Accurately Calculate Cut-and-Fill Volumes! How the Latest 3D Surveying Technologies Are Changing Construction Management

Table of Contents

• What are cut and fill?

• Traditional volume calculation methods and their challenges

• Benefits of earthwork calculation using the latest 3D surveying technologies

• How does construction management change with 3D surveying technologies?

• New surveying technologies expanding with drones and smartphones

• Easy high-precision surveying for anyone with LRTK

• FAQ

In earthmoving works at construction sites, "cut" and "fill" are performed routinely. Cut and fill refer respectively to excavating and removing soil from the ground and building up soil to form the ground. Cut and fill are indispensable in any civil engineering work such as land development, road construction, and housing-site preparation. Accurately calculating cut volumes and fill volumes (the volumes of soil excavated or placed) is extremely important for construction planning and cost control. Inaccurate volume calculations can cause significant impacts such as increased material costs due to shortages or excesses and schedule delays.

However, calculating cut and fill volumes using traditional surveying methods required tremendous labor and time and often produced errors. Recently, the emergence of advanced 3D surveying technologies has solved these issues, realizing dramatic efficiency gains and higher accuracy. This article focuses on calculating cut and fill volumes, explains the challenges of traditional methods and how the latest 3D technologies solve them, and details the benefits. We also examine how these technologies change construction management. Finally, we touch on a new technology called LRTK that enables simple surveying with a smartphone, showing that an era is coming in which anyone on site can easily perform high-precision earthwork measurements.

What are cut and fill?

"Cut" refers to excavating and removing soil from high ground such as mountains or hills. Conversely, "fill" refers to building up soil to raise low ground. Simply put, cut is the work of removing ground, and fill is the work of piling up soil. For example, in road construction, high parts are cut (cut) and low parts are filled to level the surface according to the road alignment to secure the specified ground elevation. In civil engineering, cut and fill are combined to shape the terrain and proceed with development or foundation work.

When performing cut or fill, it is necessary to estimate and plan in advance how much soil (volume) will be moved. It is also important after construction to compare and verify the design quantity on the drawings with the actual volume moved on site (as-built quantity). Soil shortages or excesses directly affect construction costs and transportation planning, so accurately grasping cut and fill volumes is essential for overall construction efficiency and proper management.

Traditional volume calculation methods and their challenges

Traditionally, the volume (earthwork) of cut and fill has been calculated mainly using planar survey maps and cross-section drawings. The representative method is the "average end-area method." This method is based on multiple cross-sections measured on site at fixed intervals: the cut area and fill area of each cross-section are calculated, and the volume for each segment is obtained by multiplying the average of adjacent cross-sectional areas by the distance between sections. Summing the volumes of each segment yields the total cut and fill volumes. This method is simple and can be done by hand calculations or spreadsheet software, so it has long been the standard method for earthwork calculations.

However, the average end-area method has several challenges. First, it requires significant time and labor. Personnel must measure heights at survey points on site at fixed intervals, produce drawings to calculate cross-sectional areas, and repeat calculations for each segment—an enormous effort on large sites. In terrain with many undulations, the number of survey points increases, and surveying alone can take days to weeks. Second, there are accuracy limitations. The average end-area method does not account for fine terrain variations between survey points. For example, if survey points are spaced every 20 m (65.6 ft), deep depressions or rises between those points will not appear in the cross-sections and thus will not be reflected in the calculation, becoming a source of error. In other words, the weakness of 2D drawing-based methods is that they tend to overlook terrain changes between points.

Moreover, traditional surveying requires post-excavation as-built surveying using batter boards and staffs, and producing as-built drawings to calculate volumes, which also imposes a burden on site work. Survey crews must walk the site thoroughly to measure after construction, raising safety risks and concerns about labor shortages. As described above, using traditional methods to accurately calculate and confirm cut and fill volumes required substantial time and effort and could not avoid certain errors.

Benefits of earthwork calculation using the latest 3D surveying technologies

To solve these challenges, new earthwork calculation methods utilizing 3D surveying technologies have emerged. Specifically, photogrammetry using drones, 3D laser scanners, or LiDAR-equipped smartphones acquire detailed point cloud data of the ground surface, and volumes are calculated from differences between pre- and post-construction terrain models. The ground before construction and the ground after construction (post-cut/fill) are each recorded as 3D models (digital terrain models), and the difference in volume between them is calculated to determine the actual amount excavated or filled. This entire process can be automatically processed by dedicated software, and complex calculations are executed quickly by computers.

The point cloud data (a collection of innumerable measured points) obtained by 3D surveying can represent terrain details at high density, dramatically improving the accuracy of volume calculations. Point clouds can capture fine irregularities that traditional cross-section methods could not. For example, if the ground surface is measured with a high-density point cloud at about 10 cm (3.9 in) intervals, volume estimation can be expected to be far more accurate than traditional methods. In one large-scale development site, a task that previously required four people working seven days (28 person-days) for earthwork measurement and calculations was switched to drone photogrammetry and completed by 2 people in 1 day (2 person-days). Moreover, the as-built quantities calculated were nearly as accurate as those obtained by traditional methods, with errors around 1%. Thus, by introducing 3D surveying technologies, there are cases where work time was reduced to about one-tenth or less while maintaining sufficient accuracy.

