Five precautions when verifying excavation positions with a total station

In excavation work, mistakes in the starting position, excavation limits, depth, slope location, or interfacing with structures can lead to rework and safety risks. A total station is an effective surveying instrument for advancing excavation while confirming positions on site, but errors in setup, handling of control points, checking against construction drawings, or responding to changes during excavation can mean that, even though measurements are being taken, they do not necessarily lead to correct decisions. This article organizes the precautions that practitioners should keep in mind when checking excavation positions with a total station, from a site perspective.

Table of Contents

• Confirm the reference point and coordinate conditions before excavation.

• Carefully perform the machine installation and post-installation inspection.

• Verify the excavation extent and depth against the construction drawings

• Ensure line of sight and a safe measurement position during excavation

• Record measurement results to prevent rework

• Summary

Confirm reference points and coordinate conditions before excavation

When checking excavation positions with a total station, the first thing to pay attention to is verifying the reference points and coordinate conditions used on site. No matter how carefully you measure the excavation position itself, if the reference points or the assumptions about the coordinates are off, the verification results will also be off. Especially in excavation work, because the ground surface before commencement, existing structures, temporary works, and the design excavation line are intricately related, it is important to sort out the assumptions before surveying.

First, confirm which drawings or survey results the reference points used on site are based on. On sites where construction control points, temporary reference points, existing points, and temporary stakes are mixed, it is dangerous to decide "this point is fine to use" based on appearance alone. You must cross-check the point name, coordinate values, elevation, installation date, and whether it has been verified, and make clear that it is a point that may be used for confirming the excavation location before beginning work.

Checking excavation locations involves not only planimetric position but also elevation control. This is because it includes information that directly affects the as-built condition after excavation, such as the floor level, excavation depth, the heights of slope toes and crests, and the interface with the bottom surface of structures. Even when verifying position with an optical surveying instrument, confirming that benchmarks and temporary leveling points used as elevation references have been correctly carried forward makes it easier to reduce discrepancies in later stages.

Be careful not to confuse coordinate systems. Depending on the site, coordinates close to the public coordinate system may be used, or local coordinates used only within the work section may be employed. Furthermore, the origin and axis orientations of the coordinate system may differ among design drawings, construction drawings, survey results, and construction control/as-built management data. When confirming excavation positions, it is essential to verify in advance that the coordinates entered into the total station and the excavation lines indicated on site are based on the same reference frame.

In excavation work, simply transferring the lines on the drawings directly to the site can be insufficient. You need to sort out whether the points that should actually be checked are the design lines or lines that include construction allowances, taking into account excavation gradients, over-excavation, working space, formwork and structural construction clearances, the positions of earth-retaining members, and the swing radius of heavy equipment. If this remains ambiguous when surveying, even if the measurements are correct, interpretations of the area to be excavated can vary on site.

Also, control points may have moved after being installed. Passage of heavy machinery, the placement of temporary materials, ground loosening, or the effects of rainfall can cause stakes or pins to shift slightly. Before excavation, it is reassuring to check the distances and angles between multiple control points and confirm there are no significant discrepancies with previous survey results. If there is an obvious discrepancy, it is preferable not to use that point as-is; confirm with the site supervisor or surveyor before deciding on the reference.

When verifying excavation locations with a total station, the accuracy and safety are greatly affected by the preparations made before measurements begin. Organizing reference points, coordinates, elevations, drawing revisions, and construction clearances, and ensuring a common understanding on site before measuring, reduces uncertainty when determining excavation locations.

Carefully perform machine installation and post-installation visual checks

Equally important is the setup and backsight verification of the total station. When confirming excavation locations, measurement work is often carried out in parallel with the operation of heavy equipment and the movement of workers, and quick decisions may be required. Even in such situations, if measurements are started with the instrument insufficiently set up, positional errors may be discovered later, resulting in re-excavation or the need to repeat verification tasks.

