Setup & Calibration

Overview#

The accuracy of every field measurement depends on proper instrument setup and calibration. A total station capable of 1-arc-second angular accuracy delivers meaningless results if it is poorly leveled, miscentered, or operating with an unchecked collimation error. A GNSS base station on the wrong antenna height produces coordinates that are systematically wrong on every point.

Setup and calibration are not administrative overhead -- they are the foundation of measurement quality. This guide covers the procedures, checks, and routines that every field surveyor should follow.

"The best instrument in the world cannot compensate for poor setup procedures. Centering, leveling, and calibration are non-negotiable steps that directly affect the quality of every measurement taken during the day." -- Kavanagh & Mastin, Surveying: Principles and Applications (9th Ed.), Ch. 5, p. 132

Tripod Setup Over a Point#

Procedure

Setting up a tripod over a survey point is the most basic and most frequently performed task in conventional surveying. A systematic approach prevents the frustration of repeated adjustments:

1. Position the tripod -- Spread the legs to form a roughly equilateral triangle with the center approximately over the point. The head should be approximately level and at a comfortable working height.
2. Push the legs firmly into the ground. On hard surfaces, use leg point pads or weights to prevent slipping.
3. Rough center -- Look past the tripod head (or through the optical plummet) to verify the point is approximately below the center of the head.
4. Mount the tribrach or instrument on the tripod head and secure the centering screw.
5. Level the circular (bull's eye) bubble by adjusting the tripod leg lengths. Do not move the legs laterally at this stage -- only extend or retract them.
6. Center precisely using the optical or laser plummet. Slide the tribrach on the tripod head (loosen the centering screw slightly) until the plummet is directly over the point.
7. Level the plate bubble using the leveling screws (foot screws). Follow the standard leveling procedure (see below).
8. Re-check centering -- Leveling may shift the centering slightly. Iterate between centering and leveling until both are satisfied simultaneously.

The iterative nature of centering and leveling is normal. Typically two to three iterations are sufficient. If you find yourself iterating more than that, the tripod setup is probably poor -- start over with better leg placement.

Centering Tolerance

For most conventional survey work, centering accuracy should be within 1 mm of the point. For precise control work, centering within 0.5 mm is the target. The centering accuracy achievable depends on the plummet type:

Forced Centering#

Forced centering (also called constrained centering) is a technique that eliminates centering error when multiple setups are made at the same point. The tribrach remains on the tripod, centered and leveled over the point. Only the instrument or target is changed by lifting it from the tribrach and replacing it with another device.

When to Use

• Traverse stations where both a target and an instrument must occupy the same point.
• Control surveys where multiple instrument types (e.g., total station then GNSS antenna) are used on the same point.
• Any situation where re-centering would introduce unnecessary error.

Procedure

1. Set up the tripod and tribrach over the point. Center and level precisely.
2. Place the target (prism) in the tribrach. Lock the clamping lever.
3. When it is time to occupy the point with the instrument, unlock the lever, remove the target, and place the instrument in the tribrach. Lock the lever.
4. Re-check the plate bubble -- the instrument and target may have slightly different weights, but the tribrach centering remains unchanged.

Never loosen the tribrach from the tripod during a forced centering swap. The entire benefit of the technique depends on the tribrach remaining fixed on the tripod in its centered position.

Leveling the Instrument#

Plate Bubble Method

The plate (tubular) bubble is the primary leveling reference on a total station or theodolite. The standard procedure uses the three leveling screws:

1. Rotate the instrument so the plate bubble is parallel to a line connecting two of the three leveling screws (screws A and B).
2. Turn screws A and B simultaneously in opposite directions (thumbs moving toward each other or apart) to center the bubble.
3. Rotate the instrument 90 degrees so the bubble is perpendicular to the A-B line (now over screw C).
4. Turn screw C alone to center the bubble.
5. Rotate back to the original position and check. Repeat until the bubble remains centered in both positions.
6. Rotate 180 degrees from each position as a final check. If the bubble moves off center by more than one division, the bubble needs adjustment.

Electronic Level (Tilt Compensator)

Modern total stations have a dual-axis tilt compensator that measures and corrects for residual tilt in real time. The compensator typically has a range of $\pm 3'$ to $\pm 6'$. If the instrument is leveled to within this range, the compensator applies corrections to horizontal and vertical angles automatically.

However, the compensator is not a substitute for proper leveling. Operating near the edge of the compensator range reduces measurement reliability. Always level the instrument so the compensator reading is as close to zero as possible.

Total Station Setup Procedures#

Standard Setup Sequence

1. Set up and level the tripod over the point.
2. Mount the instrument and precisely center/level.
3. Turn on the instrument and allow it to initialize (compensator settling, internal checks).
4. Enter the station point coordinates (or point ID for later assignment).
5. Measure and enter the instrument height (HI) -- slant height to the tilting axis mark or vertical height, as the instrument requires.
6. Sight the backsight point. Enter the backsight coordinates or set the azimuth.
7. Verify the backsight by measuring the distance and comparing to the known value. The difference should be within the expected accuracy for the distance.
8. Begin the survey.