With the new method, the acquired point cloud data can be used to automatically calculate volumes via mesh (grid) methods, so once the data is collected you can flexibly recalculate or compute volumes for other areas without additional surveying. For example, if a design change occurs during construction, you can immediately obtain the new cut-and-fill volumes by comparing the old and new ground surfaces on the existing point cloud model without extra field surveys. This is a major advantage not available with traditional methods. Also, data obtained by 3D surveying is easy to store and share digitally and, as described later, can be utilized in various aspects of construction management. As a technology that excels in both accuracy and efficiency, it is rapidly spreading in the field of earthwork calculation.

How does construction management change with 3D surveying technologies?

High-precision earthwork calculation using 3D surveying technologies not only streamlines calculation tasks but also brings major changes to on-site construction management (as-built management). Traditionally, survey results were compiled and reported after the work, but with 3D point cloud data you can grasp as-built quantities on the spot immediately after construction. For example, in excavation work you can instantly compare the planned design volume with the soil actually removed to confirm whether the work was performed without shortage or excess. In fill work, you can similarly verify on the same day whether the placed volume matches the planned value. If there is a shortage, you can promptly arrange additional soil; if there is an excess, you can quickly plan disposal or reuse. Because as-built quantity verification that used to be done days to weeks later can now be performed nearly in real time, construction progress control and decision-making speed improve dramatically.

Furthermore, 3D models generated from point clouds become easy-to-share digital materials among clients and construction management engineers. Replacing paper drawings and numeric tables with visualized and stored 3D data increases the persuasive power of evidence. For example, presenting a three-dimensional model as proof of "this is how we excavated/filled" during an as-built inspection makes quantity verification with the client smoother. Archiving point cloud data after completion is also useful for future maintenance planning and monitoring terrain changes. As part of the digital transformation (DX) of construction sites, such data-driven management methods are attracting attention.

Reducing the burden on site staff is another important benefit. By utilizing 3D surveying, tasks that previously relied on experienced surveyors for as-built measurements can be performed by site personnel using simple procedures. The need to carry heavy surveying equipment or stand on dangerous steep slopes holding rods is reduced. For example, drone aerial photography can measure areas where people cannot enter, and smartphone surveying (described later) enables anyone to take measurements easily. 3D surveying technologies are expected to contribute to improved safety and mitigation of labor shortages.

New surveying technologies expanding with drones and smartphones

Not long ago, high-precision 3D surveying required laser scanners costing several million yen or specialized operators for drone aerial photography. However, those barriers have been significantly lowered in recent years. Particularly noteworthy are drone photogrammetry and smartphone LiDAR scanning.

Drone photogrammetry captures numerous photos from the air with a drone-mounted camera and processes them to generate high-density point clouds and orthophotos. It allows rapid surveying of large areas and safely acquires data from above even in dangerous areas where people cannot enter. Recently, as part of the Ministry of Land, Infrastructure, Transport and Tourism's "i-Construction" initiative, using drones for as-built measurements has been promoted and demonstrated at many sites.

On the other hand, LiDAR scanning with smartphones and tablets is revolutionary because it enables easy 3D surveying with familiar devices. For example, LiDAR sensors are built into iPhone Pro models from iPhone 12 onward and iPad Pro, and with dedicated surveying apps these devices can record the surrounding terrain and structures as point clouds with tens of millions of points. Simply walking around the object while holding the smartphone completes the scan in just a few tens of seconds to a few minutes. It is truly an era in which "professional-level point cloud measurement is possible with a device in your pocket." In fact, supervisors at construction sites have begun to routinely use smartphones to measure earthwork volumes. The significance of enabling detailed surveying that once required drone operation or expensive equipment with a single smartphone is great, and an environment where measurement is “anytime, anyone, immediately” is taking shape.

However, point clouds obtained by a smartphone alone are convenient but require attention regarding data positional accuracy (positioning accuracy). Ordinary smartphone GPS can have errors of several meters, and additional measures are needed to assign accurate coordinates to the acquired point cloud data. This is where the solution called LRTK introduced next comes into play.

Easy high-precision surveying for anyone with LRTK

LRTK is attracting attention as a new solution that turns a smartphone into a high-precision surveying instrument. LRTK (※a high-precision positioning system provided by Refixia Co.) is an integrated RTK positioning device for smartphones; by attaching a dedicated small antenna to an iPhone or similar device, network RTK enables centimeter-level positioning accuracy (half-inch accuracy). RTK (Real Time Kinematic) is a technique that corrects errors in satellite positioning such as GPS, and it can improve smartphone GPS—which normally has errors of several meters—down to a few centimeters. Using LRTK, you can attach highly accurate position coordinates in real time to point clouds acquired by the smartphone’s built-in LiDAR or camera. As a result, high-precision 3D surveying that previously required a drone + GNSS base station or an expensive laser scanner can be realized with just a smartphone.

