Improving Construction Quality with RTK Surveying:
A Guide to Precise Measurements on Civil Engineering Sites

Why Precise Measurements Lead to Improved Construction Quality

Precise positioning through RTK surveying is directly linked to quality control in civil engineering projects. When traditional surveying errors are within several meters, even if construction follows the design plans, discrepancies in the final product (as-built) can occur. However, with RTK's centimeter-level positioning, it becomes easier to place the structure at the intended position and height, ensuring the quality is maintained according to the design. For example, by accurately positioning structural elements such as bridge piers or tunnels, misalignments at the joints or junctions of components can be reduced, leading to improved durability and safety of the entire structure. Additionally, in road construction, accurately managing the thickness of subgrade or pavement ensures a smooth finish with minimal bumps, improving the durability and longevity of the surface. Enhanced surveying accuracy allows for the early detection and correction of rework or construction mistakes, ultimately preventing quality issues and shortening project timelines, resulting in cost savings.

Furthermore, real-time positioning with RTK offers the advantage of obtaining measurement results immediately during construction, making it easier to perform quality checks on-site. For example, during excavation, the depth can be instantly verified to ensure it meets specifications, and additional excavation or backfilling can be instructed as needed. This prevents situations where errors are discovered later during additional surveying, avoiding the need for rework and ensuring that the construction meets quality standards from the outset. In short, the "peace of mind of knowing immediately after measuring" that precise measurements bring reduces on-site decision-making errors and significantly improves the accuracy of quality management. To build high-quality infrastructure, precise surveying down to the millimeter level is essential.

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Specific Use Cases of Precision Measurements with RTK
The power of RTK surveying is demonstrated in various civil engineering construction scenarios. In this section, we will focus on four key areas where RTK is increasingly utilized: benchmark surveying, as-built management, excavation and backfilling management, and pavement thickness management. We will introduce specific examples for each of these cases.

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Utilization in Benchmark Surveying – Accurate Benchmarks Are Key to Construction

Every construction project begins with setting up a benchmark surveying point (benchmark) on-site. With RTK surveying, this benchmark surveying can be performed quickly and with high precision. Traditionally, time-consuming methods such as traverse surveying were used to establish new benchmark points within the surveying range from known points. However, with RTK, coordinates can be directly transferred from electronic reference points or known coordinate values to the site.

For example, using the network-based RTK (VRS method) provided by the Geospatial Information Authority of Japan, real-time correction data can be obtained from the nationwide network of electronic reference points, allowing for centimeter-level benchmark surveying at any point on-site. This enables multiple benchmark points to be set up quickly, even in large construction areas, ensuring smooth subsequent surveying and construction processes.

The benefits of using RTK in benchmark surveying are significant in terms of both personnel and time savings. In traditional total station (TS) triangulation surveying, a two-person team is usually required. However, with RTK-GNSS, one worker can set up the survey points simply by carrying an antenna and walking around the site. Even in forests with poor line-of-sight or in urban areas with limited visibility, benchmarks can be set up as long as an open view of the sky is available. Furthermore, the risk of accumulated surveying errors between known points is reduced. Since RTK always performs positioning linked to the global coordinate system (GNSS), errors are less likely to accumulate over long distances, making it easier to maintain uniform benchmarks across multiple remote sites. By quickly obtaining an accurate benchmark, the precision of subsequent as-built measurements and stakeout work is enhanced, contributing to an overall improvement in construction quality.

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Utilization in As-Built Management – Real-Time Verification of Finished Work

As-built management is the process of verifying that the completed structure or construction component matches the design dimensions and shapes, and conducting quality checks. RTK surveying plays a key role in this as-built management. For example, after road subgrade shaping or the completion of embankment and excavation work, measuring the surface height and slope with RTK immediately reveals any discrepancies with the design values. Since RTK can detect errors down to a few centimeters in real time, if there is any deviation from the standard values, corrections can be made on-site, and the measurements can be rechecked with RTK, all within the same day. This approach is far more efficient compared to the traditional method of taking measurements with total stations (TS) or leveling equipment, where data would need to be brought back and analyzed later.

In recent years, the Ministry of Land, Infrastructure, Transport and Tourism has been promoting "ICT Earthworks," and the use of 3D as-built management data has been advancing. By using numerous point cloud data collected with RTK-equipped tablets and surveying instruments, 3D models of the as-built structure can be quickly generated. This allows for the assessment of large-scale structures such as roads and dams, enabling an overview of the as-built condition across a wide area and checking for uniformity and flatness. For example, when continuously measuring the as-built condition of a dam slope with RTK-GNSS, creating a 3D color-coded error map allows you to easily identify any elevation discrepancies, which is very useful for quality management. However, it’s important to note that RTK’s vertical measurement accuracy requires careful attention. For public works, the allowable error in height is often set to ±5 cm, and to ensure accurate verification, efforts must be made to keep surveying errors to within ±1 cm. If necessary, combine RTK-based as-built measurements with leveling surveys to precisely correct the height, ensuring highly reliable as-built management.

