Table of contents

• Introduction

• What is as-built management? Its importance

• What is layout setting (reverse staking)? Its importance

• Issues with conventional methods and causes of rework

• What is RTK? Basics of high-precision positioning

• Preventing as-built and layout errors using RTK

• Further efficiency through digital technology

• Benefits RTK brings to the site

• Conclusion: aiming for zero rework

• FAQ

Introduction

On construction sites, even when work seems to proceed according to plan, redo work (“rework”) often occurs afterward. Rework not only wastes labor but also leads directly to material loss and schedule delays, representing unnecessary costs the site wants to avoid. Rework caused by surveying-related mistakes in particular can have a large impact on schedule and cost. To reduce such needless rework and improve site efficiency, it is important to eliminate the errors themselves in processes such as as-built management and layout setting during construction. This article reviews common challenges and causes of errors in as-built management and layout setting, and explains the use of RTK (high-precision GNSS positioning technology), which is seen as a key solution. From a field perspective, we will look at how the latest digital surveying DX (digital transformation) technologies can improve quality and reduce rework.

What is as-built management? Its importance

As-built management is one of the construction management processes for checking and recording whether completed structures and formed terrain match the shapes and dimensions specified in the design documents. Especially in public works, contractors must demonstrate that completed road widths, thicknesses, heights, and structure dimensions fall within the standards defined by the client (national or local government). Because the results of as-built management determine the pass/fail of inspections and are prerequisites for handover, it is a crucial element for ensuring quality.

For long-term or large-scale projects, confirming and recording intermediate as-built conditions during the work is also essential. Construction status of underground buried items that will later be backfilled and hidden, or the reinforcement state before concrete placement, must be reliably documented with photos and measurement data during construction, otherwise it becomes impossible later to prove that work was performed according to the design. Therefore, sites carefully measure as-built conditions at the completion of each process and, if deviations from standards are found, correct them early to prepare for final inspection.

What is layout setting (reverse staking)? Its importance

Layout setting (also called survey layout or reverse staking) is the work of accurately reproducing the positions and elevations of structures shown in the design drawings on the actual site. Surveying is performed from control points or origin points, and design points are indicated on site by staking and marking. The accuracy of layout setting at the start of construction can determine the quality of the entire subsequent project. Conversely, if errors occur during layout setting, they can lead to position shifts or dimensional discrepancies of structures and cause substantial rework in later stages. Therefore, layout setting is, alongside as-built management, a critical process for ensuring construction quality.

Issues with conventional methods and causes of rework

However, performing layout setting and as-built management with conventional surveying methods involved a lot of effort and risk. Typical layout work required mounting expensive surveying instruments such as total stations and conducting the work with a team of two or more people, including a surveyor and a helper. Skilled technicians calculate angles and distances from control points, while a helper guides and marks stake positions on site using a prism. This analog method leaves room for human error during measurement and marking. When reading distances with a tape measure while holding paper drawings, numbers may be misread or instructions misheard, resulting in position shifts that often lead to repeated re-measurements.

For as-built management (measurement and inspection of as-built conditions), the traditional approach was for staff to measure heights with a level, measure thicknesses and widths one point at a time with a tape or caliper, and handwrite those values on paper forms. With such analog work, increasing the number of measurement points requires enormous time and labor, so only a very small portion of the site can be sampled. Consequently, differences from the design in unmeasured areas can be missed. Also, during transcription of site notes later in the office or when transferring data into reports, copying errors and omitted records are unavoidable. In busy work environments, forgetting to take photos or losing data is common, and there are many cases where necessary evidence for inspection is not available and panic ensues.

Thus, conventional surveying and as-built management methods that rely on manual work have weaknesses such as “limited measurable points,” “prone to recording and communication errors,” and “dependence on skilled workers.” As a result, rework is caused when discrepancies with the design are discovered after completion, or when incomplete records lead to rework orders during inspection. In public works especially, a single failed inspection can cause large delays and cost increases across the entire schedule, so reducing rework is a major challenge for sites.

