7 Things to Check Before Measuring Existing Structures with an Electro-Optical Total Station

In surveying existing structures, because the subject is already present on site, it can at first glance seem like "just measuring what you can see." However, in practice the points that need to be measured and the conditions that must be recorded can vary greatly depending on the structure’s shape, surrounding obstructions, the location of control points, the purpose of the measurements, and how the deliverables will be used. A total station is a surveying instrument that determines positions based on distances and angles, but if you start measuring without adequately addressing setup conditions, line of sight, handling of reflectors, and organization of the coordinate system, you may need to re‑survey or revise deliverables in later stages. This article organizes seven items that practitioners should check before surveying existing structures with a total station.

Table of Contents

• Confirm the purpose of the survey and how the deliverables will be used

• Organize the scope of existing structures and the points to be measured

• Confirm the handling of reference points and coordinate systems.

• Verify the installation position and line-of-sight conditions of the total station.

• Confirm the effects of reflectors and ranging conditions.

• Confirm the conditions for the surrounding environment and safety measures.

• Confirm the recording method and the post-processing flow.

• To reduce re-surveys in surveys of existing structures

Confirm the survey objectives and how the deliverables will be used

Before measuring an existing structure with a total station, the first thing to confirm is the purpose of the survey. Even when measuring an existing structure, the required density of survey points and the way required accuracy is considered change depending on whether you are checking positions for renovation work, assessing deviations from the design drawings, preparing as-built or current-condition documentation, or checking interfaces with adjacent works. If you start measuring without a clear purpose, you can easily end up with many measured points that are useless for decision-making or with having failed to measure the locations you actually needed.

For example, even when confirming the position of an existing retaining wall, what to emphasize—such as the alignment of the top, locations near the foundation, the inclination of the front face, how the ends tie in, or the relationship with drainage facilities—depends on the project objectives. If it will be used to make decisions about design changes, survey points that allow comparison with reference lines or planned positions on the design drawings are required. If it will be used to check for conflicts before construction, survey points that clarify the separation and elevation relationship with the new structure are important. Even if the information is to be kept only as a site record, representative points and supplementary explanations should be provided so that anyone who looks at them later can understand the positional relationships.

Before surveying, confirm whether the deliverables will be reflected in plan drawings, used to create cross-sections, used for quantity verification, or attached to meeting documents.

Knowing how the deliverables will be used makes it easier to decide not only which points to collect, but also which points should be named, which should be matched with photos, and whether to leave field notes. When surveying existing structures, it is important to decide in advance how the measured results will be used in decision-making, rather than focusing solely on the measurements themselves.

Also, confirming the required accuracy is essential. Not all survey points necessarily require the same level of accuracy, and by clarifying high-priority points—such as corners of structures, centerlines, boundary-related locations, and locations that affect construction positioning—you can improve on-site work efficiency. Conversely, allocating an excessive number of points to areas where a rough check is sufficient lengthens work time and complicates post-processing. Total stations are convenient instruments for managing position and elevation, but it is essential to choose a measurement method appropriate to the objective.

When confirming the purpose of a survey, it is also important to align understanding among the client, the designer, the contractor, and the surveyor. Even with the same phrase "confirmation of the position of existing structures," the designer may be thinking in terms of drafting, the contractor may be considering interference with heavy machinery or formwork, and the manager may be expecting inspection documentation. If you share, before work begins, what decisions the survey is intended to inform, you can more easily prevent rework caused by missing survey points or insufficient explanations.

Organize the scope of existing structures and the points to be measured

The next point to confirm is which parts of the existing structure will be included in the survey. Unlike new structures, existing structures do not necessarily match the drawings. Aging, repair marks, additional works, partial loss, buried sections, and changes in the surrounding ground can make it difficult to grasp the whole from appearance alone. For that reason, rather than increasing survey points on an ad hoc basis after arriving on site, it is important to clarify in advance the scope of the survey and the approach to selecting representative points.

When surveying existing structures, you should check not only corners and edges but also points where the alignment changes, locations where elevation changes, steps, joints, openings, and the positions of attachments. The features to focus on vary depending on the type of object, such as retaining walls, gutters, abutments, curbs, drainage facilities, building foundations, and pavement edges. Because the measurement points obtained with a total station remain as coordinate values, you must select points that capture the structure’s characteristics so the shape can be reproduced later when plotted on drawings.

When organizing the points to be measured, it is not sufficient to assume that taking as many points as possible is best. If there are too many points, it becomes difficult during post-processing to determine what each point represents. This is especially true for existing structures, where similar corners or steps may occur in succession; if point names or notes are unclear, the reliability of the deliverables decreases. It is important to adopt an approach of selecting representative points and, where necessary, adding supplementary points so that the intent of the measurements is clear.

