Traverse comes from a Latin word meaning “passing across,” and that is exactly what a traverse survey does: it passes across the ground as a connected series of measured lines. Surveyors record the length of every line with a tape or chain and the direction of every line with a compass or theodolite, building a framework of known points for later detail surveys. Those two measurements, distance and direction, provide the core two-dimensional data needed to compute land areas and to establish horizontal control. When the results of a leveling network for the same site are added to the traverse, the survey becomes a three-dimensional representation of the ground surface. Traversing remains one of the most widely used surveying methods in construction, and modern GPS receivers using RTK and PPK surveying technologies have made control surveys faster without changing the underlying geometry.
What Is Traversing in Surveying?
Traversing in surveying is the operation of determining the lengths and directions of consecutive lines that connect a series of survey stations. A traverse is run for two general purposes: to locate features that already exist in the area, such as buildings, boundaries, and utility lines, and to establish new points by predetermined measurements, which is how setting out works on construction sites. The fieldwork has three parts: reconnaissance, picking up details, and booking the field notes, the permanent record of every reading. Because traversing fixes positions in the horizontal plane only, a leveling survey is run alongside it whenever elevations are needed, and the types of leveling in surveying range from simple differential leveling to precise trigonometric methods.
Principle of Traversing
If the length and direction of every line in a chain of connected lines are known, the position of every point in the chain can be computed. Starting from a station of known coordinates, each new station is located by adding the distance and direction of the next leg to the previous station. This step-by-step process, called running a traverse, continues station by station until the loop is complete.
Quantities Measured During Traversing
Two quantities are measured for every leg: the horizontal distance and the angle or bearing that fixes its direction. Every other value in a traverse, from station coordinates to land area, is derived from these two.
Horizontal Distance
Horizontal distances are measured with a steel tape or chain on short traverses, with a total station using electronic distance measurement on longer ones, and with GPS receivers on open terrain. Distances are always reduced to their horizontal equivalent, so a slope distance measured on a hillside is corrected for the angle of inclination before booking.
Types of Traverse: Open and Closed
Every traverse is one of two types: open or closed. An open traverse starts at a known point and ends at a point checked by no other measurement. A closed traverse either returns to the starting station or starts and ends at two separately fixed points, so its geometry can be checked.
Open Traverse
An open traverse does not return to its starting point. It cannot be checked or adjusted because no second fixed point exists to close against. Open traverses are the standard method for surveying long, narrow strips of country, including the path of a highway, railway, canal, pipeline, coastline, and transmission line. Field errors accumulate without detection, so surveyors keep legs short and measure each one twice.
Closed Traverse
A closed traverse returns to its starting point, forming a loop, or it starts and finishes at two different stations whose coordinates are already known, a form called a link traverse. Because the final position can be computed independently, the closing error is measurable and the traverse can be adjusted. Closed traverses are used for boundary surveys, site surveys, and control networks that construction will be built from.
| Feature | Open Traverse | Closed Traverse |
|---|---|---|
| Return to starting point | No | Yes, as a loop or as a link between fixed points |
| Independent check | None | Closing error is measured |
| Adjustment | Not possible | Graphic, Bowditch, or transit method |
| Typical use | Highways, railways, canals, pipelines, transmission lines | Boundaries, site control, setting out |
Long linear projects often leave no place to close, so an open traverse is the only option, while site control is almost always a closed traverse. The distinction also shapes how the earth’s curvature is treated: the choice of plane surveying vs geodetic surveying decides whether the ground can be treated as flat, the usual assumption for closed traverses on construction sites.
Methods of Traverse Surveying
Traverse surveys are named after the instruments used to measure directions. Four methods dominate practice: chain traversing, compass traversing, theodolite traversing, and plane table traversing.
Chain Traversing (By Chain Angle)
In chain traversing, all lines are measured with a chain and tape, and angles are fixed by chain offsets or by measuring the sides of triangles. The method belongs to the family of chain surveying, in which the whole framework is built from linear measurements alone. It suits small, open, fairly level sites where nothing blocks the line of sight.
Compass Traversing (By Free Needle)
Compass traversing measures the direction of each line with a prismatic or surveyor’s compass, reading the magnetic bearing directly from the free needle. It is fast and inexpensive, and its accuracy depends on keeping the needle away from local magnetic attraction. Each bearing is measured independently, so an error in one line does not carry into the next.
