7.4 Traverse Adjustment: Compass Rule and Transit Rule

Compass Rule, Transit Rule, and Correction Signs

After a traverse closure check, an exam problem may ask for adjusted latitudes and departures. Adjustment does not make field work perfect. It distributes the mathematical misclosure using a rule stated in the problem. The FS exam may use this to test proportional reasoning, signs, and whether you understand the difference between distance-weighted and component-weighted methods.

The compass rule, also called Bowditch adjustment, assumes angular and distance errors are balanced enough that corrections should be proportional to course length. If the total latitude misclosure is +0.40 ft, the adjustment must add corrections totaling -0.40 ft to the latitudes. A course that is 300 ft in a 3000 ft traverse receives 10 percent of the total correction, or -0.04 ft in latitude. The same idea applies separately to departure.

The transit rule distributes latitude correction in proportion to the absolute latitude of each course and departure correction in proportion to the absolute departure of each course. It is often associated with conditions where angular precision is relatively strong compared with distance precision, though exam problems normally tell you which rule to apply. Do not decide based on preference if the wording specifies the method.

Correction signs matter more than the formula name. If the algebraic sum of departures is -0.25 ft in a closed traverse, the departure corrections must sum to +0.25 ft. A correction is not automatically positive because a course is long. It takes the sign needed to remove the total misclosure. After applying all corrections, the adjusted latitude and departure sums should match the required closure condition, within rounding.

A compact compass-rule workflow is:

1. Compute total traverse length.
2. Compute latitude and departure misclosures.
3. Reverse the sign of each misclosure to get the total correction needed.
4. For each course, compute course length divided by total length.
5. Multiply that ratio by the total latitude correction and total departure correction.
6. Add corrections to the original latitudes and departures.
7. Accumulate adjusted coordinates and confirm closure.

For example, a 400 ft course in a 2000 ft traverse receives 20 percent of each total correction under the compass rule. If the total latitude correction needed is -0.30 ft and the total departure correction needed is +0.50 ft, that course receives -0.06 ft latitude correction and +0.10 ft departure correction. Those values are added to the original components for that course.

Adjustment should not hide blunders. If one course has a transcription error, a wrong quadrant, or an impossible distance, proportional adjustment merely spreads the problem through the entire traverse. On the exam, if the misclosure is large or one course is inconsistent with the sketch, first look for a blunder before applying a routine adjustment. In real work, field notes, control checks, instrument records, and crew communication matter as much as the final adjusted numbers.