Streamlining As-Built Surveys with Network RTK: Achieving Both Labor Savings and Improved Accuracy

In recent years, digital technology adoption has accelerated at civil engineering construction sites to boost productivity. As-built surveying is an indispensable process for quality control, but traditional methods have required significant manpower and time. Meanwhile, shortages of skilled surveyors and the need for improved work efficiency mean the key challenge on sites is how to achieve both labor savings and enhanced accuracy. With the Ministry of Land, Infrastructure, Transport and Tourism promoting the i-Construction initiative, construction DX using ICT and GNSS is rapidly advancing. Among these advancements, a new surveying method that leverages high-precision satellite positioning—network RTK—is attracting attention. This article explains how network RTK streamlines as-built surveys and the key points for simultaneously achieving labor savings and improved accuracy.

Challenges in As-Built Surveying and Traditional Methods

As-built surveying measures the finished shape of structures or developed land and verifies deviations from design drawings. It is a crucial step to confirm whether road base thicknesses, slope gradients, dam or embankment volumes, and other post-construction features meet specified shapes. Traditionally, this work has been performed by staff using levels (optical automatic levels) or total stations, measuring heights and positions from construction control points one by one to create cross sections. It typically required one to two experienced surveyors; many survey points had to be measured and recorded manually, making the process very labor-intensive. On large sites or complex terrain, surveying could take several days, placing a heavy burden on construction management engineers.

Another issue with traditional methods is that only limited points per cross section are measured, making it difficult to accurately grasp the as-built condition of the entire site. For example, when cross-sections are taken at regular intervals during earthwork, localized depressions or excesses between the sections can be missed. Records were often kept in field notebooks or entered manually into PCs, and a lot of staff-hours were consumed in transferring and organizing measurement results into drawings and inspection forms as part of inspection preparation. Thus, for many years the question of how to efficiently understand as-built conditions while ensuring accuracy has been a persistent challenge.

Utilizing GNSS and RTK Positioning Technologies

A trump card that has emerged to solve these challenges is labor-saving as-built surveying using satellite positioning technology (GNSS). GNSS positioning, typified by GPS, measures ground positions using signals from satellites; recently, using multiple satellite constellations such as GLONASS and QZSS (Michibiki) has improved positioning accuracy. Standalone GNSS receivers have errors on the order of several meters, but a method that corrects these errors to achieve real-time centimeter-level accuracy is RTK positioning. RTK (Real Time Kinematic) computes high-precision relative positions by correcting satellite signal errors between a base station with known coordinates and a rover. Previously, a local base station needed to be set up near the site and correction information sent by radio, but now network RTK services provide this correction mechanism broadly via the Internet.

Network RTK uses a network of multiple reference stations deployed across the country to establish a virtual reference station (VRS) near the user and deliver the correction data in real time. Users can receive online correction information (e.g., via NTRIP) through mobile communications and perform high-precision positioning without providing their own base station. In Japan, centimeter-level positioning anywhere on site can be achieved by using the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations, private GNSS reference station services, and even QZSS CLAS signals. In other words, the time-consuming tasks of locating control points and stopping machinery to set up base stations for each survey are eliminated, and precise coordinates can be obtained immediately with just a receiver.

Labor Savings in As-Built Surveying with Network RTK

Introducing network RTK dramatically improves the efficiency of as-built surveying. The first advantage is that a single person can complete the survey work. A surveyor carrying a rover GNSS receiver (high-precision GNSS antenna) can obtain three-dimensional coordinates on the spot simply by pressing the measurement button at each point. The traditional workflow—one person holding a target while another operates equipment—is no longer necessary. Even on large development sites, a minimal crew can efficiently measure many points while walking.

Shorter equipment setup time also contributes greatly to labor savings. Total stations required repeated setups, backsighting, and ensuring line-of-sight for each measurement point, which consumed a lot of time. With GNSS surveying, as long as the sky is visible, starting the rover and achieving measurement readiness takes just a few minutes. Continuous measurements while moving make it possible, for example, to cover an area that used to take half a day in 1–2 hours. Because coordinate values are obtained in real time, it is easy to flexibly measure additional missing points on site after checking results.