The location for setting up a total station should not only provide a clear view of the excavation area to be measured, but also allow the instrument to be installed stably. Around excavations, the ground may be loose, vibrations from heavy machinery can be easily transmitted, and there may be heavy foot traffic near temporary access routes. If the ground where the tripod is placed is soft, it can settle slightly during measurements, changing the sighting direction and height. The basics are to choose firm ground, firmly plant the tripod legs, and secure a position that is unlikely to be touched during work.

After installation, carefully level the instrument. When checking the excavation position, height is often verified along with horizontal angle and distance, so inadequate leveling affects the measurements. The more rushed the situation, the more you should check the bubble and the electronic leveling display and be mindful of any shift during measurement. Especially at sites where excavation progresses and the ground surface changes, the tripod feet may sink or loosen from vibration, so recheck as necessary even during measurements.

Backsight confirmation is also indispensable. A backsight is the process of sighting a known point or reference direction to align the orientation of the instrument and the coordinate reference. If the backsight point is selected incorrectly, a point name is mistaken, or the prism height is not entered, subsequent measurement results can be significantly off. When confirming excavation locations, decisions may be made based on only a few measurements, so the initial backsight confirmation is extremely important.

After performing the backsight, it is advisable to measure another known point to verify. Measure a different point using the established reference, and by checking whether its coordinates and distances are consistent with the existing results, you can confirm there are no major errors in the setup or the backsight. Even when time on site is limited, carrying out at least a minimal check can reduce the risk of major rework later.

Care must also be taken in handling prisms and targets. When checking excavation positions, it is common for a worker to move while holding a mirror and for a surveyor to confirm the position with an optical surveying instrument. If the prism constant, mirror height settings, or the pole’s verticality are not correct, measurement errors will occur. Especially when confirming heights, a tilted pole will affect both the vertical (height) and horizontal (planar) measurements. It is important that the worker holding the mirror also understands to place the pole at the center of the point to be checked and to keep it vertical by watching the bubble level.

At excavation sites, the measurement targets are often soil surfaces, which makes point locations prone to ambiguity. Unlike measuring a clear point such as a stake or nail, features like the slope shoulder, slope toe, floor surface, and excavation edge require deciding in advance which spot to treat as the point. If the surveyor and the nearby worker have different understandings—“measure this corner,” “check the outside of this line,” “check this position on the excavation bottom”—their judgments will diverge for reasons unrelated to the performance of the total station.

Furthermore, it is important to regularly check that the instrument has not moved during measurements. After heavy equipment passes nearby, after a tripod leg becomes loose, or after a worker passes close to the tripod, it is reassuring to reconfirm the backsight direction. Checking excavation positions tends to be a repetitive task performed in line with construction progress rather than a one-off survey, so making on-the-job checks a habit makes it easier to maintain accuracy.

Verify the excavation extent and depth against the construction drawings

When checking excavation positions with a total station, you should not only look at the measured values but also carefully verify which conditions on the construction drawings those values correspond to. In excavation work there are multiple elements to confirm, such as plan position, excavation width, excavation length, bedding elevation, slope gradient, and separation from structures. If you judge that the "position is correct" by looking at only one of these elements, inconsistencies may arise with other conditions.

First, when confirming the excavation limits, it is important to distinguish between the design excavation line and the construction verification line. The outline of the structure shown on the design drawings and the outer line that will actually be excavated are not necessarily the same. Take into account formwork, working space, crushed stone and base materials, earth retention, over-excavation, and so on, and clarify which line is being checked on site. Even when confirming the excavation edge by following coordinates with an optical survey instrument, it is important not to confuse whether those coordinates correspond to the design line or the excavation line laid out for construction.

When checking depth, you look to see whether the bottom elevation of the excavation is approaching the design elevation. During excavation, heavy equipment is used for major digging and finishing is often done by hand or with small machines at the end, so it is necessary to share at which stage and to what extent excavation will be carried out. When measuring elevation with an optical surveying instrument, confirm that the instrument height, prism height, reference elevation, and the position of the measurement point are correctly set. Rather than judging solely by the numbers, it is important to understand the difference from the required finished elevation on site to prevent over-excavation or under-excavation.

