Six Precautions before Conducting Tunnel Surveys with a Total Station

Table of Contents

• Prerequisites to check before using an optical total station for tunnel surveying

• Verify the reliability of underground reference points and known points.

• Determine the line of sight and installation position first.

• Confirm the effects of temperature, humidity, and atmospheric pressure on distance.

• Set up an observation environment that can handle low-light conditions, dust, and water ingress.

• Avoid interference between safety routes and construction machinery.

• Set rules for observation records and data management

• Enable on-site management that doesn't rely solely on total stations.

Prerequisites to confirm before using a total station in tunnel surveying

When conducting surveys inside tunnels, many conditions differ from those at open, aboveground sites. A total station, a surveying instrument that determines positions based on angles and distances, is used in a variety of situations such as construction management, as-built verification, centerline checks, management of internal cross-sections, and verification of equipment locations. However, inside tunnels visibility is limited, it is dark, humidity and dust are likely to occur, and there is frequent movement of construction machinery and workers; if work is started without sufficient preliminary checks, measurement variability and rework are likely to result.

Total stations are convenient instruments, but simply setting them up does not guarantee correct results. Preparation is important: which point to use as a reference, which directions to check, where to place the instrument, and how to handle meteorological conditions and the underground environment during measurement. Especially inside tunnels, it is often difficult to use satellite positioning directly, and the controls brought in from outside or the reference points installed inside the tunnel form the foundation of the entire survey. If reference points are handled ambiguously during work, it becomes difficult later to decide which measured values to adopt.

Also, in-tunnel surveying is closely linked to the construction workflow. As trades progress—excavation, support works, lining, invert, drainage facilities, lighting systems, communication systems—the targets to be measured and the required accuracy also change. Before using an optical distance meter, it is essential to clarify the day’s surveying objectives and choose an observation method appropriate to the construction stage. Rather than simply measuring, deciding in advance what judgments the measurements are intended to support makes it easier to organize the necessary survey points, the number of observations, and the information to be recorded.

This article explains, divided into six points, the precautions to check before conducting tunnel surveying with an optical total station. Focusing on control points, line-of-sight, distance correction, in-tunnel environment, safety, and data management—areas where field personnel often get confused—we organize the content so it is easy to verify during work planning and on-site meetings.

Verify the reliability of underground reference points and known points

The first thing to verify in tunnel surveying is the reliability of control points and known points. A total station determines positions based on reference points and directions, so errors in the coordinates or elevations of control points will affect the entire subsequent survey results. In tunnels in particular, you cannot easily use multiple points while broadly surveying the surroundings as you can above ground. Because surveying proceeds in a confined space, it is necessary to check in advance the placement, protection status, and history of use of the control points.

When bringing coordinates into the tunnel from outside, confirm which reference point and which route were used. It is important to check not only the survey records but also on site whether the control point markers have moved, whether the surrounding area has been affected by excavation or vehicle traffic, and whether visibility is preserved. Tunnel construction involves considerable vibration and movement of materials, and points that had no problems at installation can develop slight shifts or damage over time. Do not assume everything is safe just because coordinates are listed in the results table; always cross-check the field conditions against the records.

When setting the instrument station and backsight using known points, it is also important to confirm the backsight direction. If the backsight is chosen improperly, a slight angular error can appear as a large positional displacement at the forward end. Continuing to work using only short-distance backsights tends to weaken direction verification, so it is desirable to use points with as much distance as possible within practical limits. However, in tunnels, visibility and work constraints may make ideal placement difficult. In such cases, perform checks using different points before and after the observation to confirm there are no major inconsistencies in the survey results.

Height management must not be overlooked. Inside tunnels, there are many items to check regarding elevation, such as changes in the roadbed, temporary works, drainage gradient, and the position of the lining surface. When using reference leveling points or temporary benchmarks as height references, note their installation date, the date of verification, and their protection condition. When handling heights with an optical total station, instrument height, reflector height, and the installation condition of targets directly affect the results, so it is necessary to manage not only the heights of the reference points but also the values entered on site. Instrument and reflector heights should not be handled from memory on the spot; it is desirable to record them in a form that can be kept in the observation records.

In-tunnel reference points may be extended as construction progresses. If new points are installed and work proceeds without verifying consistency with existing points, accumulated errors can become difficult to detect. Before using extended reference points, perform comparisons with the previous survey, closure checks, and verifications from different directions to determine whether the points are suitable as references. In tunnel surveying, speed is important, but omitting verification of the reference can lead to major corrections in later stages.

Determine line of sight and installation location in advance

A total station assumes a clear line of sight in the direction to be measured. In a tunnel, even sections that appear straight can have their line of sight obstructed by construction machinery, piping, cables, materials, temporary supports, or lighting fixtures. If you discover after setting up the instrument that the line of sight is blocked, you will need to redo the setup or wait for work to clear. Therefore, before starting surveying it is important to confirm on site the positional relationships of the instrument point, the backsight, the survey point, and the reflector, and to determine which positions will allow stable observations.

