RTK cross-section diagram use case: A civil site that dramatically streamlined as-built control

Table of Contents

• Introduction: The importance of cross-section diagrams in as-built control

• Conventional as-built control and the challenges of cross-section measurement

• What is RTK positioning? A new technology for high-precision cross-section measurement

• Site case study: How as-built control changes when RTK cross-section diagrams are adopted

• Main benefits of RTK adoption (efficiency, improved accuracy, safety, etc.)

• Conclusion: Simple surveying anyone can start with LRTK

• FAQ

Introduction: The importance of cross-section diagrams in as-built control

On civil engineering sites, confirming that completed structures and formed ground match the design—known as as-built control—is indispensable. Among the drawings used for as-built evaluation, cross-section diagrams that show transverse shapes such as roads and slopes are particularly important. By looking at cross-sections, one can intuitively grasp vertical information, slopes, and thicknesses on site. Therefore, in road works they are used to check pavement width and slope gradients; in land development works they are used to calculate fill and cut volumes; and in bridges and tunnels they are used to check for deformations. Cross-section diagrams are widely used from pre-construction planning through construction management to post-completion inspection and maintenance.

However, creating cross-section diagrams by conventional methods required a great deal of time and labor. This is because one had to measure terrain at regular intervals using instruments such as levels or total stations, and draw the cross-sectional shape by connecting point to point. For example, if you create cross-sections for a 100 m (328.1 ft) road segment by measuring every 10 m (32.8 ft), setting up and observing the survey points can take half a day, and drafting the drawings in the office can take another half day, so the total man-hours can amount to several days. Surveying on steep slopes or in areas with poor footing can be hazardous, and on small sites it is not uncommon to have no dedicated survey technicians and therefore to have to outsource. Thus, cross-section measurement for as-built control has been a major burden for site engineers.

Now, attention is focused on the use of RTK positioning technology. With RTK (Real-Time Kinematic), anyone can perform positioning and surveying with centimeter-level accuracy (half-inch accuracy), and the transverse measurement process for as-built control is undergoing a major transformation. This article explains RTK from basic principles to the challenges of conventional methods, and concretely describes how on-site operations are improved by using RTK cross-section diagrams. We also touch on the features of the latest smartphone RTK solution "LRTK" and present key points that enable dramatic efficiency gains on site. At the end of the article we also guide you on how to get started with simple surveying using LRTK.

Conventional as-built control and the challenges of cross-section measurement

First, let’s organize the workflow of transverse measurement (cross-section creation) in conventional as-built control and its challenges. Conventionally, surveying relied mainly on manual measurement and manual drafting. Typically, survey staff set up benchmarks (height references) and measure heights point by point from reference points using tape measures and levels. For roads and slopes, heights and positions are measured at 10 m (32.8 ft) or 20 m (65.6 ft) intervals, or at points where the terrain changes, and those point clouds are connected to draw cross-section lines. When calculating earthwork volumes by measuring multiple cross-sections, one must calculate the area of each section and estimate volume using the average cross-section method. After these steps, the as-built cross-section drawings and reports are finally completed.

The greatest challenge of conventional methods is the large amount of time and labor involved. When an experienced surveyor makes precise measurements with an optical total station, it is not uncommon for multiple people to take more than a full day including equipment setup and post-processing. Manual surveying limits the number of points that can be obtained at once, so covering a wide area thoroughly requires more days and personnel. On sites suffering from labor shortages, this surveying work was a major burden. Also, on small sites without a dedicated surveyor, surveying had to be outsourced, incurring costs and scheduling hassles.

The issues of accuracy and comprehensiveness cannot be ignored either. Because only representative points tend to be measured, fine undulations and irregularities between points are often overlooked. The cross-section line drawn on the as-built drawing can differ subtly from the actual terrain, and errors may later be discovered—for example, a section thought to meet specifications may actually lack fill, or part of a slope may be gentler than the design. Even if main survey points are measured, critical spots can be “forgotten,” leading to last-minute rework after inspectors point out discrepancies at final inspection. Even when you think you've checked everything beforehand, unexpected remarks during inspection can cause anxiety.

There are also safety risks. Survey staff often have to enter below slopes or into areas with operating heavy equipment, so the risk of falls or collisions is always present. Surveying in areas with poor footing or high structures is a physical burden on workers. In addition, as-built control involves record-keeping such as photography, and on busy sites photographs may be missed or records may contain human errors.