Key points of labor savings with network RTK are summarized as follows:

• Enables one-person operation, improving personnel allocation and reducing manpower

• No need to set up base stations or perform post-processing, so surveying can start immediately

• Allows continuous measurement while moving, enabling quick understanding of as-built conditions over wide areas

• Data are recorded electronically, reducing paper-based recording and streamlining subsequent processes

In this way, network RTK reduces the work volume of surveying itself and provides a system to obtain more measurement data with less effort. Construction managers can quickly check as-built conditions when needed, giving more leeway in overall schedule management.

Ensuring As-Built Quality through High-Precision Positioning

When you hear “labor savings,” you may worry “will accuracy be sacrificed?” However, positioning with network RTK can secure the coordinate accuracy required for as-built management. With a high-quality GNSS receiver and a network RTK service, planar accuracy can typically be kept within a few centimeters, and elevation can be stably measured to an accuracy of several centimeters to the low centimeter range. For example, as-built management standards in civil works often set allowable errors around ±5 cm (±2.0 in) for embankment or base thickness, and it is said desirable that the measuring instrument itself ensures around ±1 cm (±0.4 in) accuracy for such judgments. RTK-GNSS can meet these measurement accuracy requirements and obtain data robust enough for quality control.

Indeed, using RTK surveying can reduce variability in as-built quality. Because traditional methods measured only a limited number of points, differences from the design could be understood only partially. If network RTK enables a large increase in the number of points, localized excesses or shortages are less likely to be missed. The higher resolution of as-built data reduces quality variability. Moreover, by using high-precision positioning during construction to check finishes periodically, nonconforming areas can be detected early and corrected. Reducing rework while ensuring quality is another major benefit of network RTK.

Leveraging 3D Point Cloud Surveys and Promoting i-Construction

The spread of network RTK, together with the increasing use of 3D point cloud surveying, is significantly changing as-built management. Under i-Construction measures, the use of point cloud data that captures wide areas as surfaces—such as drone photogrammetry and laser scanner measurements—is recommended. RTK-GNSS becomes powerful when combined with these point cloud measurements. For example, in drone photogrammetry, if the coordinates of ground control points installed on site are measured by RTK, orthophotos and point cloud models can be given absolute coordinates. Whereas post-processing used to require adjustment to control points, using RTK allows seamless linkage between field surveying and photo processing, improving point cloud processing efficiency.

New point cloud measurement methods combining mobile devices and GNSS are also emerging. For instance, by combining a smartphone or tablet’s built-in LiDAR scanner with an RTK receiver and scanning the surroundings while walking, anyone can easily obtain a high-precision point cloud with absolute coordinates. The acquired point cloud can be directly compared with 3D design models or BIM/CIM data to verify as-built conditions or compute earthwork volumes. Detailed point clouds covering wide areas enable accurate understanding of terrain undulations and the shapes of structures—visualizing areas that traditional cross-section surveys might have overlooked—and thus increase the reliability of as-built management.

The i-Construction as-built management guidelines explicitly permit using RTK-GNSS-measured point clouds as as-built measurement data (TIN files). In other words, RTK measurement results are accepted as official deliverables, and the era in which as-built submissions are made not only as paper drawings and tables but also as 3D data has arrived. Combining network RTK and point cloud technologies makes it possible to perform rapid and accurate as-built management based on detailed 3D information acquired on site.

Efficient Data Integration and Report Generation

As-built surveying with network RTK also brings major benefits from the perspective of site DX (digital transformation). Positioning data obtained by GNSS equipment and related software are stored and managed in digital formats, making subsequent data integration easy. Coordinate data for measured points can be directly imported into CAD drawings or CIM models, or shared immediately with stakeholders via the cloud, eliminating information transmission lag. This smooths communication between construction managers, designers, and owners, helping prevent mismatches in understanding or missed confirmations regarding as-built conditions.