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Utilization in Excavation and Backfilling Management – Precision Checking of Invisible Areas

In civil engineering, RTK surveying plays a crucial role in managing underground processes such as excavation and backfilling. For example, when excavating foundations on a construction site, RTK can be used to continuously measure whether the depth and shape of the excavation meet the design specifications, preventing over-excavation or under-excavation. Depth management, which was traditionally done based on the intuition of workers or heavy machinery operators' visual checks, can now be handled by RTK, ensuring that deviations of just a few centimeters are not overlooked. By confirming the shape of the excavation bottom and sidewalls, RTK helps avoid errors such as excessive excavation, which would increase the amount of concrete backfill (leading to cost increases).

Similarly, in backfilling, RTK is used to measure whether materials are filled and compacted to the required height. By checking the height after compaction of each layer and comparing it with the planned height, RTK helps prevent settlement and excessive filling. Especially in the construction of roadbeds and railway tracks, careful management of layer thickness is crucial to prevent future settlement. RTK surveying allows for multiple height measurements over large areas in a short time, making it easy to evaluate the degree of settlement caused by compaction across a surface. For example, after laying embankment and compacting it, RTK can be used to measure at the moment of compaction and again the next day after settlement has stabilized, with differences analyzed to understand overall settlement trends. This helps determine whether additional compaction is needed and ensures that the required ground strength and density are met with the appropriate thickness.

Additionally, collaboration between heavy machinery and surveying is key in excavation and backfilling management. Modern ICT construction machinery (equipped with 3D machine control) uses GNSS to automatically control the height of buckets and blades. Many of these machines enhance their accuracy with correction data from RTK base stations. By combining rough construction by heavy machinery with precise surveying by personnel, the quality of the construction in areas that are not visible can be guaranteed.

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Utilization in Pavement Thickness Management – Supporting Road Longevity from the Ground Up

RTK surveying plays a critical role in road pavement construction. Pavement thickness management involves checking whether the finished thickness of asphalt or concrete pavement meets the design standards. Typically, the height of the subgrade and base course is measured before paving, and the surface height is measured again after paving to calculate the actual pavement thickness by the difference between the two measurements. With RTK, these pre- and post-paving height measurements can be conducted quickly and with high reproducibility. If the error is within a few centimeters, the accuracy is sufficient for evaluating the pavement thickness, which is usually several tens of centimeters.

Specifically, during the base course finishing, RTK is used to conduct as-built measurements and obtain longitudinal and transverse profiles. After paving, the same measurement points are re-measured with RTK. While RTK is sometimes said to lack high reproducibility, its effects are minimized when measuring the same points multiple times in a short period. The difference between the measured subgrade height and surface height is used to verify the pavement thickness at all points, checking for any insufficient or excessive thickness. Excessive pavement thickness leads to material waste, while too-thin pavement results in premature damage, so ensuring the appropriate thickness is critical for maintaining quality.

The advantage of using RTK for pavement thickness management is that it allows the fresh pavement surface to be measured without workers walking on it. Typically, freshly paved surfaces are hot and soft, and walking on them can damage the surface. With RTK, workers can extend a pole from the shoulder of the road or equip a vehicle with an antenna to measure without placing direct load on the newly paved surface. Recently, drive-by surveying (vehicle-mounted GNSS surveying) technology has also advanced, and methods such as continuously measuring the road surface height while driving a vehicle equipped with an RTK receiver have become practical. These technologies allow for safe and efficient verification of pavement thickness, contributing to quality assurance.

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Tips and Considerations for Improving Accuracy with RTK Surveying

To make the most of RTK surveying, it's essential to be aware of several tips and precautions for improving accuracy. To achieve high and stable positioning precision, the following points should be kept in mind.

• Ensure Satellite Reception Environment:
  In RTK, stable reception of GNSS satellite signals is crucial. Set up the antenna for both the base station and the rover in open areas where the surroundings are visible to ensure clear sky visibility. RTK surveying is particularly effective in environments with no obstructions overhead (e.g., large airport areas). On the other hand, in areas under the shadow of buildings or trees, the number of visible satellites decreases and errors increase, so it's necessary to either avoid these areas or adjust the observation time. Additionally, during times when satellite geometry (PDOP) is poor, it's wise to avoid measurements. Checking the PDOP value on the receiver’s screen and temporarily postponing measurements when it's too high is effective for maintaining accuracy.

