Improving On-site Efficiency with Reverse Sequencing: Dramatically Shortening Construction Time with Smartphone Surveying × DX

What “reverse sequencing” means: onsite meaning and background

“Reverse sequencing (gyakuuchi)” refers to performing tasks in an order that is essentially the reverse of the usual one. In construction, for example, the reverse-sequence construction method (top-down construction) is well known. By constructing underground floors sequentially from the upper floors, this method allows aboveground and underground work to proceed in parallel, which can significantly shorten the overall construction period. In surveying, the work of laying out design coordinates on site (staking out / marking) is sometimes called “reverse-sequence surveying.” This is the work of placing stakes at prescribed distances and directions from control points using instruments such as total stations, and it is a process that requires skill.

This article focuses on applying this “reverse-sequence” thinking to as-built management. Specifically, the approach is to consolidate the as-built recording and surveying, which were traditionally performed sequentially at each construction stage, and perform them collectively at the end of the process. Typically, in civil engineering works, after each stage is completed the as-built (the completed shape and dimensions) is checked against the design and recorded with photos and drawings. However, with conventional methods, each of these checks requires staff presence and interruptions to work, which can impede site progress. Therefore, a movement has emerged to improve efficiency by adopting a “reverse-sequence” style where as-built surveys are performed in bulk at the end of the process. Recent advances in ICT technology have made it increasingly possible to perform high-precision as-built measurements after construction is complete and finish recording and inspections in a short time. First, let us look at what problems existed with the conventional approach.

Problems with conventional as-built recording and attendance

Conventional as-built management processes have been criticized for being inefficient due to stopping work for measurements and attendance at each step. The main issues include the following:

• Work fragmentation and waiting times: After each stage is completed, personnel must perform measurements and obtain the client’s (supervisory staff’s) on-site confirmation, causing the sequence of work to be interrupted each time. Work is put on hold until an inspector can come to the site, and heavy equipment or workers often stand idle. Waiting times can become long—especially for large-scale works or when staff have multiple responsibilities—leading to delays in the overall schedule.

• Duplicate work and rework: In as-built inspections, double measurements often occur: the contractor first conducts self-checks, and then the supervisor rechecks extracted points. This doubles the effort of measuring the same locations, and if opinions differ, additional re-measurements or re-attendances become necessary. Even if inspections are performed at each stage, if it is later discovered that dimensions are not correct, rework such as removing and rebuilding already constructed parts may be required, resulting in wasted cost and time.

• Limited measurement coverage and risk of omissions: Due to time constraints, conventional practice typically measures only selected locations at each stage. For example, measuring the fill thickness or concrete thickness at a few representative points may fail to capture overall variability and risk overlooking deficient areas. Manual point-by-point measurement methods have coverage limits, posing quality-management risks.

• Dependence on personnel and experience: Operating surveying equipment and creating record drawings require experienced technicians, and securing capable staff is a challenge when manpower is scarce. In particular, as-built surveying using total stations often requires a surveying team of two or more, and there are many situations that rely on the seasoned intuition of skilled workers. Increasing workload for experienced personnel and stalled generational transition have also hindered on-site efficiency improvements.

As described above, traditional as-built recording and on-site inspections inherently contain causes of work stoppages and inefficient duplication. So how can performing as-built recording in bulk by reverse sequencing solve these issues? The next section summarizes the benefits.

Benefits of introducing reverse sequencing (reduced personnel, shortened construction time, standardized record quality)

Introducing a reverse-sequence approach that consolidates as-built recording and inspection at the end of the process brings three major benefits to the site.

• Reduced manpower (fewer personnel): By leveraging digital technology, as-built management tasks that previously involved multiple people such as surveyors and supervisors can be completed by a small team. For example, with an RTK-GNSS survey instrument one person can sequentially acquire measurement points, so verification of many pavement elevation points that used to be measured by teams of two can be handled by a single technician. The need to have heavy-equipment operators or assistants accompany measurements is reduced, easing personnel coordination. Reducing manpower means inspections can be handled even on sites suffering from labor shortages.

• Shortened construction period: Reverse sequencing reduces interruptions for inspection waiting, which in turn shortens the overall construction period. It minimizes schedule coordination for on-site inspections and allows construction and inspection to proceed in parallel and more efficiently. For example, if inspection processes that used to require several days for each completed stage can be completed in one to two days at the end, the total construction period can be compressed accordingly. Like the top-down construction method used in large urban projects, reversing the process and parallelizing stages accelerates the overall progress. If inspection becomes less of a bottleneck, the project is more likely to finish smoothly as planned.

