Traverse Surveys

Part of Cartography & Surveying

How to conduct traverse surveys — the systematic measurement of connected lines and angles — to precisely locate points and produce accurate maps.

Why This Matters

A traverse survey is the workhorse of practical land surveying. Where triangulation requires elevated viewpoints and clear sightlines to distant features, traversing simply follows the ground — along roads, boundaries, and paths — measuring each leg’s direction and distance. It is the method used to map property boundaries, survey road routes, locate buildings, and establish the control network that supports all other surveying work.

The power of a traverse lies in its self-checking nature. A closed traverse — one that returns to its starting point or connects two known points — allows you to detect accumulated errors. This quality control distinguishes professional survey work from guesswork, even when using simple instruments.

Mastering traverse surveys means mastering the core technique from which most practical mapping flows.

Types of Traverses

Closed loop traverse: Begins and ends at the same known point. After measuring all the legs, the computed closing position should match the starting position. The difference is the closure error.

Closed link traverse: Begins at one known point and ends at a different known point. The traverse is “closed” by having known positions at both ends. Less effort than a loop but requires two independent control points.

Open traverse: Begins at a known point with no return to a known endpoint. Cannot be checked for angular or linear error. Acceptable only for preliminary reconnaissance — never for maps that will be used for planning or legal purposes.

Radiation: A hybrid method where multiple points are located by measuring direction and distance from a single occupied station. Not a true traverse (no chain of connected legs) but useful for quickly mapping scattered features within sight of one position.

Equipment Checklist

Before starting a traverse:

• Compass or theodolite (calibrated, known declination if magnetic)
• Measuring tape or chain (known length, recently checked)
• Chaining pins (10–20)
• Ranging rods (3–4 minimum)
• Plumb bob
• Field book and pencil
• Tripod (for theodolite)
• Hand level or clinometer (for slope correction)
• Calculator or trig tables (for office reduction)

Field Procedure

Step 1 — Reconnaissance: Walk the traverse route before measuring. Identify each station location (stable, with clear sightlines to adjacent stations). Avoid locations under power lines, near metal objects, or with obscured sightlines for important angles.

Step 2 — Station setup: At each station, set up the instrument over a precise ground mark. For a compass traverse, the mark can be a chaining pin pushed into the ground; for a theodolite traverse, use a more permanent mark (wooden peg, spike, painted mark).

Step 3 — Angle measurement at each station:

• Face the backsight (previous station) and read the instrument.
• Face the foresight (next station) and read the instrument.
• The difference is the interior angle at this station, or alternatively, record the absolute bearing directly.
• Repeat the measurement at least twice and average, checking for consistency.

Step 4 — Distance measurement:

• Measure each leg while the angle reading is still fresh (reduces chance of misidentifying which leg the measurement belongs to).
• Note the slope if steep; measure or calculate the horizontal equivalent.
• Record the distance twice (forward and back if time allows) and average.

Step 5 — Notes: Record station number, backsight direction and reading, foresight direction and reading, leg distance, slope angle (if measured), and a brief description of each station for future reference (“iron pin at NE corner of Smith field”).

Computing a Traverse

Raw field measurements become useful positions through calculation:

Bearing calculation: Starting from a known bearing (from a fixed reference direction or the bearing of the first leg from a known azimuth), add or subtract the measured turning angles at each station to obtain the bearing of each successive leg.

Latitude and departure: For each leg, compute:

• Latitude (northing change) = distance × cos(bearing)
• Departure (easting change) = distance × sin(bearing)

Be careful with signs: north and east are positive; south and west are negative. In bearing notation (e.g., S45°E), the sine/cosine signs follow the bearing’s quadrant.

Running coordinates: Starting from the known starting coordinates, add each leg’s latitude and departure to obtain the running coordinates of each station.

Closing check: At the end of the traverse, compare the computed final position with the known closing position. The difference in northing and easting is the linear misclosure.

Closure ratio: Calculate the total traverse length (sum of all leg distances). The closure ratio = misclosure distance / total traverse length. A ratio of 1:500 means the closure error is 0.2% of the total distance. Generally acceptable standards: 1:200 for rough work, 1:1,000 for topographic mapping, 1:5,000 for property surveys.

Distributing Misclosure

If the closure is within acceptable limits, distribute it proportionally across all stations before using the coordinates for mapping.

Bowditch rule (most common): Distribute the misclosure in proportion to each leg’s length as a fraction of the total traverse length.

Correction to northing of a station = (total northing misclosure) × (cumulative length to station / total length)

Apply the same logic to eastings.

After distribution, recompute all station coordinates using the corrected values. The corrected traverse exactly closes on the known endpoint.

When misclosure is too large: Do not distribute it — find the error. Re-measure the angles and distances of the legs where the error is most likely (long legs amplify small angular errors; short legs require precise distance measurement). Compare field measurements against expected geometry. Recalculate each leg systematically.

Connecting to Existing Control

Whenever possible, tie the traverse to existing control points rather than starting from scratch.

Orientation check: From the first station of your traverse, take a bearing to an existing control point. Compare with the calculated bearing from the known coordinates of both points. The difference is the orientation error of your traverse.

Position check: Occupy a known control point with your traverse if possible. The computed position of that station from your traverse should agree with its known coordinates. Discrepancy indicates either traversing error or an error in the control point’s recorded position.

Adding new control: After computing and adjusting your traverse, the stations become new control points, available for future surveys. Add them to the community’s control network documentation with their coordinates, descriptions, and the date and method of their determination.

Traverse surveying, done systematically and checked rigorously, is the most versatile single technique in the surveyor’s toolkit — and the foundation on which all community mapping rests.