When confirming the survey extent, consider not only the visible parts but also the surrounding areas that may affect construction and design. Even if the position of an existing structure itself is known, it can be difficult to use for construction planning or interface coordination if the surrounding ground elevations, adjacent structures, road edges, boundary areas, temporary structures, drainage directions, and so on are not known. As needed, measuring not only the outline of the structure but also surrounding reference points at the same time makes later explanations easier.

Also confirm in advance how to handle parts of a structure that are difficult to see, such as the backside or underside. A total station measures points with a line of sight, so it may not be able to directly measure behind obstructions or into the depths of narrow gaps. In such cases, you need to consider using different measurement methods, such as measuring from another instrument station, supplementing with a tape measure or leveling survey, or adding photographic records. Forcibly filling in unmeasurable parts by estimation can lead to misunderstandings later, so it is important to record and distinguish between the ranges actually measured and those treated as estimates or references.

Confirm the handling of reference points and coordinate systems

When surveying existing structures, not only the measurement points themselves but also the reference used for the measurements are critically important. Even if you obtain coordinates with a total station, if the handling of control points and the coordinate system is ambiguous you will not be able to correctly overlay them onto design drawings or other survey results. Especially when comparing the positions of existing structures with design drawings or verifying the planned positions for new construction, confirmation of control points needs to be completed before surveying.

First, what you should check are the positions, names, coordinate values, and elevation information of the reference points to be used. If there are multiple reference points on site, clarify which points will be used and which will be treated as check points. Even if reference points installed during past works remain, they may have been moved or damaged, so do not trust them based on appearance alone; it is advisable to verify the consistency of distances, directions, and elevation differences between known points. Omitting confirmation of reference points risks producing results in which, although you thought you had measured the structure correctly, the entire thing is shifted.

Also confirm which coordinate system will be used to compile the outputs, such as the plane rectangular coordinate system, a site-specific coordinate system, or arbitrary coordinates on the design drawings. In surveys of existing structures, past drawings may have been created in arbitrary coordinates, or the reference may differ by construction section. If work proceeds while coordinate systems are mixed, the numbers may appear to match on the drawings but the actual positional relationships may not. Before surveying, it is necessary to clarify the coordinate system to be used and whether any transformations are required.

Reference elevations are equally important. For existing structures, top-surface elevation, floor elevation, drainage slope, level differences, and the heights of attachment points all influence construction decisions. If the elevation reference differs from the design drawings or a temporary benchmark is used, record the rationale and its relationship. In height measurements made with a total station, instrument height, reflector height, sighting conditions, and measurement distance affect the results. When using elevation as a critical basis for decisions, the measurement method and the number of verifications should also be determined carefully.

In verifying control points, it is also effective to leave photos and notes of the points used on site. Recording only the point name can make it impossible to identify the same location later. By keeping a photo of the control point’s position, nearby landmarks, the coordinates used, the observation date, the observer, and the verification method, it becomes easier to explain the results and to recreate them during re‑survey. Survey results for existing structures may be rechecked later due to design changes or discussions, so records of the control points are as important as the survey points themselves.

Check the installation location and line-of-sight conditions of the total station

When measuring existing structures with a total station, where you set up the instrument greatly affects work efficiency and measurement quality. Around existing structures there are vehicles, temporary materials, vegetation, scaffolding, heavy machinery, level differences, traffic lanes, and so on, and it is often impossible to see all the points you want to measure from a single instrument station. Therefore, before starting work you need to plan the instrument station layout and line-of-sight conditions while checking candidate survey points.

When choosing a setup location, check not only whether the point to be measured is visible but also whether the instrument can be stably mounted. Soft ground, locations with heavy vibration, places that would obstruct passage, routes used by heavy machinery, and areas with steep slopes can cause problems securing the instrument’s stability and safety. A total station is a precision instrument, and an unstable setup reduces the reliability of the observations. Decide on the installation location after confirming that the tripod legs will not sink, that the instrument will not be contacted during work, and that it will not interfere with surrounding operations.

For line-of-sight conditions, confirm that there are no obstacles between the measurement target and the reflector. When surveying existing structures, there are often many points that are difficult to sight, such as corners, backsides, low positions, and narrow gaps. In locations where people or vehicles pass during measurement, the line of sight may be temporarily blocked. If you force measurements at points with poor line of sight, the reflector's position may shift or points may be misidentified. For hard-to-measure points, consider measures such as measuring from another instrument station, establishing auxiliary points, or adjusting the work schedule.

When establishing multiple instrument stations, also confirm the connections between those stations. To combine results measured from multiple instrument stations into a single dataset, commonly observed reference points, backsight points, or check points are necessary. If surveys are conducted independently at each instrument station, discrepancies can occur when integrating them later. When existing structures are large, or visibility is interrupted by buildings, retaining walls, or similar obstacles, it is important to plan instrument-station relocations and verify connections in advance.