• Standardization of record quality: Reverse-sequence as-built management using digital technology can standardize the quality of recorded data at a high level. When recording is done manually and partially, precision and granularity tend to vary depending on the technician’s experience and recording method. However, photogrammetry or 3D scanning allows planar measurement of entire structures, enabling complete capture of as-built conditions. Measurement results are integrated in the cloud and automatically converted into drawings and reports, reducing human error and standardizing formats. In other words, regardless of who performs the work, a certain level of detail and accuracy can be retained, eliminating variability in as-built records. Storing records as digital data also makes later review and analysis easier, improving the reliability of quality control.

In addition to these benefits, remote measurement and automation also enhance safety. If people do not need to measure directly in hazardous locations, the risks associated with high-elevation work or measurements near heavy equipment are reduced. Reverse-sequence efficiency improvements not only speed up work but also raise safety and recording accuracy across the board.

Supporting reverse sequencing with smartphone surveying (use of LRTK, RTK-GNSS, and SfM)

Digital surveying technology is the key to realizing reverse-sequence as-built management. Among these, smartphone surveying, which is easy to use on site, is an attractive means that achieves both labor savings and high accuracy. Recently, small GNSS receivers that attach to smartphones have enabled position measurements comparable to conventional surveying instruments.

For example, the LRTK system lets you attach a dedicated small antenna to a smartphone with one touch, instantly upgrading smartphone GPS—which normally has errors of several meters—to centimeter-level accuracy (half-inch accuracy). This uses RTK-GNSS (real-time kinematic positioning) technology, which applies correction information sent from a base station to cancel positioning errors in real time. LRTK devices support high-precision correction data delivered by Japan’s Quasi-Zenith Satellite System (Michibiki) such as CLAS and networked commercial GNSS correction services, so a smartphone alone can obtain position information equivalent to expensive surveying instruments. The devices are palm-sized weighing a few hundred grams and include a battery, so they can be carried and used without a surveying pole or tripod. On site, you can take the device from your pocket, attach it to your smartphone, launch an app, and instantly acquire precise coordinates of your current location. No complex settings or calculations are needed; the app automatically converts to the plane coordinate system and applies geoid-height corrections, so the operator simply points the smartphone at the point to be measured. It feels as if the smartphone itself becomes a high-precision surveying instrument, and inexperienced technicians can intuitively use it.

Furthermore, smartphones have versatile functions such as photography and AR (augmented reality) display that can comprehensively support as-built data acquisition. For example, by taking many photos with a smartphone camera and using Structure from Motion (SfM) for photogrammetry, you can generate detailed 3D models (point cloud data) of structures and terrain. Combining these with images taken from drones enables comprehensive recording of wide areas in a short time. If positioning coordinates acquired with an RTK-capable smartphone are embedded in the photos, the generated point cloud model gains accurate spatial scale, allowing the entire site to be reproduced as a digital copy in the design coordinate system. Thus, using a smartphone alone or in combination with a drone dramatically improves the efficiency of as-built measurement that was previously laborious. Measurements of areas inaccessible to people and full recording of complex shapes can be completed by simply photographing them. After shooting, automatic point-cloud generation and photo stitching occur in the cloud, so even those without specialized skills can increasingly obtain high-precision as-built data.

Implementing such smartphone surveying solutions on site strongly supports efficient as-built management via reverse sequencing. Quick pinpoint measurements with an RTK-GNSS–equipped smartphone combined with SfM photogrammetry to complement overall shape allows aggregation of all as-built information at once. When necessary, a smartphone’s AR functions can overlay design models onto the real view to verify points on the spot, preventing missed measurements or misunderstandings. It is truly a technology that symbolizes on-site DX (digital transformation).

Reducing the burden of attendance with cloud integration and automatic report generation

As-built data obtained by smartphone or drone becomes even more valuable when integrated with cloud services. Uploading photos, point clouds, and coordinate data to the cloud enables immediate site status checks from the office or remote locations and centralized information sharing among stakeholders. The Ministry of Land, Infrastructure, Transport and Tourism is promoting cloud sharing of construction management materials, and environments enabling all parties to view drawings, photos, and inspection data at any time are progressing. Aggregating the latest as-built data in the cloud allows contractors, subcontractors, clients, and supervisors to always refer to common information, preventing omissions in paper logs and communication errors.

Cloud integration is most effective for remote as-built verification and attendance inspections. Inspections that used to require supervisors to visit the site and visually check can now be conducted while viewing 3D models and photos on the cloud. For example, uploading a point cloud of a completed structure and having the client and contractor review the same model during an online meeting makes remote inspections possible. If additional measurements are required, on-site staff can take measurements with a smartphone and immediately share the data, allowing real-time responses to issues. There are actual cases where drone survey data from a mountain-area project was sent to an urban office and a remote inspection was completed the same day, and cases where as-built inspections that used to require nearly ten people on-site for an embankment were completed with one person on-site plus several remote participants. The benefits of zero travel time and small, non-contact teams are substantial, directly enabling flexible scheduling and personnel-cost reductions.