• Stabilize Initialization (Integer Fixed Solution):
  In RTK surveying, resolving the integer cycle ambiguity of the carrier wave to obtain a "fixed solution (Fix)" is necessary. Immediately after starting the measurement or after a loss of signal, the solution can become unstable. Therefore, always ensure that both the base station and rover are stationary and have sufficient satellite coverage before beginning measurements. Coordinates obtained while in a "floating solution (Float)" should not be trusted. Make it a habit to regularly check the fixed/float status of the solution using the device’s display or audio guidance to ensure the solution is fixed.

• Pay Attention to Vertical Accuracy:
  GNSS positioning generally tends to have lower accuracy in the vertical direction than in horizontal positioning. Therefore, be mindful of potential sources of error related to height, such as known height errors at reference points or discrepancies in the geoid model. For critical height measurements, it’s effective to take multiple measurements of the same point and average them, or to verify the height with a separate leveling measurement at the reference point. Additionally, Japan has developed the national gravity system (geoid heights) via electronic reference points, so it is advisable to use official geoid models and correction services whenever possible to complement vertical accuracy.

• Confirm Reproducibility of Measurements:
  As previously mentioned, RTK measurements are subject to slight fluctuations in position values over time, and identical values may not be obtained even at the same location. Therefore, it is important to take multiple measurements at critical points and check for consistency. After measuring one point, return to it after measuring other points and recheck to ensure that the difference is within an acceptable range. National guidelines recommend re-measuring the same point during RTK benchmark surveying and checking for deviations in the obtained coordinates. This practice helps detect accidental errors or equipment drift, thereby improving the reliability of the data.

• Understand the Equipment’s Characteristics and Limitations:
  RTK-GNSS equipment has its strengths and limitations. For example, it is not suitable for measuring fast-moving targets or for measurements in underground or indoor environments. Additionally, due to the nature of wireless correction information transmission, network-based RTK may not work in areas with no communication coverage (this limitation is addressed in the next section with LRTK). Be sure to understand the specifications of the equipment you are using (such as supported satellites, frequency, and wireless range), and prepare backup batteries and check the communication environment as needed. Correctly operating the equipment and understanding its limitations is the key to preventing accuracy issues.

By following these points, you can fully harness the potential of RTK surveying and achieve highly reliable precision measurements, resulting in further improvement in construction quality.

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Precision Measurements with the Latest RTK Technology "LRTK"

The technology that has significantly lowered the barriers for RTK surveying is the recently introduced "LRTK." LRTK (Lefixea RTK) is the latest RTK positioning solution provided by Lefixea Inc., which allows for RTK surveying using a smartphone. Previously, RTK required expensive, dedicated GNSS receivers and radio equipment, but with LRTK, all you need to do is attach a specialized device to your smartphone to achieve RTK surveying. This makes it possible for site supervisors and engineers, even without a specialized surveying department, to easily perform centimeter-level precision surveying.

A key feature of LRTK is its compatibility with the centimeter-level positioning correction service (CLAS) provided by Japan's Quasi-Zenith Satellite System (QZSS). The antenna-integrated device (LRTK Phone 4C) attached to the smartphone receives L6 band signals from QZSS, and by applying correction data in real-time through the dedicated app, it enables high-precision positioning that would otherwise be impossible with just the smartphone alone. Notably, it can also perform positioning even in areas without mobile network coverage. While conventional network-based RTK relies on mobile communication networks to receive correction data, in mountainous or underground areas where communication can be interrupted, LRTK continues to provide centimeter-level positioning by using direct correction signals from QZSS. This is a significant breakthrough that enables surveying in environments where GNSS surveying was previously difficult, such as tunnel construction or mountainous work.

LRTK is also extremely user-friendly on-site, with the dedicated LRTK app (LRTK App) allowing easy recording and mapping of survey points. For example, while surveying, you can take photos with your smartphone, and automatically add high-precision coordinates and azimuth information to the photos, which are then saved. It’s easy to plot photos taken during infrastructure inspections on a map, or embed positioning information into photos taken during as-built management. Additionally, a function for sharing point cloud data and 3D models via the cloud has been developed, making it an ideal platform for smooth data sharing between the field and office. As a tool that transforms your smartphone into an all-purpose surveying device, LRTK is quietly gaining popularity in the construction industry.