When checking excavation depth, it's important to consider not only the difference from the existing ground surface but also the difference from the planned elevation after finishing. Proceeding with the mindset of simply digging a certain number of meters from the current surface can lead to errors when the existing ground surface is uneven or when prior grading has changed elevations. You should organize the design subgrade elevation, the bottom of structures, the thickness of foundation materials, and so on, and clearly define which final elevation you are aiming for.

In excavations involving slopes, the relationship between horizontal position and elevation becomes even more important. Because the position of the slope crest, the slope toe, the gradient, and the presence or absence of benching are all involved, simply checking the coordinates of the excavation edge does not guarantee the entire slope is correct. There can be cases where the crest is in the right place but the toe is off, or where the bottom elevation is correct but the gradient has become too steep. You need to measure multiple points with a total station and make a conscious effort to confirm that they align as a cross-section.

In excavations around structures, attention must be paid to interfaces with existing and newly constructed structures. If the excavation line is too close, it can affect existing structures; if it is too far, it can impact the workspace for subsequent operations and the volume of backfill. Around piping, foundations, retaining walls, manholes, and side ditches, it is required not only to rely on dimensions shown on drawings but also to measure the positions of existing elements on site and confirm that there are no discrepancies between the design conditions and the actual conditions.

When verifying excavation locations, version control of the drawings used on site is also important. If construction drawings have been updated but surveying is carried out using an old drawing, even if a total station determines the position accurately, the result will differ from the location that should be constructed. Before excavation, confirm that the drawings to be used are the latest and that change instructions and the results of consultations have been reflected. If changes communicated orally are not reflected on the drawings, it is advisable for the stakeholders to confirm them before surveying and to leave a record before proceeding with the work.

Also, when showing the excavation area on site, it is important to present it in a way that is easy for workers to understand, not just by indicating measurement points. Even if only a few points are shown, if the heavy equipment operator or hand crew cannot correctly visualize the excavation line, deviations will occur during construction. As needed, make use of stakes, spray markings, offset stakes, batter boards, and markers to convey the measured positions to the site as information that can be used for construction.

The total station is an effective means of verifying on-site the positions shown on drawings, but how measurement results are interpreted depends on human judgment. By organizing the excavation extent, depth, slope, and relationships to structures, and by linking measurements with construction conditions for verification, it becomes easier to improve excavation accuracy and work efficiency.

Ensure line of sight and safe measurement positions during drilling

When confirming excavation positions with a total station, ensuring line of sight and selecting a safe measurement position are indispensable. A total station requires a clear view from the instrument to the prism or target on the machine. On excavation sites, heavy equipment, soil, temporary structures, workers, vehicles, and shoring often block the line of sight, and as excavation progresses the visible area changes. You should not decide the machine’s position solely based on ease of surveying; you need to consider whether it can be checked safely and continuously.

During measurements taken during excavation, what you want to avoid is having surveyors or prism operators enter areas within the operating radius of heavy machinery or where a landslide might occur. The ground near the excavation edge may be loosened and there is a risk of the ground collapsing underfoot. Even when entering the bottom of the excavation to take measurements, you must check the condition of the slopes, whether shoring is in place, groundwater seepage, and the condition of any footing or work platform, and ensure safe access and egress before beginning work. Rather than prioritizing merely getting close to the measurement point, it is important to choose methods that allow you to verify from a position where you can stand safely.

Consider the placement of the instrument from a safety perspective. If you set the tripod too close to the excavation edge, the ground may loosen or vibrations may make the instrument unstable. Conversely, if it is placed too far away, aiming becomes difficult and the prism’s position is harder to confirm. Choose a location that provides an overview of the entire site, is outside the path of heavy machinery, is unlikely to be contacted by workers, and allows the tripod to be set up stably.

When visibility is poor, measures such as changing the machine's placement, separating measurement points, establishing escape points, or adjusting the timing of the work are necessary. At excavation sites, temporary stockpiling of soil or the orientation of heavy equipment can temporarily obstruct the view. Just because something cannot be seen, taking measurements from awkward positions or setting up mirrors in dangerous locations not only compromises surveying accuracy but also safety. It is essential to prepare an environment where measurements can be taken before starting work.

