FW07 — Closed Traverse Survey Using a Surveyor's Compass
Learning Objectives
- Reconnoiter, mark, and observe a closed compass traverse.
- Measure magnetic direction and side length for every traverse course.
- Record fore and back bearings using consistent station IDs.
- Detect evidence of local attraction from fore/back-bearing disagreement.
- Apply documented station corrections when local attraction can be resolved.
- Compute preliminary latitudes, departures, linear error of closure, and relative precision from corrected directions.
Fore Bearing
The direction of a traverse line observed from its starting station toward its ending station.
Back Bearing
The direction of the same line observed from the opposite end. In the absence of local attraction and observational error, fore and back directions differ by exactly 180° when expressed as azimuths.
Local Attraction
A local magnetic disturbance that deflects the compass needle from the magnetic meridian, commonly caused by nearby steel, vehicles, electrical equipment, reinforced structures, or other magnetic sources.
Compass Traverse Principle
A closed traverse provides a geometric loop of connected stations. The compass supplies magnetic direction while taping or another assigned method supplies side length. Direction observations must be checked before coordinate computations are trusted because local attraction can affect all directions observed at a station.
Reconnaissance and Station Selection
Station Requirements
- Successive stations are mutually visible.
- Stations are stable, recoverable, and clearly identified.
- Traverse sides avoid unnecessary pedestrian or vehicle conflict.
- Compass stations are kept away from obvious magnetic sources where practical.
- The polygon provides a closed circuit back to the starting station.
Field Procedure
Compass Traverse Observation
- Reconnoiter the site and mark stations A, B, C, D, or the assigned station sequence.
- At the first station, center and level the compass as required by the instrument.
- Sight the next station and record the fore direction exactly as read.
- Measure the side length and record the method and units.
- Move to the next station and observe the back direction of the line just measured.
- Observe the fore direction to the next station.
- Continue around the traverse until the final course returns to the starting station.
- Before leaving, compare every fore/back pair and investigate large deviations from the expected 180° relationship.
- Record nearby magnetic objects or site conditions that may explain local attraction.
Bearing Convention
State whether the field book uses quadrantal bearings or whole-circle azimuths. Do not mix conventions within one table. Convert only in a separate reduction column when required.
Latitude of a Traverse Course
North-south component from course length and azimuth.
Variables
| Symbol | Description | Unit |
|---|---|---|
| course length | - | |
| azimuth measured clockwise from north | - |
Departure of a Traverse Course
East-west component from course length and azimuth.
Variables
| Symbol | Description | Unit |
|---|---|---|
| course length | - | |
| azimuth measured clockwise from north | - |
Linear Error of Closure
Magnitude of the preliminary traverse-coordinate misclosure.
Relative Precision
Ratio of total traverse length to linear error of closure.
Magnetic North Is Not Fixed
Compass directions are magnetic observations. They are not automatically true, grid, or cadastral bearings. Magnetic declination and local attraction must be distinguished, and the reference meridian used in the report must be stated.
Before Leaving the Site
- Every traverse station is recoverable and labeled.
- Every side has a measured length.
- Every line has the required fore and back direction observations.
- Potential magnetic disturbances are noted.
- The station sketch agrees with the field-book sequence.
- No raw bearing has been overwritten by a corrected bearing.
- Sufficient observations exist to complete the traverse reduction.
Submission Timing
Fieldwork Data is due within the scheduled 3-hour class period. The Full Fieldwork Report is due on the next class meeting. FW00 remains authoritative for field safety, raw-data integrity, and submission requirements.
FW07 — Required Data Tabulation
Use this structure as the minimum field or research data table. Add columns when the activity requires more detail.
Continue for the full traverse. If local attraction is diagnosed, preserve observed bearings and show corrected bearings in separate columns.
FW07 — Grading Rubric
Award points according to the evidence shown for each criterion.
| Criterion | Full-credit evidence | Points |
|---|---|---|
| Reconnaissance, compass use, and safety | Traverse stations are suitable; compass is handled correctly; magnetic interference and site hazards are actively considered. | 5 |
| Distance and bearing observations | All course lengths, fore bearings, back bearings, station IDs, and field notes are complete. | 6 |
| Local-attraction diagnosis and corrections | FB/BB relationships are checked, affected stations are correctly identified, and corrections are justified without overwriting raw data. | 6 |
| Traverse checks and plotting | Corrected directions are coherent, angular/graphical checks are completed, and the traverse sketch/plot is consistent. | 4 |
| Report quality and traceability | Observed versus corrected values, assumptions, magnetic conditions, field sketch, and conclusions are clearly documented. | 4 |
| Total | 25 / 25 | |
Scoring guide: full credit for complete, technically correct, traceable evidence; reduce proportionally for minor errors, incomplete work, or weak documentation.
- Fore/back checks are essential for recognizing local attraction.
- Corrected bearings belong in a separate reduction record; raw compass readings remain unchanged.
- A closed traverse permits quantitative closure checks after direction and distance reduction.
- Compass work provides useful field-control practice but does not replace higher-precision control methods when those are required.