Furthermore, digitizing measurement data enables more efficient inspection form creation. Previously, field-recorded numbers on paper were retyped at the office to create as-built management drawings and reports. Coordinate values obtained by network RTK are already in electronic form, so specialized software or templates can automatically generate reports. For example, loading measurement results into a prescribed format can instantly produce pass/fail lists for heights and thicknesses or as-built charts. This reduces calculation and drawing errors and allows preparation of submission materials for inspectors in a short time, providing leeway for inspection preparation.

With cloud-enabled systems, field-measured data can be uploaded on site and checked from the office instantly. Supervisors or owners located remotely can monitor progress in real time and issue instructions or corrections as needed, building an efficient quality-management cycle without waste. In this way, using network RTK goes beyond merely streamlining surveying work and contributes to overall productivity improvement in construction management through data utilization.

Improved Safety and Impact on Site Work

As-built surveying by network RTK is also notable from the viewpoint of improving safety. Shorter surveying times and reduced personnel exposure lower the time workers spend in hazardous site environments. For instance, as-built measurements on busy roads can minimize traffic regulation and roadside work risks if completed quickly. For high places or steep slopes, RTK-GNSS eliminates the need to set up surveying instruments in unsafe locations. By mounting a GNSS antenna on a long pole, measurements can be taken from a safe distance; combining drones or remote sensing technologies can capture shapes of areas where people cannot enter.

Transporting and setting up heavy surveying equipment was also a significant physical burden and risk for workers. Traditional surveying involved tasks such as setting up instruments under the summer sun or ensuring visibility during night work, with various safety issues. Lightweight GNSS surveying using network RTK reduces these burdens, lowering the risk of heat stroke and accidents. Labor savings and improved safety go hand in hand; establishing a system that measures efficiently and accurately ultimately contributes to a stronger safety culture across site operations.

Simple Network RTK Surveying with LRTK

Even if the advantages of network RTK are understood, some may worry, “Doesn’t this require expensive equipment and specialized knowledge?” Indeed, until recently, RTK surveying required dedicated equipment costing millions of yen and specialized surveying skills. However, solutions that enable centimeter-level positioning easily using smartphones have now emerged. A prime example is our LRTK (smartphone-mounted RTK-GNSS receiver).

LRTK is a pocket-sized GNSS receiver that attaches to a smartphone or tablet and achieves high-precision positioning using network RTK or QZSS augmentation signals with just a single smartphone. It weighs only about 125 g and includes a built-in battery, making it easy to carry on site. By launching a dedicated app and pressing a button at the point to be measured, planar coordinates and elevation are obtained instantly. Acquired data can be automatically synchronized to the cloud and immediately visualized and checked on an internal shared map.

With LRTK, each construction manager can have a “personal surveying instrument.” They can measure as-built conditions whenever needed and confirm and share results on site, making productive use of time that used to be spent waiting for surveying. LRTK also integrates with a smartphone’s built-in LiDAR camera to capture surrounding 3D point clouds simply by walking. Since absolute coordinates are attached to the point cloud in real time, 3D data can be generated instantly in accordance with as-built management guidelines. You can overlay the current point cloud model with design data and display differences in color, or calculate embankment and cut volumes on the spot—LRTK is a multifunctional tool that truly supports site DX.

By adopting simple surveying with LRTK, you can more easily bring the benefits of network RTK to your site. Network RTK technology that balances labor savings and improved accuracy is no longer limited to specialized surveying teams; it is now a tool anyone on site can use routinely. Consider actively incorporating the latest surveying DX tools at your construction site. By leveraging network RTK, you can dramatically improve the efficiency and quality of as-built management and achieve safer, more productive site operations. For more details, please also visit the [LRTK official site](https://www.lrtk.lefixea.com/lrtk-phone) and take the first step toward site DX.