On the cloud, it is also possible to automatically generate reports based on acquired data and immediately compile inspection results into electronic delivery formats. This eliminates the manual effort of creating photo logs and as-built management documents, reducing the paperwork burden on site representatives. Automatically calculating dimensions and volumes from the data and populating prescribed formats also helps prevent human errors. Paper output remains possible when required, but sharing and storing digital data on the cloud is becoming the norm.

By handling everything from data acquisition through record creation and sharing on the cloud, unnecessary waiting times and site travel associated with attendance are drastically reduced. Once data is uploaded, stakeholders can access it anytime, so situations like “missing photos that require re-shooting later” are less likely. Even if additional verification is needed, it is increasingly possible to analyze existing point cloud data remotely without visiting the site. Cloud integration and automatic report generation are mechanisms that support reverse-sequence efficiency improvements from the operational side.

Case studies and workflow in actual reverse-sequence construction sites

How does a site concretely change if it implements DX for reverse-sequence as-built management? Below we examine the differences between traditional and new workflows using an urban underground construction example. Consider a redevelopment project where a high-rise building with three underground parking levels adopts a reverse-sequence construction method (constructing the underground frame from the top down).

Traditional flow (sequential surveying and attendance)

• Reference setup and start of construction: At the start of work, a surveying crew arrives to measure and set out reference stake positions and heights with a total station. As construction progresses, each stage (retaining wall work, piling, concrete placement, etc.) is paused upon completion to measure as-built.

• As-built inspections at each stage: Supervisors are called to the site to check, for example, exposed pile head positions and structural dimensions each time. In poorly sighted underground spaces, surveying takes time and multiple people are required to reestablish and confirm measurement points. In some cases, another surveying crew performs double-checks to carefully confirm that pile-position errors are within allowable ranges.

• Rework when problems occur: If measurement results deviate from design values, corrective measures are considered on the spot. If deviations cannot be corrected with minor adjustments, parts may be repositioned or additional work performed to prevent impacts on subsequent stages. Making these decisions and corrections consumes time and manpower, and other work is forced to wait.

• Record creation: Photos and measurement values taken at each stage are organized and compiled into Excel or paper forms. Only after receiving the supervisor’s confirmation stamp does the next stage proceed. After all stages are complete, the inspection records for each phase are compiled into submission documents.

New flow (reverse-sequence surveying DX)

• Preparation: At the start of work, only the installation of control points (datum points) is performed, and otherwise only the minimum internal confirmations are made between stages. An RTK-GNSS base station is set up on site in advance and communication environments are prepared so smartphone surveying can be performed immediately. Marked reference points for photogrammetry are also placed at key locations.

• Parallel progression of construction: Construction managers monitor daily progress but do not stop the site for detailed as-built surveys. For essential stakeouts and structural alignments, smartphone AR navigation is used, and workers follow on-screen arrow guidance to place components in the correct positions. Tasks that traditionally required a surveyor for setting out pile centers can now be performed instantly with a smartphone in hand. Workers simply follow on-screen instructions, allowing precise positioning even in confined underground spaces. For example, using AR guidance to verify column center positions and angles enabled rapid and precise placement of columns along a curved underground roadway. At this stage there are no pauses for inspection; construction continues continuously while observing safety management.

• Comprehensive as-built surveying after construction completion: When the structure including the underground frame is complete, a smartphone surveying team conducts concentrated as-built measurement. Specifically, the site supervisor measures coordinates of major points with an RTK-capable smartphone while another staff member photographs the entire site with a smartphone or drone. Hundreds of photos can be taken in a short time to record every surface of the structure and ground. Narrow areas are supplemented with a handheld scanner as needed so there are no blind spots in coverage. Measurement data are transmitted to the cloud on the spot and automatically converted into point cloud models and plan views. Dimensions of critical structural elements such as piles and columns can be compared at a glance against design values on the digital model.

• Remote attendance inspection and immediate feedback: The client’s inspection staff checks the as-built data uploaded to the cloud from their office PC. A web conference connects the site and reviewers, and a remote attendance inspection is conducted while both parties view the cloud-based 3D model and photo log. Inspectors can cut arbitrary sections to measure dimensions or point out concerns. If clarification is needed, on-site staff take additional photos and upload them to the cloud immediately. What used to require taking data back for review is now fed back in real time, enabling corrective actions or additional checks to be completed the same day. Only one on-site supervisor who can operate the smartphone is sufficient, and supervisory staff can complete inspections remotely without traveling.

