A traverse is a series of connected survey lines measured in sequence to establish control coordinates. Traverse closure is the error between the computed position of the starting point (after traversing a loop) and its known position. Closure ratio = linear closure error ÷ total traverse length. Acceptable closure for construction layout is typically 1:5,000 or better; DOT second-order is 1:10,000 or better.
Why Traverse Closure Matters
A traverse allows you to extend survey control from existing monuments to areas too far from the original control for direct instrument setup. Closure tells you whether accumulated measurement errors are acceptable or whether the traverse needs to be re-run. An unclosed traverse with an unknown error can propagate that error into every layout or as-built shot made from the traverse points.
Running a Closed Traverse
- Set up the total station on a known control point and orient with a backsight.
- Measure the angle and distance to the first traverse leg.
- Set a temporary benchmark or hub at the end of the leg (the TP).
- Move to the TP; take a backsight on the previous point and measure to the next leg.
- Continue until the traverse closes back to the starting control point or connects to another known point.
- Compute the closure: sum of latitudes (northings) and sum of departures (eastings) should equal zero for a closed loop. The residuals are the closure error.
Closure Calculation
Latitude closure error (ΔN): +0.14 ft
Departure closure error (ΔE): −0.08 ft
Linear closure: √(0.14² + 0.08²) = 0.161 ft
Closure ratio: 1 : (1,250 ÷ 0.161) = 1 : 7,767
Acceptable for most construction layout (1:5,000 minimum). ✓
Bowditch (Compass Rule) Adjustment
If closure meets the specification, adjust the traverse using the Bowditch rule: distribute the closure error among the traverse legs proportional to their length. The adjustment to each leg’s latitude and departure is:
Correction to leg i departure = −(ΔE) × (leg i length ÷ total length)
After adjustment, the traverse closes exactly and the adjusted coordinates of intermediate points can be used as temporary control for layout and as-built work.
When the Traverse Fails to Close
If the closure ratio is worse than the project requirement, re-run the traverse before doing any layout from it. Common causes of poor closure: incorrect instrument orientation on the backsight, blunder in angle or distance measurement, wrong prism height at a turning point.
See also: Total Station Setup and Backsight Procedure and Resection Setup: Setting Up Over an Unknown Point.