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newSchoolWorks.com › VoTechWorks › Technology & Design › GIS / Surveying / Drone Operations › Total Station Surveying › Lesson 12
TEC-GIS45-04 · Lesson 12 · 10 clock hours · lab

Instrument Care, Calibration Checks, and Error Sources

Objective. Students perform the standard two-peg test for line-of-sight error, check the horizontal circle index, and carry out daily cleaning and storage procedures per the manufacturer's maintenance guide.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Instrument Care and Calibration Matter

A total station is a precision optical and electronic instrument capable of measuring angles to single-arc-second accuracy and distances to millimeter precision. That accuracy is worthless if the instrument is out of adjustment, dirty, or damaged. Surveying codes of practice and employer quality-assurance programs expect technicians to verify instrument condition at the start of every field day and to record the results. The NSPS CST program tests your ability to identify and correct common instrument errors before they corrupt fieldwork.

The Two-Peg (Collimation) Test

The most important daily check for a total station is the two-peg test, which reveals line-of-sight error (also called collimation error). This error exists when the line of sight through the telescope is not perfectly perpendicular to the tilting axis. It causes every vertical-angle and slope-distance reading to carry a small, systematic bias.

Setup procedure:

1. Drive two stakes (Peg A and Peg B) exactly 50 m apart on level or gently sloping ground.

2. Set up and level the instrument at the midpoint between the pegs.

3. In Face Left (direct), sight Peg A and then Peg B, reading and recording the vertical angle to each.

4. Transit (flip) to Face Right (reverse) and repeat the readings.

5. Move the instrument to a position close to one peg (within 2–3 m of Peg A) and repeat Face Left and Face Right readings to Peg B.

Calculating the error: The line-of-sight error in seconds is derived from the difference between the mean Face Left reading and the mean Face Right reading (corrected for 180°). Most manufacturers publish an acceptable collimation error limit in their maintenance manual—commonly in the range of a few seconds of arc, but always defer to the specific instrument's documentation. If the computed error exceeds the manufacturer's stated limit, the instrument must be returned to a certified repair shop; field users do not adjust internal optical alignments.

Record all readings, the computed error, the manufacturer's limit, and a pass/fail statement on the two-peg test booking form.

Checking and Adjusting the Plate Bubble

The plate bubble (or electronic tilt sensor) must be centered for the vertical axis to be truly vertical. Use the quarter-turn method:

1. Level the instrument using the footscrews until the bubble is centered.

2. Rotate the alidade 90° (one quarter turn).

3. If the bubble moves off center, it is out of adjustment.

4. Use the footscrews to remove half the apparent error, then use the bubble-adjustment capstan screws (per the manufacturer's diagram) to remove the remaining half.

5. Rotate back 90°, check again, and repeat until the bubble stays centered through a full 360° rotation.

Record whether adjustment was needed and what correction was applied.

Horizontal Circle Index Check

The horizontal circle index (orientation) can drift if the instrument is jolted. To check it, sight a clearly defined target, record the circle reading in Face Left, transit to Face Right, re-sight the same target, and record again. The two readings should differ by exactly 180°. Any consistent discrepancy indicates an index error that must be noted and may need a service technician if it persists after the face-mean approach fails to cancel it.

Daily Cleaning Procedures

Dirt and moisture on optics scatter light and degrade pointing accuracy. Follow these steps every day before use and before storage:

Storage and Transport

Always pack the instrument exactly as shown in the manufacturer's storage diagram:

1. Lower the telescope to the storage position.

2. Loosen clamps before placing the instrument in the case (prevents stress on the axes).

3. Align the instrument to the foam cutout—forcing it in at the wrong angle can crack the tribrach.

4. Secure all accessories (battery, charger, reflector) in their labeled compartments.

5. Latch the case; carry by the handle, not by the case latches.

Do not store the instrument in a vehicle trunk in high-heat conditions; temperature extremes affect electronic components and can shift calibration.

Safety Points

Connecting to Credentials

The two-peg test and bubble adjustment are tested in the NSPS CST Level 1–2 examination. Understanding systematic error sources—and how instrument checks reduce them—is foundational knowledge for any survey technician. Clean, well-maintained instruments also produce the high-quality point clouds and control data that Esri ArcGIS Pro workflows depend on downstream.

Key terms8

Line-of-sight error (collimation error)The angular error that exists when the telescope's line of sight is not perpendicular to the tilting axis; detected and quantified by the two-peg test.
Two-peg testA field procedure in which the instrument is set up at the midpoint and then near one end of a measured baseline to isolate and compute collimation error.
Face Left / Face Right (Direct / Reverse)The two 180°-opposed positions of the telescope used to average out or identify systematic instrument errors; also called Direct and Reverse.
Plate bubbleA spirit level mounted on the alidade used to confirm the instrument's vertical axis is truly vertical before observations are taken.
Quarter-turn methodA leveling check in which the instrument is rotated 90° increments to test whether the plate bubble remains centered, revealing any bubble maladjustment.
TribrachThe base plate assembly with three footscrews that mounts between the tripod head and the instrument body, used for leveling and centering over a point.
Horizontal circle index errorA systematic error in horizontal circle readings caused by the zero graduation not being precisely aligned with the instrument's reference direction.
Manufacturer's acceptable limitThe maximum collimation or bubble error, stated in arc seconds in the instrument's maintenance manual, within which the instrument is considered field-serviceable without return to a shop.