In tunnels, where the cross-section is confined, even slight differences in installation position can change how easily line of sight can be established. Placing the instrument too close to the wall can make backsights and the angle to survey points difficult, while moving it toward the center may obstruct the passage of vehicles and workers. When choosing where to set up a total station, you need to consider not only surveying accuracy but also how it will fit with work flow. Checking whether the ground can stably support the tripod, whether the location is less prone to vibration, and whether reflections from lighting will make aiming difficult will help reduce the likelihood of problems during observation.

The floor inside a tunnel is not always flat. In freshly excavated sections, temporary roadbeds, wet surfaces, and near drains, tripod legs may sink or slip. If observations are made while the tripod is unstable, the instrument may lose its level or shift position during measurement. Before setting up, check the condition of the spots where the legs will rest and, if necessary, move to a more stable position. Rather than forcing the instrument into a cramped location, taking a little extra time to choose a stable instrument point can often reduce the need for re-measurements.

In line-of-sight checks, consider not only the sightline to the survey point but also the possibility that visibility may be blocked during work. In tunnels, material delivery vehicles and workers often use the same routes, and people or machines may cross after measurements have begun. On sites where it is difficult to stop work for each survey, a prior meeting with the construction crew is necessary. Sharing information about what time of day to measure, which sections to temporarily clear, and where workers carrying reflectors will move will help the surveying proceed smoothly.

Visibility judgments become even more difficult in curved sections and sections with gradients. Depending on the tunnel alignment, situations can occur where a line of sight along the centerline cannot be obtained, survey points fall into the shadow of the cross section, or targets on the far side are difficult to see. In such cases, measures are necessary, such as dividing the instrument stations into multiple points, setting measurement points in stages, and organizing in advance the directions that can be observed. Trying to measure too far at once makes it difficult to verify measured values and can lead to incorrect judgments on site. In locations with visibility constraints, it is safer to divide the surveying section and proceed while confirming the connection to the reference in each section.

Confirm the effects of temperature, humidity, and air pressure on distance

When measuring distances with an electro-optical distance meter (EDM), the state of the air affects the measurements. Inside tunnels, temperature and humidity tend to vary more than in the outside air, and conditions can differ near the portal, near the face, close to ventilation equipment, or in areas with water inflow. For surveys requiring high accuracy, it is necessary to check conditions such as temperature, humidity, and air pressure, and to pay attention to instrument settings and the way corrections are applied.

Inside a tunnel, air conditions change due to ventilation airflow and the operation of construction machinery. Humidity may increase the deeper you go into the tunnel, and temperature may rise locally from the exhaust heat of operating equipment. For short-distance surveys the effects can be small, but when measuring long distances or performing tasks that form the foundation of survey results—such as control point extension—environmental conditions cannot be ignored. Before surveying, it is important to check the site temperature and atmospheric pressure and reflect the necessary settings in the instruments.

However, simply applying correction values does not solve everything. You also need to consider whether the conditions entered at the time of measurement truly represent the entire survey line. If there is a temperature difference between the area near the portal and the working face, judging based only on the conditions near the instrument can differ from the state of the entire survey line. Decide in advance which location’s environmental conditions to adopt and how thoroughly to verify them according to the required accuracy and distance on site; this will make the survey records easier to explain later.

When using reflectors, attention must also be paid to the prism constant, the type of reflector, and the installation orientation. If the instrument settings and the reflector actually being used do not match, a consistent offset in distance measurements may occur. On sites where multiple people conduct surveying, mistakes are likely to happen—such as using a different reflector than before without changing the settings, or observing with settings left from another work crew. Before beginning tunnel surveying, it is important to confirm the reflector to be used and ensure that the instrument settings and observation records match.

In addition, inside the mine, water droplets and dust can affect the line of sight. Even if it appears that you have a clear sighting, a lot of mist-like moisture or dust along the measurement line can reduce the stability of measurements. When measured values fluctuate unnaturally, do not immediately rely only on coordinate calculations; check the environmental conditions and any obstructions along the measurement line. Measuring the same point multiple times to assess the stability of the values is also effective. If you move on to the next task while measurements remain unstable, it will be difficult to trace the cause later.

How temperature, humidity, and atmospheric pressure are handled also affects how results are explained. In inspections and internal reviews, you need to record the conditions at the time of surveying to explain why a measured value can be trusted. In addition to the observation data from the total station, recording the observation date and time, the underground location, environmental conditions, equipment used, reflectors, operators, and verifiers makes it easier to assess the results later.