Coordination between surveying work and heavy equipment operations is also important. When confirming positions during excavation, it is necessary to clarify the timing for temporarily stopping the equipment, the area the mirror attendant will enter, and the signal indicating that measurement is complete. If signals between the surveyor, the spotter, and the heavy equipment operator are ambiguous, the equipment may move during measurement or the mirror attendant may enter a dangerous position. Even for brief checks, it is important to be diligent about verbal communication and signaling.

As excavation gets deeper, visibility from the total station to the excavation bottom becomes difficult. The bottom of the excavation can fall into shadow, the prism on the slope face can become hard to see, or the line of sight from the instrument can be blocked by shoring. In such cases, rather than forcing measurements from above, consider relocating the instrument to another safe position. However, if you do relocate, carefully perform backsight checks and reference-point checks at the new instrument station so that the measurements before and after remain consistent.

Weather and site conditions also affect measurements. After rain the ground can loosen, and excavation edges may become more prone to collapse. Strong sunlight or reflections, dust, steam, raindrops, and the like can also make sighting and distance measurements unstable. When readings are unstable, you should not make a judgment based on a single measurement; it is advisable to take multiple measurements to confirm any variation in values. If the measurement environment is poor, do not rush the work—conditioning the measurement conditions leads to safer and more reliable construction.

When confirming excavation locations at night or in dark areas, securing lighting is also important. If the mirror position, excavation edge, the area around workers' feet, or the surroundings of the machine are difficult to see, the risk of mistaking measurement points or falling increases. When installing lighting, position it so that it does not obstruct the machine's line of sight and so that the target is not obscured by the worker's shadow. Do not assume it is safe just because there is lighting; it is important to ensure that both the surveyor and the mirror operator can confirm the measurement points.

Surveying at an excavation site is a task that requires both accuracy and safety. An optical surveying instrument is effective for confirming excavation positions, but if measurements are forced in areas with poor visibility or hazardous locations, it becomes difficult to obtain stable results. Even when you want to minimize the time the site is stopped for surveying, preparing the measurement environment and a safe standing position is ultimately the quickest way to prevent rework and accidents.

Record measurement results to prevent rework

After verifying the excavation location with an optical total station, it is important to record the measurement results and leave them in a form that can be shared on site. Because excavation work changes the terrain daily, if it becomes unclear when, at what location, or by which reference the verification was made, you will not be able to determine that information later. Even if there seemed to be no problem at the moment of measurement, when height or position checks are required in the next process, insufficient records make re-measurement or rework more likely.

What should be recorded are not only the coordinates and elevations of measurement points. Also important are the reference points used, the instrument points, backsight points, measurement date and time, the drawings checked, the surveyor, site conditions, and the decisions made.

Especially when confirming excavation positions, it is important to record not only what the measured results were but also what construction-related judgment was made based on those results. Whether it was judged acceptable to continue excavating, that additional excavation was required, or that work was stopped to avoid over‑excavation—recording these makes later verification easier.

Measured values should also be converted into expressions that are easy to understand on site and shared. Simply recording coordinate values does not necessarily mean heavy equipment operators or construction managers can immediately use them to make construction decisions. Conveying information in a way that makes clear what to do next on site—such as the deviation from the design position, the remaining distance to the design subgrade elevation, the offset from the excavation line, and the direction to correct slopes—makes it more likely that survey results will be applied to the construction.

During checks while excavating, the same location may be measured multiple times. The purpose differs depending on when the measurement is taken—during rough excavation, before finishing, after finishing, before inspection, etc. In rough excavation the objective is to prevent large positional shifts and over-excavation, while at the finishing stage the goal is to check the design elevation and the excavation edges in detail. If you don't record which stage a measurement was taken at, you may later misinterpret what the values mean.

