Traverse Design, Layout, and Field Observations

Traverse Design, Layout, and Field Observations

A traverse is a series of connected lines whose lengths and directions are measured in the field. When the last line returns to the starting point the traverse is closed-loop (or closed-polygon). Closed-loop traverses are the backbone of control surveys because they allow you to check your own work mathematically before you leave the site.

Planning the Traverse Network

Before you pick up the total station, spend time on reconnaissance. Walk the area and identify five or more hub locations that satisfy three rules:

1. Intervisibility — you must be able to sight from any station directly to its two neighbors without obstruction.

2. Geometry — avoid very acute interior angles (less than about 30°); poorly shaped figures amplify errors.

3. Stability — hubs should be on firm, undisturbed ground away from foot traffic and vehicle paths.

Once you have chosen the locations, pace the distances between adjacent hubs and sketch the network in your field book. Label each hub (A, B, C, D, E for a five-station loop) and record your paced estimates. These estimates are your first check: if a measured distance later differs from your pace by more than 10 %, look for a blunder.

Setting Hubs and Flagging

Drive a wooden hub flush with the ground surface or just proud enough to accept a tack on top. A tack or fine pencil mark on the hub top defines the exact point. Flag each hub with a visible stake and surveyor's flagging so the rod person and instrument operator can find it quickly. Clear any tall grass or brush in the line of sight between adjacent stations.

Occupying Each Station — The Booking Routine

Set up the total station over the hub using the optical or laser plumb and level the instrument carefully. The centering error at setup is a systematic error that cannot be removed in post-processing, so take the time to center precisely — typically within 1–2 mm of the hub mark.

For each station follow this sequence:

1. Backsight the previous station. Set the horizontal circle to zero (or to any convenient reference value) on the backsight.

2. Measure the interior angle by direct and reverse face:

• Face Left (Direct): Read the horizontal angle to the foresight. Record.
• Plunge (transit) the telescope: Rotate 180° in azimuth to Face Right (Reverse). Re-sight the foresight. Read the horizontal angle. Record.
• Average the two faces. Double-face observation cancels most instrumental errors, including collimation error and horizontal axis tilt.

3. Measure slope distance to the foresight hub. Record the slope distance and the vertical angle so you can later reduce to horizontal distance.

4. Record the instrument height (HI) and reflector height (HR) for every setup — these are required for elevation calculations even if today's task is planimetric.

Enter every value in the traverse booking form at the time of observation, in pencil, with no erasures. If you make an error, draw a single line through it and write the correct value beside it.

Computing Angular Misclosure in the Field

For a closed polygon, the theoretical sum of interior angles is:

> (n − 2) × 180°

where n is the number of sides (stations). For five stations: (5 − 2) × 180° = 540°.

After completing the loop, add up your averaged interior angles. The difference between your sum and the theoretical value is the angular misclosure.

Acceptable misclosure depends on instrument class and project specification. A common rule of thumb for a 5-second theodolite or total station on a control traverse is:

> Tolerance = ±(instrument least count) × √n

For a 5" instrument with five stations: ±5" × √5 ≈ ±11". Always check the project specification, which takes precedence. If misclosure exceeds tolerance, re-observe the suspect station before leaving — re-mobilizing to the site costs far more than a few extra minutes on site.

Safety on the Survey Site

• PPE: Hard hat and high-visibility vest are required whenever there is any possibility of vehicle traffic. Steel-toed boots protect against dropped tribrachs and prism poles.
• Traffic control: If work is adjacent to a roadway, follow the project's traffic control plan. Never assume drivers see you.
• Tripod stability: Always carry the total station secured in the case or cradled upright with legs spread — never slung over one shoulder while the instrument is attached.
• Sun and heat: Work outdoors in summer heat demands hydration, sunscreen, and recognition of heat exhaustion symptoms in yourself and your crew.
• Electrical storms: Total stations and prism poles are metal and elevated. At the first sign of lightning, break down equipment and move to a vehicle or structure.