• Automatic report creation and submission: After inspection, as-built management documents and photo logs are automatically generated in the cloud. The responsible person only needs to review and electronically approve them; there is no need to transfer values to Excel or paste photos manually. The complete as-built dataset is organized as electronic delivery data and submitted/shared online. There is no need to collate and mail paper documents; records are stored entirely as digital data.

With this new workflow, time spent on surveying and inspection was dramatically reduced. Stakeout work for pile coordinates was completed in about 1/6 the time compared with optical methods【※】, and because schedule coordination for as-built inspections was no longer necessary, the project achieved overall schedule reductions on the scale of several weeks. The number of on-site personnel required to handle surveying through inspection was reduced to 1–2 people (whereas traditionally a cumulative dozen or more people were involved across stages). As a result, improved personnel allocation efficiency and reduced safety-management burden were major outcomes. Combining the inherent benefits of reverse-sequence construction with the effects of digital surveying DX makes next-generation construction management possible that balances speed, accuracy, and labor savings.

Points of caution and tips for successful implementation

To smoothly introduce reverse-sequence as-built management, there are several points to keep in mind. Below are tips for success.

• Prepare high-accuracy control points (datum points): Before performing surveying with smartphones or drones, install control points with known coordinates on site. Ground control points are essential to achieve accuracy in photogrammetry, and cross-checking with site control points is important for RTK-GNSS positioning accuracy management. To avoid discrepancies with public coordinate systems, measure and set a sufficient number of control points around the construction area.

• Plan surveying to avoid blind spots: Each measurement method has blind spots—drones cannot photograph directly under structures or covered areas, while ground photos cannot capture an overhead view. Compensate for these blind spots by combining methods: supplement high or rear areas with terrestrial laser scanners or pole-mounted cameras, or analyze drone and ground photos together. Aim for no blind spots in data acquisition by using multiple means. For narrow tunnels or buried sections, integrate partial measurements taken during construction with post-completion overall measurements to prevent omissions in as-built information.

• Standardize photo-taking rules: To stabilize point cloud generation from SfM and the quality of record photos, establish unified photography rules. Fix photo resolution and exposure as much as possible, and take photos from various angles with an overlap rate of around 60–80%. Standardizing shooting methods within the team ensures that consistent-quality models are obtained regardless of the photographer. Also set rules for file names and folder organization in advance so cloud organization and search are easy for subsequent processes.

• Train members and run trials: When introducing new tools, pre-training and pilot operations for site staff are essential. Conduct smartphone surveying and cloud operation training before construction and test on a small scale. Gaining initial small successes builds confidence in DX use across the site. When everyone can measure with a smartphone, younger staff can collect data even in the absence of veteran technicians, enabling better use of human resources.

By planning with the above points in mind, you can maximize the benefits of reverse-sequence as-built management. The key is “prepare well up front, then rely on digital technology.” With initial settings and team agreements in place, smartphones and the cloud will automatically consolidate the results.

Conclusion: Accelerating on-site DX with smartphone surveying and LRTK

The reverse-sequence approach to as-built management is expected to be a trump card for solving on-site challenges such as labor shortages and pressure to shorten construction time. Using ICT centered on smartphone surveying enables a small team to efficiently perform high-precision checks, making construction workflows that overturn conventional wisdom a reality. DX-driven transformation of construction management is no longer limited to advanced sites; it is becoming required across a wide range of projects.

That said, many sites may hesitate to adopt expensive 3D scanners and ICT due to high initial costs. This is where lightweight, high-precision surveying tools using smartphones and small GNSS receivers—LRTK—are worth attention. Because LRTK uses smartphones as the platform, it is easy to handle, and staff with limited surveying knowledge can intuitively operate it to obtain precise as-built data. Tasks that used to rely entirely on veteran surveyors can be delegated to younger staff using LRTK, greatly contributing to labor savings on sites with manpower shortages. LRTK is more mobile than dedicated instruments and can safely survey at high elevations or on slopes with poor footing, reducing workload and risk. Acquired data can be shared in real time to the cloud, enabling operations that share information with inspectors on the spot. Although the latest technology may sound daunting, LRTK uses familiar smartphones, lowering psychological barriers and easing site adoption.

In this way, LRTK is a tool that dramatically lowers the barrier to ICT use in as-built management. It realizes an environment where “anyone, anywhere, easily” can perform high-precision surveying and strongly supports efficiency gains through reverse sequencing. Start digital measurement with smartphone surveying even in a small part of routine site operations. With LRTK, you can take the first step toward on-site DX as early as tomorrow. Those aiming to streamline sites with reverse sequencing should consider adopting such easy-to-use ICT tools—the future of the site is sure to change significantly.