When recording, it is effective to combine photos with field notes. Photographing the surroundings of measurement points, the condition of excavation edges, the positions of stakes and markings, and the progress of heavy equipment work can help clarify situations that are difficult to understand from coordinates or numerical values alone. However, because photos alone may be insufficient as evidence for exact positions or elevations, it is important to manage them in association with the measured values.

The results of verification of excavation locations must be shared promptly with relevant parties. Even if only the surveyor understands the results, if they are not conveyed to the construction team the site may proceed in the wrong direction. Share the verification results and points of caution through morning briefings, pre-work meetings, verbal reminders during work, and construction management records. It is especially important to clearly convey on site any locations where there is a large discrepancy from the design position, locations requiring additional excavation, and locations that must not be excavated any further.

When managing measurement results, pay attention to data overwriting and mix-ups. If points with similar names, similar excavation locations, or measurement data from multiple construction sections are mixed together, it can become unclear later which data should be used. Organize the naming of points, date entries, section names, and the measurement purpose so that they remain understandable when viewed later. Survey data used on site should not merely be stored; it is important to manage them in a usable state.

Also, if you feel something is off with the measurement results, confirm the cause on the spot. If the position on the drawings does not match the site conditions, if there is a large discrepancy from the previous measurement, or if repeated measurements at the same point do not produce stable values, do not immediately proceed with excavation; check the reference points, backsight, mirror height, input coordinates, drawing conditions, and the method used to take the measurement points. If you proceed while any anomaly remains, major corrections may be required later.

In excavation work, survey results directly determine construction decisions. Therefore, correctly recording the results confirmed with an optical total station, sharing them in a clear and easy-to-understand way, and keeping them accessible for review when needed helps prevent rework. Making the measured results usable on site is as important as measuring accurately.

Summary

When confirming excavation positions with an optical surveying instrument, it is important to consider not only setting up the instrument and taking measurements, but also the reference points, coordinate conditions, installation and setup, backsight checks, verification against construction drawings, maintaining safe lines of sight, and record management as a single integrated process. In excavation work, the terrain and working environment change as work progresses, and the measurement conditions change as well. Even if the initially confirmed position is correct, vibrations during excavation, changes in line of sight, drawing revisions, and discrepancies in on-site judgment can cause errors in the construction location.

First, before excavation, confirm the reference points and coordinate conditions to be used, and clearly determine which lines will be treated as excavation positions for this work. If the design lines, construction clearances, extra excavation, and interfaces with structures remain ambiguous, construction decisions can deviate even if the measured values are correct. Next, carefully set up the total station and perform a backsight check to ensure there are no errors in the starting point of measurement. By confirming that the instrument is stable, that you have not mistaken the backsight point, and that the mirror height and prism conditions are correct, you can prevent major rework.

When verifying the extent and depth of excavation, it is important to check not only the plan position but also elevations, slopes, and section consistency. Bedding elevation, top of slope, toe of slope, and separation from existing structures should be cross-checked against the construction drawings and translated into the decisions required on site. Furthermore, because heavy equipment operations and surveying work occur close together during excavation, maintaining line of sight and prioritizing safe positions is essential. A measurement plan is required that considers not only ease of measurement but also ground stability, the operating range of heavy equipment, and worker movement paths.

After measurements, it is essential to record the results and share them with relevant parties. By documenting when and where measurements were taken, by which standards, and how decisions were made, it becomes easier to connect them to subsequent process checks and as-built management. In excavation work, small positional shifts or overlooked elevations can affect backfilling, foundation construction, structure installation, and responses to inspections. Verification work using a total station should be regarded not as a one-off measurement but as a management task that supports the quality of the entire construction.

At some sites, combining measurements from an optical total station with photo records, measurement point lists, as-built control documents, and incorporation into construction drawings makes confirmation results easier to share. The wider the excavation area or the more measurement points there are, the more important it is to decide in advance how measurements will be taken, which documents they will be recorded in, and who will make the decisions. Ensuring that optical total station measurement results are reliably linked to on-site construction decisions is fundamental to preventing excavation position deviations and rework.