What is RTK? Basics of high-precision positioning

RTK (Real-Time Kinematic) is a high-precision positioning technology that dramatically improves the accuracy of satellite positioning (GNSS/GPS). Normally, GPS used in smartphones and car navigation has positioning errors on the order of several meters (several ft), but RTK applies correction information from a reference station (base station) in real time to determine position with an accuracy within a few centimeters (a few in). Specifically, correction data is received via the Internet from a mobile base station installed near the worksite or from GNSS reference station networks provided by organizations such as the Geospatial Information Authority of Japan, and the GNSS receiver applies those corrections to its position measurement. In Japan, the Quasi-Zenith Satellite System “Michibiki” also provides high-precision positioning services (from sub-meter to centimeter-level (cm-level)), and with compatible receivers it is possible to obtain high-precision position information without preparing a dedicated base station. In recent years, promoted by the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* (ICT construction) initiatives, high-precision surveying using RTK has been rapidly spreading in the civil engineering and construction industry.

Position information obtained by RTK is acquired in real time as absolute coordinates referenced to known site control points. This makes it possible to directly indicate survey points on site with GNSS positioning, which were previously derived by measuring distances from control stakes with tape. Without needing to carry heavy tripods and total stations to maintain line of sight, wide-area surveying can be performed in a short time by placing an antenna and receiver where the sky is visible. In other words, by utilizing RTK, layout and as-built measurement tasks that previously required skilled personnel can be performed with simpler procedures and high accuracy.

Preventing as-built and layout errors using RTK

So how exactly does onsite use of RTK help prevent errors? For layout work, using an RTK-compatible GNSS receiver together with a smartphone allows you to confirm the coordinates from the design drawings on site and perform staking and marking accordingly. Even without a skilled surveyor, workers can accurately identify points by observing the discrepancy between the target point coordinates displayed on the device screen and their current position. For example, selecting a target point in a surveying app on a smartphone will display the direction and distance from the current location to that point. The worker moves in the indicated direction and stakes or marks the point where the distance reaches zero, which corresponds exactly to the design position. Moreover, because operations that used to require both a surveyor and a helper can be completed by a single person, there is no risk of instruction communication errors or downtime losses. Device guidance reduces reliance on human intuition or experience, significantly lowering the risk of position shifts due to misreading a tape or manual calculation errors.

RTK also excels in as-built measurement. Where previously only a few points were measured, an RTK receiver carried by hand can measure many points in a short time simply by walking the site. For example, when checking the road subgrade elevation, instead of measuring a few spots every several tens of meters (every several tens of ft) with a level and estimating average thickness, RTK makes it easy to obtain surface elevation data across the entire road by walking. All as-built data measured with RTK are recorded in a unified coordinate system and stored in the cloud, so later automatic error checks against design data can be performed. It is also possible to immediately determine whether a measured value meets the specified standards on site; if an out-of-spec area is found, it can be corrected immediately and re-measured. In this way, RTK enables an environment where measurement omissions and oversights are prevented and verification can be performed on site in real time. As a result, the risk that “only part of the structure lacked thickness” discovered later is significantly reduced.

Further efficiency through digital technology

Alongside high-precision positioning with RTK, digitizing recordkeeping through cloud services and mobile apps advances efficiency. Coordinates of measured points and results of as-built measurements are automatically saved to the cloud without manual note-taking. Because data recording completes simultaneously with measurement, concerns about transcription errors or missing records are eliminated. Cloud data can be used to automatically generate forms, reducing the wasted effort of repeatedly re-entering site-obtained values. Photos of as-built conditions taken on smartphones or tablets can be stored in the cloud with geolocation and timestamps, so there is no confusion later about “where on site was this photo taken?” and photos can be managed together with drawings and measurements. Introducing these digital technologies creates an environment where site-collected information can be shared and utilized immediately. Cloud-aggregated data can also be shared instantly with office staff or remote stakeholders, enabling remote site monitoring or remote witness inspections (online attendance). This shortens lead times for reporting and inspection considerably.

Benefits RTK brings to the site

By introducing surveying DX centered on RTK as described above, construction sites gain the following benefits:

• Significant reduction in rework: Improved surveying accuracy and on-the-spot verification minimize the risk of rework after construction.

• Quality assurance: Detailed measurements covering the entire site and automatic recording allow variation in construction quality to be detected and corrected.

• Improved operational efficiency: Time spent waiting for surveying teams and organizing paper forms is eliminated, enabling one person to quickly complete surveying and recording.

• Prevention of human error: Automatic saving of measurements and digital guidance prevent input errors, misreads, and missed measurements.

• Response to labor shortages: Tools that can be used without special expert skills enable younger staff or personnel from other disciplines to perform surveying tasks.

• Improved safety: Shorter surveying times in hazardous areas and reduced work near heavy machinery or at height lower the risk of occupational accidents.