Instrument Height Measurement

Errors in measuring the instrument height (HI) propagate directly into every elevation computed from that setup. Measure HI carefully:

• Use the manufacturer's specified measurement point (usually the tilting axis mark on the instrument).
• Measure with a calibrated tape or rod to the nearest millimeter.
• For critical work, measure on two sides of the instrument and average.
• Record the measurement in the field notes and in the data collector.

A 10 mm error in HI produces a 10 mm error in every elevation measured from that station. This is a systematic error that cannot be detected by measurement redundancy at that station. Always double-check HI.

GNSS Base Station Setup#

Procedure

1. Select a base station location with clear sky visibility above 15 degrees elevation on all sides. Avoid proximity to large structures, fences, and overhead wires.
2. Set up the tripod over the control point. Center and level carefully.
3. Mount the GNSS antenna on the tribrach (or direct-mount on the tripod).
4. Measure the antenna height to the antenna reference point (ARP) -- typically the bottom of the antenna mount. Record the measurement method (vertical, slant) and height to the nearest millimeter.
5. Enter the known coordinates of the control point in the base receiver, specifying the datum and coordinate system.
6. Configure the broadcast settings (radio channel, power, correction format -- RTCM 3.x is standard).
7. Begin broadcasting corrections.
8. Verify by occupying a second known control point with the rover and checking the coordinates. The differences should be within expected RTK accuracy.

Antenna Height -- Critical Detail

Antenna height errors are the most common source of systematic error in GNSS surveys. They affect every point measured from that base.

Always record which measurement method was used. A slant height entered as a vertical height (or vice versa) introduces a systematic vertical error on every point in the survey.

Level Setup and Peg Test#

Setting Up a Level

1. Extend the tripod legs to a comfortable viewing height and push firmly into the ground.
2. Mount the level on the tripod head.
3. Center the circular bubble by adjusting the leg lengths (not the foot screws on a self-leveling instrument).
4. The compensator will establish a truly horizontal line of sight once the circular bubble is centered.

The Peg Test (Two-Peg Test)

The peg test determines whether the line of sight of a level is truly horizontal -- that is, whether a collimation error exists. Collimation error causes the line of sight to slope slightly upward or downward from horizontal.

Procedure:

1. Set two points (A and B) approximately 30 m apart on relatively flat ground. Drive stakes or place turning points.
2. Setup 1: Set the level at the midpoint between A and B. Read both rods. Because the instrument is equidistant from both points, any collimation error affects both readings equally and cancels in the elevation difference.

$\Delta h_{\text{true}} = r_A - r_B$

3. Setup 2: Set the level near point A (within 2--3 m). Read both rods. The collimation error now affects the far rod (B) much more than the near rod (A).

$\Delta h_{\text{observed}} = r_A' - r_B'$

4. Compare: If $\Delta h_{\text{true}} \neq \Delta h_{\text{observed}}$, collimation error exists. The magnitude of the error is:

$e = \frac{(\Delta h_{\text{observed}} - \Delta h_{\text{true}})}{D}$

where $D$ is the distance from the second setup to the far rod.

5. If the error exceeds the manufacturer's specification (typically $\pm 2$ mm over 30 m), adjust the cross-hair reticle.

Run the peg test at least once per week during active leveling campaigns, and always after the instrument has been dropped, jarred, or transported over rough terrain.

Collimation Checks for Total Stations#

Horizontal Collimation (Line-of-Sight Error)

Horizontal collimation error causes the line of sight to deviate from the perpendicular to the horizontal axis. It is eliminated by measuring in both face-left and face-right and averaging, but it should still be checked and minimized:

1. Sight a well-defined target at approximately the same elevation as the instrument, at a distance of 100+ m.
2. Read the horizontal angle in face left (FL).
3. Transit the telescope and read the horizontal angle in face right (FR).
4. The theoretical difference is exactly 180 degrees. Any departure is twice the collimation error:

$c = \frac{(\text{FL} - \text{FR}) \pm 180°}{2}$

5. If $c$ exceeds $\pm 5"$ for most work (or $\pm 2"$ for precise control), adjust the reticle.

Vertical Collimation (Index Error)

Vertical index error causes vertical angle readings to be offset from true. Check by:

1. Read the vertical angle to a target in face left ($V_{\text{FL}}$).
2. Read in face right ($V_{\text{FR}}$).
3. For a zenith angle system, the sum should equal 360 degrees:

$i = \frac{(V_{\text{FL}} + V_{\text{FR}}) - 360°}{2}$

4. If $|i|$ exceeds the tolerance, apply the instrument's electronic adjustment routine.

EDM Calibration on a Baseline#

Purpose

EDM calibration verifies that the distance-measuring component of a total station is reading correctly. It checks for zero error (constant offset), scale error (proportional to distance), and cyclic error (periodic with the modulation wavelength).

