Table of contents

• The importance of as-built management and volume measurement

• Conventional volume measurement methods and their challenges

• 3D scanning technology using smartphones

• Achieving centimeter accuracy for smartphone measurement with RTK-GNSS

• As-built volume measurement anyone can do with LRTK

• Conclusion

• FAQ

What if you could accurately measure the volume of embankments or excavations at a construction site with just a single smartphone?

Measuring earthwork volumes for as-built management is a crucial process for tracking construction progress and confirming quality, but for a long time it required specialized surveying teams and expensive equipment. Now, by combining a smartphone with the latest technologies, an era has arrived in which anyone can quickly and accurately calculate volumes on site.

This article explains, step by step, the importance of volume measurement in as-built management, the challenges of conventional methods, solutions using the latest 3D scanning technologies, and simplified surveying enabled by the RTK-GNSS solution “LRTK.”

The importance of as-built management and volume measurement

In civil engineering and construction, there is a process called “as-built management” for checking and recording whether completed structures and terrain match the design in shape and dimensions. This is an important task to demonstrate, with measurement data, that the actual finish falls within the tolerance defined by the client, and it is indispensable for quality assurance. Especially for public infrastructure projects such as roadworks and site formation, the results of as-built management are conditions for inspection approval and handover, so rigorous measurement and record-keeping are required.

Calculating earthwork volume also has important meaning in as-built management. For embankment and excavation works, it is necessary to accurately measure whether the specified volume of soil has been placed or removed. The volume values directly affect progress management (work quantity tracking), pass/fail judgments in as-built inspections, and even construction payments, so their accuracy greatly influences the fairness and economics of a project. For example, embankment and excavation volumes have traditionally been calculated by surveying the ground before and after work and computing the difference in volume, or by manual calculations on drawings using cross-sectional area and length via the average-end-area method. However, methods that measure only a few discrete points are prone to missing local irregularities, overlooking overfilled or under-excavated areas, and causing rework later. Especially in large-scale earthworks, manual surveying has limits, and a new measurement method that is efficient and comprehensive for volume measurement has been sought.

Conventional volume measurement methods and challenges

In traditional volume measurement, it has been common to measure key points of the terrain using leveling or total stations, then calculate areas and volumes on drawings. Specifically, one measures elevation differences between the current situation and the design or post-construction terrain at various points, creates cross-sections and longitudinal sections, and then computes the volume. This method has the following issues:

• Labor and time: Large sites require many survey points, and surveying, drafting, and calculation take a long time.

• Errors from point data: Measurable locations are limited to discrete points or section lines, so surface irregularities or local depressions and bulges may be missed. As a result, calculated volume values tend to accumulate errors.

• Limits of manual work: Surveying on steep slopes or in areas with poor footing is hazardous, and it can be impossible to approach and measure in detail. In large earthworks, covering the entire area with only manpower is difficult.

• Risk of rework: If missed overfilled or under-excavated areas between survey points are discovered at inspection, remedial work will occur, affecting schedule and cost.

Thus, conventional methods have limits in efficiency and accuracy, and the field has needed a solution that can measure volumes more quickly and comprehensively.

3D scanning technology using smartphones

To meet these needs, 3D scanning technology using smartphones has attracted attention in recent years. In particular, iPhone 12 Pro and later models are equipped with LiDAR sensors, enabling easy, high-precision 3D measurement. LiDAR is a technology that uses infrared lasers to measure the distance to objects, and the iPhone’s LiDAR can acquire distance data up to about 5 m (16.4 ft). Working with the smartphone camera, it records surrounding shapes as a collection of many points (point cloud data). A point cloud is data that represents the site’s shape with countless coordinate points and can reproduce the space as a realistic 3D model like a photograph.

For smartphones that do not support LiDAR, or when you want to measure a wider area in detail, photogrammetry is also effective. This technique reconstructs three-dimensional shapes from multiple images of the site taken with a smartphone camera. The same principle used in drone aerial photogrammetry or DSLR photogrammetry can be achieved with a smartphone; by recording video with a dedicated app and analyzing it in the cloud (which can take minutes to tens of minutes), you can create 3D models that include areas beyond the reach of LiDAR. LiDAR has the advantage in real-time responsiveness, but photogrammetry can cover large areas and fine details at high density if you allow more processing time. By selecting between these smartphone scan methods according to site scale and required precision, you can efficiently obtain point cloud data.