• Contribution to DX promotion: Utilizing the latest technologies in line with the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* initiative supports site digitalization and productivity improvement.

Conclusion: aiming for zero rework

To eliminate unnecessary rework on construction sites, improving the accuracy of surveying processes such as as-built management and layout setting and promoting DX are indispensable. Introducing the latest surveying technologies, including RTK, enables tasks that used to rely on craftsmen’s intuition and effort to be performed accurately by anyone. In practice, a new style of “one smartphone per person for surveying” is being realized at sites across the country, and AR combined with RTK allows the experience of “bringing the drawing directly to the site and digitizing the site as-is.” The emergence of handheld, high-precision smartphone RTK solutions offers a trump card to address labor shortages and rising quality demands.

In particular, using smartphone-compatible high-precision positioning tools such as LRTK dramatically simplifies formerly complex layout and as-built measurement tasks, making it possible for a single person to perform accurate surveying. Why not proactively adopt the latest technologies, aim for a zero-rework site, and take on the DX challenge in quality management?

FAQ

Q: What is the difference between RTK and regular GPS positioning? A: Regular GPS can have position errors of several meters (several ft) due to satellite signal errors, but RTK uses correction data from a reference station to reduce errors to about a few centimeters (a few in). Simply put, RTK is a system that makes GPS more accurate and is powerful in applications that require high precision, such as civil engineering surveying.

Q: What equipment and environment are needed to use RTK? A: RTK surveying requires GNSS receivers that support centimeter accuracy (cm level accuracy (half-inch accuracy)) and a base station that provides correction information (or a reference station service via a network). For example, you can set up a portable base station on site or use paid correction services provided by mobile carriers or the Geospatial Information Authority. Recently, small RTK-capable receivers that attach to smartphones are commercially available, and with a smartphone and a dedicated app you can easily start RTK positioning.

Q: How accurate is RTK positioning? A: It depends on the equipment and environment, but generally RTK achieves horizontal position errors of about 2–3 cm (0.8–1.2 in) and vertical (elevation) errors of a few centimeters to about 5 cm (a few in to about 2.0 in). In open areas with a clear view of satellites, the accuracy is sufficient for most civil engineering as-built management. However, accuracy declines or positioning may become impossible in environments where satellite visibility is obstructed, such as between tall buildings or under tree cover.

Q: Can RTK be used in rain or mountainous areas? A: Basically, as long as the sky is visible outdoors, RTK positioning is not significantly affected by light rain. However, extreme weather such as thunderstorms or typhoons can make radio propagation unstable and may cause positioning instability. Also, in mountainous areas outside mobile communication coverage, network-type RTK correction data cannot be received, so you need to set up a temporary base station on site in advance or secure communications.

Q: Can less-experienced technicians use RTK effectively? A: Yes. Modern smartphone RTK systems are designed with user-friendly interfaces, and with basic training site supervisors and junior engineers can use them effectively. Unlike total station surveying, which requires advanced specialist skills, RTK devices provide navigational guidance based on coordinates, so users with basic knowledge of coordinates and surveying can intuitively perform measurements. Creating an environment where everyone on site can perform surveying prevents work from concentrating on specific personnel and reduces waiting time and improves staff allocation efficiency.

Q: Isn’t total station surveying more accurate? A: Indeed, for cases that require millimeter-level precision—such as aligning steel frames or machine equipment—optical total stations and levels remain indispensable. However, for typical civil engineering as-built management and staking, RTK positioning with centimeter accuracy (cm level accuracy (half-inch accuracy)) is sufficient in most cases. RTK also has the advantage of being able to position without line of sight and cover wide areas by a single person, so it is effective to use RTK for routine surveying and combine optical surveying only for processes that require millimeter verification.

Q: What is “LRTK” mentioned in the article? A: LRTK is a solution that enables centimeter-level surveying using a smartphone (such as an iPhone). A compact GNSS receiver connects to the smartphone and, via a dedicated app, provides RTK positioning for layout setting, 3D scanning, and AR-guided staking. With cloud integration, coordinate data and photos collected on site can be shared and managed instantly. LRTK was developed as a tool to support as-built management DX compatible with *i-Construction*. It has been introduced at many construction sites and supports the new style of “one smartphone per person for surveying.” By enabling centimeter-level surveying—previously requiring expensive equipment and specialized knowledge—with a “single smartphone,” it aims to make surveying DX accessible to everyone.