Transit Instrument

Part of Surveying

Building and using a pivot-mounted sighting instrument capable of measuring both horizontal and vertical angles.

Why This Matters

A transit is a step above the simple goniometer: it measures angles in two planes simultaneously — horizontal (azimuth) and vertical (elevation). This dual capability allows you to calculate the height of objects from a distance, run level lines across terrain, and perform the full range of surveying tasks that a flat-disc instrument cannot handle.

For rebuilding communities, the transit unlocks critical applications: determining the fall gradient of a water channel, establishing the alignment of a road over hilly ground, calculating the height of a tree or cliff without climbing it, and laying out building foundations that are both level and square across long spans.

A transit does not need to be a precision instrument to be useful. The principles are simple — a sighting telescope (or open sight) mounted on a horizontal pivot, which itself rotates on a vertical axis — and the whole assembly can be built from wood, bone, and salvaged hardware.

Anatomy of a Transit

Understanding the parts helps you build or improvise one.

Part	Function
Vertical axis (main spindle)	Allows rotation left-right (horizontal angles)
Horizontal axis (trunnion axis)	Allows the sighting tube to tip up-down (vertical angles)
Horizontal circle	Graduated disc for reading horizontal angles
Vertical arc or circle	Graduated arc for reading vertical angles (elevation/depression)
Sighting tube or telescope	The optical or open sight
Leveling base	Three or four adjusting screws that level the instrument
Tripod	Stable support; should be heavy and wide-legged

Building a Wooden Transit

The Tripod

A transit tripod needs three qualities: stability, levelability, and a flat top plate.

1. Cut three legs from straight-grained hardwood, approximately 1.5 m long and 4 cm square.
2. Taper the lower ends to points for ground penetration.
3. Join the tops to a triangular head plate (a 20 cm equilateral triangle of hardwood) using mortise-and-tenon or lapped joints with bolts.
4. Spread the legs to about 1 m radius at ground level.
5. Add cross-bracing near the head for rigidity — a single diagonal brace on each pair of legs.

Stability is Everything

A transit that vibrates or shifts during a reading is worthless. Plant the tripod legs firmly, press them into the ground if soft, and place a heavy stone over the foot of each leg on hard surfaces.

The Leveling Base

Between the tripod head and the main instrument body, you need a leveling base with at least three adjusting points.

Simple version: three screws through a flat plate, each bearing on the tripod head. Turning a screw raises or lowers that corner of the plate. A bubble level (a curved glass tube with liquid and an air bubble) tells you when the plate is level. A bubble level can be improvised from a small piece of transparent animal gut or glass tube, sealed with beeswax, filled with water leaving a small air pocket.

The Vertical Axis and Horizontal Circle

1. Drive a hardwood or metal spindle vertically through the center of the leveling plate. This is the vertical axis.
2. Mount a graduated horizontal disc (made as described in the Goniometer article) around this spindle so it can be clamped at any position.
3. The instrument body (the part that carries the sighting tube) rides above the disc on the same spindle, free to rotate.
4. A clamp screw locks the instrument body to the disc when you want to record a horizontal angle.

The Trunnion Axis and Vertical Arc

The sighting tube must pivot up and down:

1. Construct two upright supports (called “standards”) rising from the instrument body, set apart by the width of your sighting tube plus clearance.
2. A horizontal pin or bolt through both standards and through the sighting tube forms the trunnion axis.
3. Attach a graduated arc (a quarter-circle or semicircle of wood or metal, marked in degrees) to the side of the sighting tube. As the tube tilts, an index mark on the standard points to the vertical angle.

The Sighting Tube

The simplest option is an open tube — a hollow cylinder 30–40 cm long with a cross-wire or notch at each end. More effective is a tube with a peep hole at the eye end (1 mm diameter) and a fine vertical wire at the objective end. If you can salvage a lens, a simple magnifying objective improves both range and accuracy.

Cross-wire construction: Stretch two fine threads (spider silk works best) at right angles across the objective end of the tube, secured with wax or resin. These cross-wires allow precise alignment on a target staff.

Using the Transit

Leveling Up

1. Set the tripod, press the legs firm.
2. Roughly level the instrument by eye.
3. Check the bubble level. Turn two opposite leveling screws in opposite directions simultaneously to move the bubble toward center in that axis. Rotate 90° and repeat with the other pair.
4. Check both axes again; repeat until the bubble stays centered through a full rotation.

Measuring Horizontal Angles

1. Set the horizontal circle to 0° while sighting your first target (backsight).
2. Clamp the circle to the instrument body.
3. Unclamp the instrument body from the circle and rotate to the second target (foresight).
4. Read the horizontal angle from the circle.

Measuring Vertical Angles

1. Read the vertical arc while the sighting tube is horizontal — this is your zero reference.
2. Tilt the tube up or down to sight the target.
3. Read the vertical arc. Angles above horizontal are elevations (+), below are depressions (−).

Calculating Height by Vertical Angle

If you know the distance to an object and the vertical angle to its top:

Height = Distance × tan(vertical angle)

For example: distance 50 m, vertical angle 20° → height = 50 × 0.364 = 18.2 m.

Without a table, you can calculate tangent by constructing a right triangle on paper at the measured angle and measuring the opposite side against the adjacent.

Eliminating Systematic Errors

The Two-Face Method

Every well-used transit should be observed in both “face left” and “face right” positions. The instrument is flipped 180° on its vertical axis between the two readings. If the sightline is truly perpendicular to the trunnion axis, both faces will give the same angle. If they differ, the mean of the two readings cancels the collimation error.

Formula: Corrected angle = (Face Left + Face Right ± 180°) / 2

Index Error (Vertical Angle)

When the sighting tube appears horizontal, the vertical arc may not read exactly 0° — this offset is the index error. Measure it by sighting a point at exact eye level and reading the arc. Subtract the index error from all subsequent vertical angle readings.

Common Field Problems

Problem	Cause	Fix
Bubble won’t stay centered	Soft ground, vibration	Re-seat legs, add weight
Two-face readings disagree by more than 2°	Collimation error	Adjust sighting tube or accept mean
Vertical arc moves when horizontal circle is clamped	Loose trunnion	Tighten pivot; pack with leather
Target drifts out of view between setups	Tripod settling	Allow 5 min for settling after setup

Maintenance

• Oil all moving pivots with animal fat or plant oil after each use in damp conditions.
• Check that the bubble level tube has not lost fluid (evaporation can occur over months).
• Re-check the two-face agreement whenever the instrument has been transported.
• Store disassembled in a padded bag, not leaning against a wall where it may warp.

A transit built with care and maintained well can serve decades of field work. In a community rebuilding survey capacity, even one working transit — made from wood and salvaged hardware — is a transformative tool.