Worked examples4

1. During a two-peg test, the mean Face Left vertical angle to Peg B from the mid-point is 91° 00′ 08″ and the mean Face Right vertical angle (reduced to the same sense) is 90° 59′ 52″. Calculate the collimation error in seconds.
  1. Write down the two mean readings: Face Left = 91° 00′ 08″; Face Right = 90° 59′ 52″.
  2. Find the difference: 91° 00′ 08″ − 90° 59′ 52″ = 0° 00′ 16″.
  3. The collimation error is half the difference (because each face contains one instance of +c and one of −c): 16″ ÷ 2 = 8″.
  4. Compare 8″ to the manufacturer's stated acceptable limit. If the limit is, for example, 10″, the instrument passes; if the limit is 5″, the instrument must be returned for service.
Answer: Collimation error = 8 arc seconds.
2. You move the total station to a position 2 m from Peg A (50 m from Peg B). In Face Left you read a vertical angle of 89° 59′ 54″ to Peg B; in Face Right the reduced reading is 90° 00′ 06″. What is the collimation error, and does it agree with the mid-point result of 8″?
  1. Face Left = 89° 59′ 54″; Face Right = 90° 00′ 06″.
  2. Difference = 90° 00′ 06″ − 89° 59′ 54″ = 0° 00′ 12″.
  3. Collimation error = 12″ ÷ 2 = 6″.
  4. The near-end result (6″) and mid-point result (8″) are close but not identical because small pointing and reading errors exist in each set; the two values bracket the true collimation error. Average the two estimates: (8″ + 6″) ÷ 2 = 7″ as the best estimate. Record both sets of observations on the booking form.
Answer: Near-end collimation error ≈ 6″; best combined estimate ≈ 7″.
3. During the quarter-turn bubble check, the bubble appears 4 divisions off center after a 90° rotation. Using the half-correction rule, how many divisions should you remove with the footscrews, and how many with the adjustment capstan screws?
  1. Total observed error = 4 divisions.
  2. Half-correction with footscrews: 4 ÷ 2 = 2 divisions. Bring the bubble 2 divisions toward center using the footscrews.
  3. Remaining error = 4 − 2 = 2 divisions off center.
  4. Remove the remaining 2 divisions using the bubble adjustment capstan screws (not the footscrews) to physically re-center the bubble vial.
  5. Rotate back 90°, verify the bubble stays centered. Repeat if needed.
Answer: Remove 2 divisions with footscrews, then 2 divisions with the adjustment capstan screws.
4. A technician sights a target in Face Left and reads a horizontal circle value of 34° 17′ 22″. In Face Right the reading to the same target is 214° 17′ 30″. What is the horizontal circle index error?
  1. Face Right reading reduced to the Face Left sense: 214° 17′ 30″ − 180° = 34° 17′ 30″.
  2. Difference between Face Left and reduced Face Right: 34° 17′ 30″ − 34° 17′ 22″ = 8″.
  3. Index error = 8″ ÷ 2 = 4″.
  4. Record 4″ as the horizontal circle index error and note whether it falls within the manufacturer's tolerance.
Answer: Horizontal circle index error = 4 arc seconds.

The math in this lesson1

Learners halve the 16-second difference between face-left and face-right readings to find an 8-second collimation error.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Execute the two-peg (collimation) test: set two stakes 50 m apart, level the instrument mid-way and at one end, record the line-of-sight error in seconds, and compare to the manufacturer's acceptable limit · Check the plate bubble adjustment using the quarter-turn method and record whether the bubble requires adjustment; demonstrate correct field adjustment if out of tolerance · Clean the optics using lens tissue and isopropyl alcohol only, inspect and clean the tribrach centering cone, and pack the instrument into the carrying case following the manufacturer's storage diagram
Materials: Total station, manufacturer's maintenance manual, two-peg test booking form, lens tissue, isopropyl alcohol, soft brush, instrument case, stakes and tape for test setup
Check: Two-peg test results recorded and compared to spec; cleaning and storage observed by instructor—all steps performed in correct sequence for pass

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NSPS CST – Instrument Adjustment and Error Sources
1. What is the primary purpose of the two-peg test?
NSPS CST – Instrument Adjustment and Error Sources
2. During a two-peg test, the Face Left vertical angle mean to the far peg is 90° 00′ 18″ and the Face Right mean (reduced) is 89° 59′ 46″. What is the collimation error?
NSPS CST – Instrument Adjustment and Error Sources
3. When the plate bubble is found to be 6 divisions off center during the quarter-turn check, how many divisions are corrected using the footscrews?
NSPS CST – Equipment Care and Maintenance
4. Which material is correct for cleaning smears from a total station's objective lens?
NSPS CST – Instrument Adjustment and Error Sources
5. Which action should a field technician take if the computed collimation error exceeds the manufacturer's stated limit?
NSPS CST – Instrument Adjustment and Error Sources
6. What does a horizontal circle index error of 0° 00′ 00″ between Face Left and Face Right (corrected for 180°) indicate?
NSPS CST – Field Safety
7. Which PPE requirement is most critical when setting up a total station along an active roadway?
NSPS CST – Instrument Adjustment and Error Sources
8. A technician completes a two-peg test at the midpoint and calculates a collimation error of 9″. She then performs the test from the near-end position and computes 11″. The manufacturer's acceptable limit is 10″. What is the best course of action?
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