Prepare an observation environment suitable for low-light, dust, and groundwater seepage conditions

In tunnel surveying, coping with darkness is essential. Because total stations involve sighting work, when targets, reflectors, or survey point markers are difficult to see, work efficiency declines. Forcing sighting in dark areas can lead to mistaking the target or failing to align the center of a reflector accurately. Before surveying, it is important to check the lighting conditions of the work section and, if necessary, prepare temporary lighting or portable lighting.

However, it's not as simple as just increasing the lighting. Depending on the direction of the lighting, reflections and shadows can occur, making sighting difficult. When wet lining surfaces or metallic temporary materials reflect, the contours of the target can become hard to see. If lighting shines directly into the field of view of an optical surveying instrument, operators' eyes can tire easily, and prolonged observations may lead to decreased concentration. It's important to position lighting so that measurement points and reflectors are easy to see while not impairing the instrument's visibility.

In sections with a high concentration of dust, take care to both protect the equipment and maintain the stability of measurements. Immediately after excavation, spraying, or materials delivery, fine particles may remain in the air. Observing under dusty conditions can reduce visibility and make measurements less stable. In addition, dust adhering to equipment lenses or reflectors can affect sighting and distance measurement more than it appears. Before and after surveying, check equipment and reflectors for dirt and establish cleaning procedures as part of site rules.

Seepage and condensation are also elements that are easy to overlook. Inside a tunnel, droplets can fall from the walls or crown, and water can pool on the road surface. You should avoid locations where droplets could land on equipment or where tripod feet could become slippery. If reflectors or survey point markers are wet, their visibility can decrease. In areas with water, workers also need to watch their footing when moving while holding reflectors. Checking the observation environment is important not only for surveying accuracy but also to prevent falls and contact accidents.

The way survey point markers are made must be designed with dark and humid conditions in mind. Marks that are clearly visible at aboveground sites can become difficult to see underground due to darkness and dirt. Making the survey point name, survey point location, installation date, and relationship to the reference clear will make later re-measurement and verification easier. If markers are too small, there are multiple similar marks, or marks fade easily during work, this can cause misidentification. Inside tunnels, where similar scenery continues, measures to make survey points easy to find directly affect work quality.

Preparing the observation environment is not just to make the surveyor’s work easier. It is preparation to enhance the reliability of survey results and to avoid confusion in construction decision-making. By recognizing conditions such as darkness, heavy dust, and the presence of water, and by adjusting lighting, cleaning, signage, instrument placement, and observation timing accordingly, you can more effectively utilize the performance of optical surveying instruments on site.

Avoid Interference between Safety Circulation Routes and Construction Machinery

In tunnel surveying, safety management needs to be considered as part of the surveying plan. The interior of a tunnel has limited workspace, and construction machinery, material transport vehicles, workers, and surveying personnel use the same space. If the location where an optical total station is set up or the movements of workers carrying reflectors overlap with construction traffic routes, it can cause not only interruptions to surveying work but also collisions. Before surveying, it is important to check the overall work schedule on site and clarify which sections can be surveyed at which times.

The position where the instrument is set up should be chosen not only for accuracy but also so that it is easily visible from the surroundings. In a dark underground area, for example, a tripod protruding into a passage may be difficult for vehicles or workers to notice. Take site-appropriate measures such as placing markers around the tripod, sharing information about the work area, and avoiding times of heavy traffic. Because an optical total station is a precision instrument, even a slight displacement of the device caused by contact may require redoing observations. It is necessary to consider both protection of the equipment and the safety of workers at the same time.

Attention must also be paid to the movements of workers carrying reflectors. In situations such as passing close to construction machinery to stand at a survey point, moving over ground with poor footing, or approaching the crown or sidewalls, the surveyor should not make decisions alone but coordinate with the site supervisor and the construction crew. Especially near the working face and in sections immediately after work, confirming the entry limits and refuge locations is indispensable. Even if there is a point to be surveyed, if safety cannot be ensured, it is necessary to review the work sequence.

Sound echoes inside tunnels and machinery noise is amplified, so signals given only by voice can be difficult to convey. Deciding in advance how the person operating the optical surveying instrument and the person holding the reflector will communicate can reduce operational misunderstandings. If work proceeds with unclear signals, problems such as moving before being observed, standing at the wrong survey point, or waiting in a hazardous location can occur. It is important to standardize the means of communication, the signals, and how survey points are referred to.

Interference from construction machinery also affects the line of sight. If large machines or materials block survey lines, observations must be interrupted. When surveying for short periods between work tasks, first sort out which survey points to prioritize and which survey lines are likely to be obstructed. Trying to measure everything at once can leave you unable to keep up with the movement on site. Dividing the survey area and conducting observations in stages to match the flow of construction results in a realistic work plan.

