other (checking both faces of the column). A column that is plumb in one direction but not the other will still fail.

Setting corner posts: When setting posts for a building or fence at exact right angles, plumb each post in both directions after setting and before backfilling. A post that appears straight to the eye may be off-plumb by several degrees — enough to cause problems as the structure is built up.

Dealing with Wind

Wind is the main enemy of precise plumb bob work. Several techniques reduce its effect:

Shielding: The simplest approach. Stand with your back to the wind, holding the bob in your shadow. For very precise work, use a board, cloth, or other windbreak.

Immersion: Suspend the plumb bob in a bucket of water or oil. Fluid damping stops oscillation almost immediately and the fluid’s surface tension reduces the bob’s sensitivity to air currents. Mark the bucket bottom directly below where the string enters the water — this is the plumb point.

Heavy bob: A heavier bob is less affected by a given wind speed. For precision work in exposed locations, use a bob of 500 g or more.

Waiting: In calm morning conditions, even a light bob settles quickly. Wind often diminishes at dawn and dusk — schedule precise plumb work for these times.

Center of swing: If the bob cannot be calmed, observe its swing for several cycles and estimate the center. The center of a uniform swing is the true plumb point.

The Two-Mark Check

To verify that your plumb bob string is truly taut and vertical (not bent by catching on something), look at the bob from two perpendicular directions. If it appears directly below the attachment point in both views, it is plumb. If it appears offset from one direction only, it is swinging or the string is catching on a surface.