Baseline Procedure

NGS maintains a network of calibration baselines across the country -- sets of monumented points at precisely known distances. The standard test procedure:

1. Set up the total station at each baseline pillar in sequence.
2. Measure the distance to every other pillar.
3. Compare the measured distances to the published values.
4. Compute the zero error and scale factor from the residuals.

A minimum of four pillars (six independent distances) is recommended to separate zero, scale, and cyclic errors.

Frequency

• Calibrate at least once per year under normal use.
• Calibrate immediately if the instrument has been repaired, dropped, or shows suspicious distance results.
• For instruments used on critical projects (e.g., deformation monitoring, tunnel alignment), calibrate at the beginning and end of the project.

Atmospheric Corrections#

Why They Matter

The speed of light through the atmosphere depends on temperature, pressure, and humidity. An EDM measures distance based on an assumed speed of light, and deviations from the assumed atmospheric conditions introduce systematic errors.

The atmospheric correction is expressed in parts per million (ppm):

$\Delta D = D \times \text{ppm} \times 10^{-6}$

Temperature and Pressure

Most total stations compute the atmospheric ppm correction internally when the operator enters the current temperature and pressure. The standard atmosphere assumed by most instruments is 15 degrees C and 1013.25 hPa (sea level).

Practical Guidelines

• Measure temperature and pressure at the instrument location at the start of work and whenever conditions change significantly.
• Use a quality field thermometer (aspirated or shaded) -- not the instrument's internal temperature sensor, which reads the instrument body temperature, not the air temperature.
• For distances under 500 m, atmospheric corrections are typically less than 1 mm and can be ignored for low-precision work.
• For long-distance EDM work (1 km+), accurate atmospheric data is essential. Consider measuring conditions at both ends of the line and averaging.

At 1 km distance, a 10 degrees C error in the assumed temperature produces approximately a 10 mm distance error. Always measure atmospheric conditions -- do not guess or use yesterday's values.

Routine Maintenance Schedule#

Daily Checks

• Inspect tripod legs for tightness and worn points.
• Check tribrach clamp and optical/laser plummet function.
• Clean lenses with a blower and lens tissue (never wipe a dry, dusty lens).
• Verify battery charge level. Charge or swap batteries as needed.
• Check data collector screen, buttons, and Bluetooth connectivity.
• Test communication between total station and data collector (or rover and base).

Weekly Checks

• Perform a peg test on levels.
• Check horizontal and vertical collimation on total stations.
• Inspect prisms for chips, cracks, or loose housings.
• Verify prism constant setting matches the prism in use.
• Test GNSS antenna cables for damage or loose connectors.
• Back up data collector files to office storage.

Monthly Checks

• Clean and inspect all carrying cases and latches.
• Check radio/cellular modem antennas and power cables.
• Inspect survey rods and tapes for damage, bent sections, or illegible graduations.
• Verify firmware versions and update if needed.
• Review and replenish field supply kit (batteries, spray paint, flagging, nails, caps).

Annual Checks

• Send total stations for factory calibration and certification.
• Calibrate EDM on an NGS baseline.
• Send levels for factory collimation check and certification.
• Replace worn tripod shoes and leg clamps.
• Replace battery packs that no longer hold adequate charge.
• Review and update field procedure documentation.

Maintenance Log

Maintain a log for each instrument that records:

Key Takeaways#

• Centering and leveling are iterative and must both be satisfied before taking measurements. Target 1 mm centering accuracy for standard work, 0.5 mm for precise control.
• Forced centering eliminates centering error during traverse and should be standard practice for all control work.
• Instrument height errors propagate to every measurement from a station. Measure carefully, record clearly, and double-check.
• GNSS antenna height is the most common source of systematic error in GNSS surveys. Always record the measurement method (vertical vs. slant).
• Peg tests should be performed weekly on levels. Collimation checks should be performed weekly on total stations during active use.
• EDM calibration on an NGS baseline should be done at least annually and after any instrument incident.
• Atmospheric corrections become significant at longer distances. Always measure temperature and pressure -- do not estimate.
• Routine maintenance prevents equipment failures that cost far more in lost field time than the time spent on the checks themselves.

References#

1. Ghilani, C.D. & Wolf, P.R. Elementary Surveying: An Introduction to Geomatics (13th Ed.). Pearson, 2012. Chapters 6--8, 11.
2. Kavanagh, B.F. & Mastin, T.B. Surveying: Principles and Applications (9th Ed.). Pearson, 2014. Chapters 4--6.
3. National Geodetic Survey. "Calibration Base Line Information." NOAA/NGS. https://geodesy.noaa.gov/CBLINES/
4. Caltrans. Surveys Manual. California Department of Transportation. Chapter 6 -- Instruments and Equipment.
5. Leica Geosystems. Total Station User Manuals -- Instrument calibration and adjustment procedures.
6. Trimble. GNSS Receiver User Guides -- Base station setup and antenna height measurement.