A surveying plan that accounts for safe movement routes helps build overall confidence in the site. By creating conditions in which surveyors do not obstruct construction and can reliably deliver the necessary results, coordination with other work crews becomes easier. In tunnel surveying, it is important not to consider surveying accuracy and safety separately, but to verify them within the same plan.

Establish rules for observation records and data management

The data obtained with a total station are used for on-site decision-making and as-built verification. Therefore, it is important to establish data management rules before surveying. Even if the measured values themselves are correct, if the point names, observation date and time, instrument point, backsight, prism height, operator, and survey purpose are not known, it becomes difficult to verify the results later. In tunnels, similar section names and point names tend to appear in sequence, so if recording rules are ambiguous, data mix-ups are likely to occur.

First, standardize how measurement point names are assigned. On sites with many items to measure—centerlines, left and right sidewalls, top surfaces, roadbeds, equipment locations, shoring locations, etc.—it is important that measurement point names carry meaning. If anyone can tell what position a point refers to and which construction stage it belongs to, later verification becomes easier. Avoid situations where measurement point names differ by worker, abbreviations are not mutually understood, or the same name is used for different locations.

Next, decide where to store the observation data and how to name the files. On site, data collected during hurried surveys is often saved temporarily and organized later. However, postponing this can make it unclear which data set is the final deliverable. Use file names that indicate the date, section, survey purpose, and work stage to make them easier to find later. It is also important to manage separately the data output from the total station, the processed/calculated data, and the deliverables for submission.

It's useful to record site conditions in observation logs. Inside tunnels, temperature, humidity, line-of-sight conditions, dust, water inflow, and the operating status of construction machinery can affect surveying. You don't need to write down every detail, but recording conditions that are likely to affect measurements will make explanations easier later. For example, points that were remeasured because of poor line-of-sight, points where installing reflectors was difficult, and sections where work was interrupted should be kept with the survey results as a basis for later decisions.

You should also establish procedures for checking survey results on site. If you wait until you return to the office to check the measured data for the first time, it may be difficult to re-survey if you discover anomalous values. If possible, perform a preliminary check on site to verify consistency with control points and to ensure measurement points were not swapped. In particular, for control point extensions and important as-built confirmations, checking immediately after observation reduces rework.

Data management matters when personnel change. In long tunnel projects, surveyors and construction crews may be rotated. If records are kept in a way that only the predecessor understands, the meaning of the results may become unclear after handover. To make records traceable by anyone, it is important to record survey point names, file names, storage locations, verifiers, and revision history according to consistent rules.

Surveying with an electronic total station is not only a task of obtaining numerical data in the field but also a task of preserving that data in a usable form. If you concentrate too much on the observations themselves and postpone organizing the records, the valuable survey results cannot be fully utilized. By establishing rules for recording and management before surveying, it becomes easier to stabilize the quality of tunnel surveying.

Towards on-site management not solely dependent on total stations

Before using a total station for tunnel surveying, it is important to check reference points, line of sight, distance corrections, the underground environment, safety, and data management. A total station can be a useful instrument for position verification and construction control inside a tunnel when correct references and stable observation conditions are in place. On the other hand, using it without taking into account constraints specific to the underground environment can lead to measurement variability, work interruptions, re-measurements, and confusion in records.

In practice, it is important to clarify the purpose of the survey from the outset. Whether it is to check the centerline, verify the internal cross-section, confirm equipment locations, or manage as-built control, the required survey points and the accuracy to be checked will differ. Increasing the number of survey points while the purpose remains vague can lengthen work time and still leave you lacking the information needed for decision-making. Before using a total station, organizing who will use the survey results and for what decisions will make the necessary preparations clearer.

In addition, inside tunnels it is important not to rely solely on a total station to capture all conditions, but to manage data in combination with photographic records, point clouds, as-built records, construction history, and so on. Even when numerical values alone cannot easily convey site conditions, linking surveying results with site photos and three-dimensional data makes later verification and discussion easier. Because many areas become inaccessible as construction progresses, keeping records at the necessary times helps prevent rework in the future.

Combining position data, photographs, and point clouds captured on site makes survey results easier to understand and manage. On sites with restricted visibility and working space, such as inside tunnels, it is effective to have a system that, in addition to control management using total stations, records site conditions on the spot and makes them easy to share with stakeholders. By creating a situation in which not only surveyors but also construction managers, quality control staff, and client-side inspectors can make decisions while viewing the same information, the burden of verification work can be reduced.

Before conducting tunnel surveys with a total station surveying instrument, it is essential not only to check the instrument’s performance but also to organize site conditions and operational rules. Confirm reference points, ensure line of sight, record environmental conditions, maintain safe movement routes, and retain data in a traceable form—these practices lead to reliable survey results. In addition, using site photos, as-built records, and point cloud data as needed makes post-survey verification and sharing easier.