When using LRTK, you can complete the workflow from on-site point cloud scanning to earthwork calculation almost in real time. For example, if you scan the cut or filled terrain with an LRTK-compatible smartphone app, the volume is automatically calculated from the acquired 3D point cloud and the result is displayed on the smartphone screen immediately. Since the point cloud data is tagged with high-precision position information from the start, differences from the reference surface (design surface) can be accurately calculated on site. This greatly shortens the step that used to require analyzing point clouds and calculating volumes on a computer after data acquisition, enabling you to know the as-built quantities immediately after scanning on site. This makes it possible to decide within the day whether residual soil disposal is required, or to confirm the required backfill volume and order additional material immediately, greatly increasing the responsiveness of construction management.

LRTK also has cloud integration features, allowing on-site data to be shared and archived instantly. Point cloud data and high-precision photos (photos with positional information) acquired with the smartphone LRTK app are automatically uploaded to the cloud, making it easy to review details on office PCs or share data with other team members. By saving multiple point clouds in a time series, you can understand construction progress as changes in a 3D model. Surveying tasks that were previously outsourced to specialist departments or surveying companies can be performed daily by on-site personnel themselves, significantly improving the accuracy and speed of construction management. If easy high-precision surveying devices like LRTK become widespread, a true era of “one survey instrument per person” may arrive. Instantly digitally recording all as-built quantity data generated on site and using it immediately for the next decision—that kind of new construction management is becoming a reality. Why not try point cloud scanning with a smartphone and earthwork measurement using LRTK at your site?

FAQ

Q: What are cut and fill? A: Cut is the work of excavating and removing high ground, and fill is the work of building up soil on low ground. These terms are used for works such as cutting down a hill to level it or filling a valley to raise the land. In road or site development, the series of processes that cut surplus soil and fill deficient parts is also called cut-and-fill work.

Q: How are cut and fill volumes calculated? A: Traditionally, the average end-area method is commonly used. Cross-sections measured at fixed intervals are prepared, the volume for each segment is obtained by multiplying the average area of adjacent cross-sections by the distance, and the total earthwork is calculated. The latest methods calculate cut and fill volumes by computing the volume difference between pre- and post-construction ground surface models derived from 3D terrain data acquired by drones or 3D scanners. The 3D surveying method features higher accuracy and efficiency than traditional methods.

Q: What is 3D surveying? A: 3D surveying is a method that acquires numerous points in space digitally to create a three-dimensional model instead of measuring the terrain with points and lines as in traditional methods. For example, laser scanners can scan terrain, or drone aerial photographs can be analyzed to obtain point cloud data, recording ground surface undulations as a high-density collection of points (a point cloud). This enables reproduction of terrain and structures as 3D models and high-precision measurement of volumes and distances.

Q: What are the benefits of using drones? A: Drone photogrammetry can survey wide areas in a short time, including dangerous places where people cannot enter. Aerial photography efficiently acquires terrain data, greatly shortening work time even for large-scale cut-and-fill works. Compared with traditional methods that required many ground survey points, it also improves accuracy by enabling detailed volume calculations on 3D models. However, drone-specific constraints such as flight permissions and weather impacts must be considered.

Q: Can you really do surveying with a smartphone? A: Yes. Recent smartphones (e.g., higher-end iPhone models) have built-in LiDAR scanners, and dedicated apps can measure surrounding 3D point clouds. While expensive equipment was formerly required, a smartphone alone can now perform small-scale fill volume measurements with sufficient accuracy. However, smartphone GPS has limitations in positional accuracy, so for strict positioning requirements, combining with RTK-capable devices (such as products like LRTK) can correct and improve accuracy.

Q: What is LRTK? A: LRTK is the name of a high-precision positioning device used with smartphones. It utilizes a satellite positioning correction technique called Real Time Kinematic (RTK) to enhance smartphone GPS to centimeter-level accuracy. This allows you to assign accurate positional information to point cloud data and photos captured by a smartphone and perform earthwork measurements and as-built management with accuracy comparable to professional surveying equipment. The fact that anyone can easily achieve high-precision 3D surveying with LRTK without specialized equipment or advanced skills is revolutionary.

Q: Does introducing 3D surveying technologies cost a lot? A: It depends on the technology, but the barriers to entry have fallen significantly. Purchasing drones or laser scanners or outsourcing them involves certain costs, but for small sites you can start low-cost 3D surveying by combining a smartphone with an inexpensive RTK device. Moreover, software is increasingly available as cloud services, allowing you to control operating costs by using only what you need when you need it. Many companies are beginning with pilot implementations in limited areas to assess effectiveness before full-scale adoption.