Thus, conventional as-built control methods suffered from problems such as “time and effort-intensive,” “limited measurement points leading to questionable accuracy,” and “complicated record-keeping and sharing prone to mistakes.” For reliable site quality control, a more efficient and higher-accuracy measurement method was strongly needed.

What is RTK positioning? A new technology for high-precision cross-section measurement

RTK positioning has emerged as a trump card to solve these issues. RTK (Real-Time Kinematic) positioning is a method that uses signals from multiple GNSS satellites and corrects positional errors between a base station (reference station) and a rover (mobile station) to obtain centimeter-level positioning accuracy in real time. Simply put, by comparing with a nearby known reference point, RTK cancels out GPS errors and pinpoints the current location within a few centimeters of error. While ordinary GPS positioning has errors on the order of meters, RTK can dramatically reduce that error to almost a few centimeters.

This higher accuracy enables digitalization and streamlining of construction management tasks that require precise positioning, such as as-built control (post-construction shape verification) and machine guidance for heavy equipment. For example, as-built inspections traditionally performed with levels or total stations can be done with RTK-capable devices to immediately verify required heights and thicknesses on site. High-precision as-built measurement using RTK-GNSS is an important technological element in the Ministry of Land, Infrastructure, Transport and Tourism’s ICT-driven construction initiatives such as i-Construction.

RTK surveying once required specialized expensive equipment, radio communication systems, and skilled technicians, and only a limited number of specialized contractors could handle it. However, technological advances in recent years have miniaturized and reduced the cost of RTK receivers and communication modules. In Japan, the quasi-zenith satellite system Michibiki has also started a centimeter-level positioning augmentation service (CLAS), and with compatible receivers high-precision RTK positioning is now possible even at mountain sites where internet access is poor. RTK positioning has now become accessible not only to specialists but also to general construction managers. Against this background, there is growing adoption of RTK in on-site as-built control to achieve both labor savings and improved accuracy.

Site case study: How as-built control changes when RTK cross-section diagrams are adopted

So how does a site actually change when RTK is introduced for cross-section measurement in as-built control? Here we present a hypothetical civil site example showing a case that was dramatically streamlined by using RTK cross-section diagrams.

Case: As-built control of transverse shape in road embankment work On a suburban road embankment project, conventional as-built control took a lot of time. For a 200 m (656.2 ft) road segment, the person in charge needed to perform cross-section measurements every 10 m (32.8 ft) to check slope gradients and pavement thickness. Conventionally, a two-person survey team used a total station and staff (level rod) and took a full day to conduct the survey. The next day, they would organize the survey point data in the office, create CAD drawings, and prepare as-built drawings and quantity calculation reports. In total the work took the equivalent of 2–3 days, and poor weather that interrupted surveying posed a risk of schedule delays.

So this site trialed RTK-based as-built measurement. Specifically, they used a small RTK-GNSS receiver attached to a smartphone and a dedicated app, and one person walked the site scanning the entire embankment slope. GNSS position correction and the smartphone's built-in LiDAR sensor were used to quickly obtain 3D surface data (point cloud) of the embankment. Walking and measuring the entire 200 m (656.2 ft) section took about 1 hour. The point cloud data, amounting to hundreds of thousands of points, captured fine irregularities that conventional methods could not measure.

On-site immediate confirmation and sharing of as-built cross-sections After RTK measurement, the data were uploaded to the cloud, and cross-section diagrams were automatically generated on site without returning to the office. The person in charge could draw a cross-section line at any desired position on a tablet and overlay the as-built cross-section on the design cross-section. As a result, areas with insufficient fill or gentler slopes were immediately obvious, allowing the team to identify the exact extent that required additional fill. Cutting cross-sections from the point cloud was completed with the push of a button, and more than 10 cross-section drawings could be generated in a few minutes.

A shared cloud link was also issued so supervisors in the office and the client (inspection officers) could review the as-built point cloud and cross-sections. Conventionally, as-built drawings were often submitted only shortly before inspection, leaving little time to address comments. In this case, because the as-built status could be reviewed and discussed in advance via the cloud, alignment with the client was smooth. The person in charge also reported that, instead of worrying until the inspection whether drawings and field conditions matched, they were able to approach the inspection with confidence because the digital data verified details thoroughly. As a result, there were zero comments at the as-built inspection. The cross-sections and quantity reports that had been automatically generated were submitted, and the inspection procedure proceeded without delay.