Theodolite Traversing (By Fast Needle)
The theodolite measures the included angle between successive lines, or the bearing of a line by referencing a fixed bearing on the backsight, a procedure called the fast needle method. Theodolite traversing is more accurate than compass work and is the standard choice for control surveys. A total station performs the same job with electronic angle reading and distance measurement in one instrument.
Plane Table Traversing (By Fast Needle)
Plane table traversing plots the traverse directly on paper in the field. The table is oriented at each station, rays are drawn toward the next station, and distances are scaled along the rays. It is a graphical method, quick for small areas, checked as it is drawn.
Traverse Procedure: From Reconnaissance to Plotting
Running a traverse follows a fixed sequence:
- Reconnaissance: walk the site and plan the lines.
- Station selection: choose, mark, and reference each station.
- Linear measurement: measure every leg twice.
- Angular measurement: read angles at every station.
- Booking: enter all readings in the field notes with sketches.
- Plotting and adjustment: compute coordinates and close the traverse.
Linear and Angular Measurements
Leg lengths are measured twice when accuracy matters, and the two readings are compared before the mean is booked. Angles are read with the instrument in both face positions to cancel instrumental errors. Line directions are usually expressed as bearings, and the field procedures of bearings and compass surveying remain the fastest way to establish a preliminary control network before the main angular work begins.
Plotting the Traverse
Once the fieldwork is booked, the traverse is plotted. Two approaches exist: the distance and angle method, in which each leg is drawn with a protractor and scale, and the coordinate method, in which each station’s latitude and departure are computed and plotted from a single origin. Coordinate plotting is preferred because it exposes errors and feeds directly into area calculations.
Coordinate Method
In the coordinate method, the latitude of a leg is its projection on the north-south axis and the departure is its projection on the east-west axis. Each station’s coordinates come from adding the latitudes and departures of the legs in sequence, plotted from the coordinate grid. The method is more accurate and is the standard for engineering traverses.
Closing Error and Methods of Adjustment
However carefully the fieldwork is done, a closed traverse rarely returns exactly to its starting coordinates. The gap between the computed position of the closing station and its known position is the closing error, also called the error of closure. It has two parts: the linear closing error and the angular error, the difference between the measured angle sum and the theoretical sum.
Adjustment of Closing Error by Graphic Method
In the graphic method, the closing error is plotted as a vector and corrections are distributed by drawing parallels proportional to the distances from the starting station. It is quick for field checks but approximate, rarely used for final adjustment.
Bowditch’s Method
Bowditch’s method, also called the compass rule, distributes the correction to each latitude and departure in proportion to the length of the corresponding leg, so long legs receive larger corrections. It is the most commonly used adjustment for traverses because it works well when the angular and linear errors are of similar size.
Transit Method
The transit method distributes the correction in proportion to the latitude and departure of each station from the starting point rather than to leg lengths. It suits surveys where the angular measurements are more accurate than the linear ones, concentrating the correction on the longer legs.
| Method | Correction basis | When to use |
|---|---|---|
| Graphic method | Vector plot of the closing error | Quick field checks |
| Bowditch (compass rule) | Leg length | General control surveys |
| Transit method | Distance from the starting station | Angular work more accurate than linear work |
Applications and Importance of Traversing
Traversing underpins most engineering surveys. It establishes control for roads, railways, pipelines, bridges, buildings, and boundaries, and it is the framework onto which detail surveys and setting out are tied. It delivers horizontal control almost anywhere, without line of sight to distant triangulation stations, using anything from a tape and compass to a robotic total station.
Where Open and Closed Traverses Are Used
- Open traverses: highway centerlines, railway routes, canal and pipeline alignments, coastline mapping, and transmission line routes.
- Closed traverses: property boundaries, building site control, subdivision surveys, and control for detail mapping.
Modern Practice
Field practice has changed with electronics. Total stations read angles and distances in a single pointing, data loggers book the notes automatically, and adjustment software runs Bowditch and least squares solutions in seconds. GPS receivers now fix many stations directly, and automated data processing with AI in surveying is taking over the routine parts of computation, error checking, and traverse adjustment.
For small sites and quick jobs, the older methods still earn their keep. A two-person crew with a steel tape can run a boundary check faster than a GPS receiver warms up, and the field arithmetic of traditional chain survey work remains the best way to understand what the instruments are doing. Whichever method is chosen, the goal is the same: a closed, checked, adjusted network the next surveyor can trust.