It is also important to avoid placing the instrument in positions that bias the measurement angle or distance too much. If you measure an object from an extremely oblique angle, it can become difficult to align the point positions. For long-distance measurements, aiming errors and reflector setup errors can have a relatively large impact. From positions that are too close, you may not be able to view the entire structure and it can be difficult to understand the connectivity of measurement points. Depending on site conditions, using positions that allow you to grasp the whole and positions that let you check details makes it easier to achieve consistency in the results.

Verify the impact of reflectors and ranging conditions

In surveying with an optical total station, how the reflector is handled affects the measurement results. When measuring existing structures, you may place the reflector directly against an edge or face of the structure, but if the center of the reflector is offset from the actual point you want to measure, the coordinates will be shifted. In particular, for wall faces, column corners, curb edges, the inside of drainage channels, and step corners, you need to decide in advance which orientation to place the reflector and which position to define as the survey point.

When using reflectors, check the reflector constants and the measurement mode settings. If the settings do not match the reflectors actually used, distance measurements can be affected. On site, multiple reflectors may be used, but in that case verify that the instrument settings and the observation records match. For surveying existing structures, as the number of points increases it becomes harder to notice setting errors, so it is effective to make a habit of checking before starting work and after moving the instrument station.

In locations where it is difficult to place a reflector, reflectorless measurement may be used. Reflectorless measurement is convenient where installing a reflector is hard, but it can be difficult to measure distances depending on the target surface’s material, color, angle, wetness, dirt, or irregularities. Existing structures may have deteriorated surfaces or a mix of repair materials, so it is desirable to establish verification points before adopting measurement values as-is. For critical points, verify the validity of the measurements by cross-checking with measurements using a reflector or observations from different directions.

When the point you want to measure is recessed in a surface or around the back of a corner, you need to verify that the total station is actually capturing the intended point. For example, even if you think you are measuring the corner of a wall, you may actually be measuring a front chip or protrusion. When measuring inside a gutter or a drainage inlet, the position of the reflector's tip and the tilt of the staff can also affect the result. Whether you define the survey point as the "visible corner" or as the "structural design edge" changes what should be recorded.

Weather conditions and the surrounding environment also affect distance-measuring conditions. Strong sunlight, rain, fog, dust, steam, vibration, and the like can make aiming and distance measurements unstable. In tunnels, underground structures, and confined sites, attention must also be paid to lighting conditions and how reflective surfaces appear. Because surveying existing structures cannot always be done in ideal conditions, when measurement conditions are poor consider increasing the number of observations, conducting inspection surveys, and recording the measurement times and circumstances.

Confirm the surrounding environment and safety conditions

In surveying existing structures, it is essential to confirm not only the survey target itself but also the surrounding environment and the conditions for ensuring safety. Many existing structures are located where people, vehicles, and machinery are active—such as roads, factories, rivers, developed land, building exteriors, and facilities in operation. Setting up and operating an optical total station requires remaining in one place for a certain period of time; if coordination with nearby operations is not carried out, problems can arise not only in surveying accuracy but also in safety.

First, what you need to check is the conditions for entering the work area. On roads or within facilities that are in use, traffic control, entry permits, placement of traffic marshals, restrictions on working hours, and so on may be required. If there are elevated areas, slopes, waterways, openings, or the operating range of heavy equipment near existing structures, you need to decide in advance the movement routes of the surveyor and the worker carrying the reflector. Because concentrating too much on taking survey points can make you less attentive to your footing and surroundings, it is important to share hazardous locations ahead of time.

Next, confirm that the instrument’s installation location is safe. If a tripod is set up in a passageway, workers or passersby may come into contact with it. In areas used by vehicles, an instrument placed where it is hard to see can lead to collisions. While surveying, operators may be looking through or operating the instrument, which can slow their response to movement around them. As necessary, implement site-appropriate safety measures such as traffic cones, area demarcation, verbal warnings, or assigning a spotter.

Attention must also be paid to the condition of existing structures themselves. With aged retaining walls, cracked structures, unstable covers, corroded metal members, or easily collapsing slopes, approaching the survey point itself can be hazardous. Adopting an awkward posture to aim a reflector for an optical surveying instrument, or standing in an unstable location, carries the risk of trips or falls. Do not force an approach to dangerous areas; consider methods that allow measurements from a distance or the combined use of alternative measurement means.