Dramatic efficiency gains and improved quality In this example, 3D surveying with RTK and automatic cross-section generation reduced the work that previously took more than two person-days to roughly half a day in practice. Personnel required decreased from 2–3 people to a single person, allowing others to be assigned to different tasks. Because the acquired data covered the entire site, there was no need for additional site visits due to missed measurements, resulting in zero rework. From a quality perspective, because even millimeter-level (0.04 in) irregularities were captured and corrected immediately after construction, the completed work met the design documents’ accuracy. No rework or additional construction was required during inspection, contributing to shorter schedules and cost savings. The person in charge commented, "I was surprised surveying could become this simple. With the data, the acceptability of the as-built condition is clear to anyone, so communication with the client is much easier," and the site as a whole experienced the benefits of digital surveying.

Main benefits of RTK adoption (efficiency, improved accuracy, safety, etc.)

As described above, creating cross-section diagrams using RTK brings significant effects to sites and achieves dramatic efficiency gains and quality improvements compared to conventional methods. Here are some of the most important benefits:

• Time savings and labor reduction: RTK surveying acquires a large number of points at once and automates drafting, making it far faster than manual methods. As-built measurements that previously required a survey team to spend a full day can be completed quickly by a single site staff member. Because data processing can be done immediately after measurement, there is no need to bring data back to the office and spend overtime drafting drawings. With labor shortages deepening, measurement tasks can be completed by site staff without relying on experienced surveyors, contributing to workforce reduction.

• Improved measurement accuracy and comprehensiveness: Point cloud data obtained by RTK combined with 3D scanning contain much more information than conventional surveys that only capture representative points. Because they can digitize site shape exhaustively, even subtle irregularities between points can be detected. Deviations from the design, including millimeter-level (0.04 in) depressions, can be identified and corrected early, preventing problems like insufficient fill or over-excavation. The risk of discovering "missed measurements" later or of cross-sections differing from actual conditions is almost eliminated, markedly increasing the certainty and reliability of as-built control. Since obtained data are stored as digital coordinates, you can re-cut cross-sections at arbitrary locations later if needed. Once data are captured, the number of additional site visits for follow-up surveys is reduced, eliminating concerns about omissions.

• Improved safety: Because measurements are completed with fewer people and in a shorter time, the time workers spend in hazardous locations is reduced. RTK allows measurements from a distance away from areas where heavy equipment is operating, and in situations where data can be gathered remotely, worker risk is lowered. Fewer measurements at heights or on steep slopes are required, reducing the risk of falls and equipment contact accidents. On sites that have adopted lightweight systems like LRTK, safety of measurement tasks has improved and surveyors’ anxiety has decreased. RTK-based as-built control therefore contributes not only to efficiency but also to site safety.

• Smoother data sharing and consensus building: A major advantage of digital measurement data is that visual information is easy to share. Point clouds and cross-sections can be displayed as 3D models or graphs, making it easier to convey site conditions that are hard to communicate with paper drawings or tables. For example, overlaying design models on as-built point clouds to create heat maps that show excesses and shortages by color, or overlaying design and measured lines on cross-sections, makes differences immediately understandable even to non-experts such as clients or nearby residents. On-site AR (augmented reality) comparisons between the completed model and actual terrain via a tablet are also possible, helping everyone form a common understanding faster. Sharing data on the cloud allows stakeholders to check as-built status in real time from remote locations, contributing to more efficient meetings and inspections. Such visualization and data-sharing benefits smooth communication around as-built control results and speed up consensus.

• Streamlined report production and support for digital deliverables: High-precision as-built data obtained with RTK can be used directly to create as-built drawings and quantity calculation reports. Because arbitrary plan and cross-section drawings can be automatically generated from point clouds, there is no need to manually redraw drawings. Some software can automatically calculate differences from design data and even perform pass/fail judgments, making partial automation of as-built inspections realistic. The Ministry of Land, Infrastructure, Transport and Tourism’s "Guidelines for 3D As-Built Control (draft)" specify LandXML or SIMA formats for electronic deliverables, and data obtained via RTK can be easily converted and exported to these formats. For example, LRTK’s system can export quantities and cross-section data computed from point clouds to CSV or LandXML with one click, so as-built drawings, photo logs, and quantity reports that once required experienced engineers to prepare manually can be simplified digitally. The reduction in time for creating reports and smooth support for electronic deliverables are major benefits of using RTK cross-section diagrams.