Using point cloud data acquired by a smartphone, you can understand terrain as surfaces rather than just discrete points, dramatically improving the accuracy and reliability of volume calculations. The number of points obtainable far exceeds those of manual survey points, enabling you to capture subtle bumps and hollows of embankments and excavations in full detail. For example, if you scan an entire embankment and overlay the resulting point cloud on the design’s 3D model, you can intuitively confirm elevation differences with a color-coded heatmap. Visualizing irregularities and errors makes it easy to correct shortfalls by adding fill or to trim excessive areas before inspection. The acquired 3D data can be stored and shared in the cloud as records, providing persuasive evidence for as-built results. Quantity calculation from point clouds is more objective than traditional cross-section calculations and is useful as explanatory material for clients. Also, sharing point cloud data via the cloud allows stakeholders in remote locations to check the site’s 3D model, helping to prevent misunderstandings and disputes.

Moreover, at excavation sites, scanning before and after work and computing the difference between the two point clouds can automatically calculate the removed soil volume (excavation volume). You can grasp the work quantity on site in just a few minutes, eliminating the need to return to the office to compute volumes as in the conventional workflow.

However, one challenge remains for smartphone-only 3D scanning: the acquired point cloud data are not tied to absolute coordinates (latitude, longitude, elevation). Point clouds recorded by a smartphone exist in a virtual coordinate system internal to the device, so manual alignment was necessary when comparing them with design drawings or other survey data. Recently, technologies that solve this problem through integration have emerged.

Achieving centimeter accuracy for smartphone measurement with RTK-GNSS

The key to further improving smartphone scan accuracy is RTK-GNSS (real-time kinematic GNSS) technology. RTK-GNSS is a method that corrects satellite positioning (such as GPS) errors in real time and can measure positions with centimeter-level accuracy. Traditionally, obtaining such positioning accuracy required expensive specialized equipment like surveying GNSS receivers and base stations. However, in recent years, miniaturization and cost reduction have progressed, and small RTK-capable receivers can now be connected to smartphones for easy use.

Combining RTK-GNSS with smartphone 3D scanning allows you to give accurate latitude, longitude, and elevation information to each point in the acquired point cloud. In other words, you can record scanned 3D models at positions that conform to real surveying coordinate systems (plan coordinates and elevation references). This prevents misalignment when comparing measurement data with design coordinate systems and eliminates the need for additional alignment work. For example, traditionally when overlaying a 3D scan onto design CAD data, you had to align them using known points measured on site. With RTK-enabled smartphone measurement, point clouds are acquired with correct coordinates from the start, allowing you to analyze differences from the design immediately without such tedious adjustments. Furthermore, point clouds obtained at multiple times share a common coordinate system, making it smooth to compare time-series changes.

With high-precision positioning information, a smartphone can be used as a practical surveying instrument. In addition to volume calculation from point clouds, tasks such as measuring coordinates of arbitrary points for drafting or checking heights can be performed with only a smartphone and a small GNSS receiver. Measurements that previously required a professional surveyor can increasingly be carried out by site personnel on the spot. Quick feedback helps prevent rework and contributes to improved quality control. We are entering a reality where having just one smartphone per person can complete necessary measurements.

As-built volume measurement anyone can do with LRTK

LRTK is a solution that makes the high-precision 3D measurement described above—smartphone plus RTK-GNSS—easily realizable. LRTK attaches a pocket-sized RTK-GNSS receiver to a smartphone (iPhone/iPad) and, using a dedicated app to perform measurements, provides positioning accuracy and 3D scanning capabilities comparable to conventional large equipment.

Simply walking around the site with a smartphone yields high-precision point cloud data, and the required earthwork volumes can be computed on the spot. For example, to measure the volume of a pile of soil on site, you can walk around it scanning with the smartphone fitted with LRTK and measure embankment volumes on the order of hundreds of cubic meters within minutes. After scanning, the volume (in cubic meters) is displayed immediately on the smartphone screen, and if the planned design volume is available, the difference can be determined right there.

The acquired point clouds integrate with cloud services so that uploading automatically overlays them with design data to analyze differences, visualize elevation differences with heatmaps, and generate volume reports. Without relying on specialized CAD software, you can intuitively calculate necessary quantities in the app or cloud, making it easy for non-surveying specialists to handle.

LRTK lowers the barrier to as-built volume measurement considerably. The benefits of simplified surveying using LRTK include:

• Ease: All you need is a smartphone and a small receiver. There is no need to carry heavy tripods or surveying instruments, and you can measure whenever the need arises.

• Speed: Scanning and volume calculation on site can be completed in as little as a few minutes. Cloud integration lets you confirm and share results immediately without returning to the office.

• High accuracy: RTK-GNSS provides positional accuracy within a few centimeters or better. You can obtain far more precise as-built data than with conventional methods and reduce rework due to errors.