Also, the surrounding environment affects the interpretation of survey results. If temporarily stored materials, temporary soil piles, structures scheduled for removal, or temporary construction works are included among the measurement targets, it becomes difficult later to determine whether they should be treated as existing conditions. When existing structures and temporary works are in close proximity, clearly record what was measured as existing. If you match photographs with survey point names, it will be easier during post-processing to exclude unnecessary points and to avoid misunderstandings in documentation.

On-site, surveying work can sometimes affect the progress of other operations. When measuring existing structures at a construction site, it is efficient to decide the order of survey points in advance to shorten downtime. It is important to consider the surveying plan and the safety plan together: measure critical points during periods of low traffic and activity, have hazardous locations verified by multiple people, and reduce the number of instrument station moves.

Confirm the recording method and post-processing workflow

Before measuring existing structures with a total station, confirm how the data acquired in the field will be recorded and which deliverables it will be compiled into during post-processing. In surveying work, attention tends to focus on obtaining coordinate values, but when surveying existing structures it is critically important to keep records that correctly convey the meaning of each point. If the person responsible for post-processing is not on site, or if there is a time gap between surveying and drafting, interpreting the data will take time unless point names, notes, photos, and measurement conditions are all provided.

First, decide how to name the measurement points. Use names that indicate the type of existing structure, the part, and the order of the measurement points so they are easier to organize later. Simple sequential numbering alone makes it hard to tell what was measured when the number of points grows. For example, if you categorize by structure and record names that clarify meanings such as top edge, corner, bottom, openings, and attachments, drafting and verification work will proceed more smoothly. However, trying to enter overly complex point names in the field reduces work efficiency, so it is practical to establish rules for abbreviations and notes in advance.

Corresponding photographic records is also important. Existing structures often have complex shapes, and similar locations can occur in sequence. Keeping close-up photos of the measurement points, overall view photos, and line-of-sight photos from the instrument station makes it easier to verify point positions later. It is effective to attach notes to the photos indicating the measurement point numbers and the parts. Because storing photos and coordinate data separately makes it time-consuming to confirm their correspondence later, it is desirable to establish rules on site for linking them.

Do not forget to record the observation conditions. The reference points used, instrument stations, backsight points, types of reflectors, reflector height, instrument height, measurement methods, weather, and any notes regarding visibility all serve as material to explain the reliability of the results. In particular, when surveying over multiple days or establishing multiple instrument stations, make sure you do not lose track of which point was measured under which conditions. If records are insufficient, it becomes difficult to trace the cause when questions arise about the figures.

Confirm the post-processing workflow before surveying. The information required on site will vary depending on whether you will place the acquired coordinates directly onto drawings, verify them together with point clouds or photographs, overlay them on existing drawings or design plans, or reflect them in cross-sections or development drawings. When creating drawings, you need the order of points to connect as lines and a distinction between the inside and outside of structures. If performing design comparisons, you need points that can be matched to design reference lines and planned elevations. If using the data to verify quantities, you must clarify the treatment of the extents and end points.

In terms of data management, it is important to consolidate survey data, photographs, field notes, and drawing data so they are not scattered and can be traced later. If file names and storage locations are not organized, you cannot retrieve the correct data when needed. Survey results for existing structures may be reused not only during construction but also for inspection, consultation, maintenance management, and additional work. Considering future use, retaining records that make clear when and where the data were measured, who measured them, and for what purpose increases the value of the results.

Reducing Re-measurements in Surveys of Existing Structures

Before measuring existing structures with a total station, it is important to check in sequence the surveying purpose, target range, reference points, instrument setup, reflectors, surrounding safety, and recording methods. If you skip these checks and start measuring, even if work appears to be progressing on site, problems are likely to occur later, such as drawings not matching, point meanings being unclear, necessary locations being missing, or an inability to explain the reference points. Surveying existing structures is a task that requires judgment adapted to local site conditions, and preparation before measurement greatly influences the quality of the results.

What's particularly important is to clarify "what you are measuring for" and "by what reference you are measuring" before taking measurements. Once the purpose is understood, the necessary measurement points are determined, and if the reference is clear it becomes easier to reconcile with other drawings and survey results. Furthermore, because there will be places on site that cannot be measured or are difficult to measure, rather than forcibly aligning numbers alone it is important to record separately the actual measured range, supplementary information, and points of caution. Survey results should not be merely a list of coordinate values; they need to be organized so they can be used to make decisions later.

Total stations are effective instruments for determining the position and elevation of existing structures, but if integration with field records, photographs, and drawing organization is lacking, the usefulness of the results is limited. If you anticipate explanations and sharing after surveying, it is effective to record and associate the on-site position data, photos, survey point names, and observation conditions, and organize them so stakeholders can confirm them with a common understanding. By establishing rules for pre-checks and recording before starting work, you can reduce re-surveys and insufficient explanations, and make it easier to use the survey results of existing structures for construction planning, consultation materials, and inspection responses.