Conclusion: Simple surveying anyone can start with LRTK

As described, RTK-based as-built control dramatically increases surveying productivity while contributing to quality assurance and safety. By utilizing 3D point cloud data, precise and wide-area shape acquisition that was difficult to achieve by manual methods is now possible, and construction management is shifting to reliable and speedy digital measurement. The recent advent of smartphone-compatible mobile surveying systems like LRTK has made centimeter-level surveying that once required specialists and expensive equipment accessible to anyone. The LRTK series is an all-in-one surveying solution combining a compact GNSS receiver, a smartphone app, and cloud services. In addition to high-precision RTK positioning, it can handle photogrammetry, LiDAR scanning, and AR display with a single device, making it a “jack-of-all-trades surveying tool” that can transform on-site as-built control.

Especially for small- and medium-sized contractors and regional sites, advanced 3D surveying has often seemed out of reach. However, easy and affordable solutions like LRTK are designed so that existing staff can operate them without specialized surveyors. The intuitive smartphone app interface allows even less-experienced technicians to use the system without confusion, building confidence that “we can do this ourselves.” The Ministry of Land, Infrastructure, Transport and Tourism encourages starting DX on site with technologies that are easy to handle, and LRTK is a prime example of “DX that’s easy to start.”

To meet the growing demands for improved efficiency and sophistication in construction, why not begin site digitalization with accessible RTK surveying? Using LRTK, anyone can easily achieve high-precision surveying and dramatically reduce the burden of as-built control. Benefits such as improved productivity from reduced work time, workforce reduction, and quality management based on solid evidence provide tremendous value compared to conventional methods. Please take this opportunity to consider introducing new surveying technology. For details, refer to LRTK’s official site and other resources to check how it can be applied to your site. With advanced RTK utilization, your civil site’s as-built control can become more reliable and efficient.

FAQ

Q: What is the difference between RTK and conventional GPS positioning? A: Ordinary GPS positioning has errors on the order of meters, whereas RTK positioning can reduce errors to a few centimeters by means of relative positioning with a base station. Because RTK provides high-precision real-time location, it can be used immediately for tasks requiring accuracy such as civil surveying and as-built control.

Q: What equipment and preparations are required for cross-section measurement using RTK? A: Basically you need an RTK-capable GNSS receiver (rover) and a base station that provides correction information. In Japan, it is common to prepare only the rover and connect to a network RTK service (reference station network) provided by organizations such as the Geospatial Information Authority of Japan. You also need a field controller or tablet for data collection and processing and surveying software. For example, with LRTK, you attach a small receiver to a smartphone and receive correction information over the network in the app to start positioning. There is no need to install dedicated base equipment in the field, making preparation relatively simple.

Q: Can beginners operate RTK surveying, or is specialized skill required? A: Compared to conventional surveying equipment, recent RTK systems have simplified operations and are easier for beginners to use. Systems that use smartphone apps like LRTK allow measurement and point cloud acquisition by following Japanese-language menu guidance on the app. Intuitive support features such as AR comparison with design data reduce the need for deep surveying knowledge. With basic training, site personnel themselves can use RTK surveying.

Q: Can data obtained with RTK be used directly for inspections and document submission? A: Yes. As-built measurement data obtained with RTK can be used in formats that meet national standards. The MLIT guidelines recommend using 3D data for as-built control and support electronic deliverables in LandXML or SIMA formats. Point clouds and cross-section data obtained with RTK can be converted into these formats as needed. On the LRTK cloud you can download cross-section images or export CAD data with one click. With proper RTK receivers and measurement methods, high accuracy that meets public works tolerance requirements can be ensured, so RTK results can be submitted as reliable deliverables at least equal to conventional level surveying.

Q: Can RTK surveying be introduced on small sites or projects with limited budgets? A: Yes. RTK surveying has become particularly affordable to introduce in recent years. While it used to be common to outsource to specialized survey companies with expensive equipment, systems like LRTK can achieve centimeter-precision surveying (half-inch accuracy) with a handheld smartphone and a small receiver. Initial investment and running costs are lower than conventional equipment in many cases, and introducing RTK can even be cheaper than repeatedly outsourcing surveys. Small sites often struggle to secure personnel with advanced surveying skills, but simple RTK tools can be operated by existing staff. Regardless of project size, you can start by piloting RTK in part of the work to verify its effects and expand gradually. Being able to start in a way that fits your company is one of the attractions of modern RTK surveying solutions.