• Safety: You can record shapes remotely even on hazardous slopes or poor footing. Minimizing site entry contributes to worker safety.

• Versatility: Beyond earthwork volumes, LRTK can measure distances and areas, perform pass/fail checks for as-built results, and support as-built management with AR overlays of design data. For example, using an AR function that overlays 3D design models on site video lets you transparently confirm the positions of buried structures or instantly detect deviations between excavation faces and design grade lines. One device can meet a wide range of surveying needs, so the benefits can outweigh the implementation cost.

With LRTK, surveying and measurement tasks that previously had to be left to specialists can be performed by anyone on site as part of daily work. Being able to check as-built volumes immediately when needed—without expensive equipment or complex procedures—will dramatically improve efficiency and quality control.

Conclusion

Volume measurement in as-built management used to require time and experience, but the fusion of smartphone 3D scanning technology and RTK-GNSS has made it possible for anyone to start doing it right now. Using point cloud data allows large areas to be captured accurately, dramatically improving earthwork calculation precision and making it easier to ensure fairness and quality in construction. Rather than estimating from a limited number of survey points as in the past, digitally measuring the entire site and leaving solid evidence provides great reassurance to both clients and contractors.

New technologies are changing construction management. The Ministry of Land, Infrastructure, Transport and Tourism has drafted as-built management guidelines using 3D measurement technologies and is promoting ICT-based as-built and work-quantity management. In this context, easy-to-use, high-precision measurement tools like LRTK strongly support on-site DX (digital transformation). In fact, high-precision measurement with smartphones plus RTK has already begun to be adopted at construction sites nationwide, and large improvements in efficiency and labor savings have been reported compared to conventional methods. As a transformative tool for site work styles, further spread is expected. If you have only tried traditional methods so far, try this smartphone surveying once—you will likely be surprised by the combination of efficiency and accuracy.

FAQ

Q: Can a smartphone alone accurately measure as-built volumes? A: By combining a smartphone’s LiDAR scan with RTK-GNSS positioning, you can calculate volumes with an accuracy on the order of a few centimeters. Since this achieves positioning accuracy comparable to conventional GPS surveying, high-precision earthwork calculations from point clouds are possible. Validation tests have shown errors contained to about 1-2 cm (0.4-0.8 in), yielding results comparable to normal field surveying (note that a smartphone’s LiDAR scan alone can produce errors of several tens of centimeters, but RTK correction dramatically improves positioning accuracy).

Q: How does this compare to measurements with drones or laser scanners? A: Drone photogrammetry and terrestrial laser scanners (TLS) excel at measuring wide areas, but they may require specialist operators, flight permissions, or large equipment. Measurement with a smartphone plus LRTK is strong in its convenience—you can measure with equipment at hand anytime. Depending on site conditions, smartphone scans can cover up to about hundreds of square meters, and combining them with drones as needed allows efficient 3D capture of entire sites. Smartphones are also effective in spaces drones cannot access, such as indoors, under bridges, or in tunnels. Portability and usability in confined spaces are major advantages.

Q: Can people without surveying knowledge or qualifications use this effectively? A: Yes. Basic operations are designed to be intuitive, and site personnel can use the system after brief training or a manual. The system automatically handles specialized settings and complex calculations, so users simply follow on-screen instructions to scan and measure. That said, basic surveying knowledge makes it easier to understand and apply the results.

Q: Is it acceptable to use this method for as-built management in public works? A: Yes. The Ministry of Land, Infrastructure, Transport and Tourism has begun recommending 3D as-built management methods as new standards in the Reiwa era, and methods for calculating volumes from point clouds are specified in guidelines. Cases of submitting point cloud survey results as electronic deliverables and as-built documents are increasing. Data obtained with LRTK can conform to required coordinate systems and accuracy management, and if used appropriately, can be treated equivalently to conventional measurement results.

Q: What do I need to use LRTK? A: Basically, an iPhone or iPad, the LRTK device, and a communications environment for RTK positioning are enough to get started. The communications environment refers to an internet connection or communication terminal to receive GNSS correction information (such as a VRS correction service). An optional monopod can be used to stabilize the smartphone for more stable measurement, but it is not required. Because the required equipment is compact, preparation and transport are far easier than with traditional surveying sets.

Q: How can measurement data be stored and used? A: Point clouds and positioning data acquired with the LRTK app are automatically saved to the cloud and securely backed up. Point cloud data can be downloaded in standard formats such as LAS or XYZ as needed. Comparison results with design data and volume calculation reports can be exported as PDFs, so they can be incorporated into drawings and reports like conventional surveying deliverables.