Table of Contents

• What are cut-and-fill works?

• Traditional volume calculation methods and their challenges

• Advantages of soil volume calculation using the latest 3D surveying technologies

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

• Emerging surveying technologies with drones and smartphones

• Easy high-precision measurement with LRTK for anyone

• FAQ

In earthworks where soil is moved on construction sites, “cut” and “fill” operations are performed routinely. Cut refers to excavating and removing soil from higher ground, while fill refers to placing soil to raise lower ground. Cut-and-fill are indispensable in all kinds of civil engineering works such as land development, road construction, and residential site preparation. Accurately calculating the cut volume and fill volume (the volumes excavated or placed) is extremely important for construction planning and cost control. Inaccurate volume calculations can have serious impacts, such as increases or decreases in material costs and schedule delays due to surplus or shortage.

However, calculating cut-and-fill volumes with conventional surveying methods has required enormous effort and time and often produced errors. Recently, new 3D surveying technologies have emerged to solve these issues, achieving dramatic efficiency gains and high precision. This article focuses on volume calculation for cut-and-fill, explaining the challenges of traditional methods and how the latest 3D technologies address them, and detailing the benefits. We will also look at how these technologies change construction management. Finally, we introduce a new technology called “LRTK” that enables simple surveying with smartphones, showing that an era is coming in which anyone on site can easily perform high-precision soil volume measurements.

What are cut-and-fill works?

“Cut” refers to excavating and removing soil from high ground such as mountainous or hilly areas. Conversely, “fill” refers to placing 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, soil from high areas is cut away and used to fill low areas so that the ground level matches the road alignment, securing the required roadbed elevation. In civil engineering works, cut and fill are combined to reshape the terrain and proceed with development and foundation works.

When performing cut and 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 designed quantities on drawings with the actual soil volumes moved on site (as-built quantities). Soil surplus or shortage directly affects construction costs and transport planning, so correctly understanding cut and fill volumes is indispensable for overall construction efficiency and proper management.

Traditional volume calculation methods and their challenges

Traditionally, the volume (soil quantity) of cut and fill has mainly been calculated using planar survey maps and cross-section drawings. The representative method is the “average cross-section method.” This involves surveying multiple cross-sections at regular intervals on site, calculating the cut and fill areas for each cross-section, and obtaining the volume for each segment by multiplying the average of adjacent cross-section areas by the distance between the sections. Summing the volumes of all segments yields the total cut and fill volumes. Because this method is simple and can be performed by hand calculations or spreadsheet software, it has long been used as the standard method for soil volume calculation.

However, the average cross-section method has several issues. First, it is labor- and time-intensive. Personnel must measure heights at survey points on site at regular intervals, draft drawings to calculate cross-sectional areas, and repeat calculations for each segment, which becomes a huge effort on large sites. Especially on highly undulating terrain, the number of survey points increases, and surveying alone can take days to weeks. Second, there are limits to accuracy. The average cross-section method does not account for fine terrain variations between survey points. For example, if survey points are spaced every 20 m (65.6 ft), a deep depression or rise between those points will not appear in the cross-section and thus will not be reflected in the calculation, potentially causing errors. In other words, a 2D drawing-based method tends to overlook terrain changes between points.

Furthermore, conventional surveying for as-built verification after heavy equipment work—using batter boards or staff measurements—also imposes a burden on site. Survey personnel must walk the site extensively to measure after construction, raising safety risks and exacerbating manpower shortages. As described above, accurately calculating and verifying cut-and-fill volumes with traditional methods requires substantial time and effort and inevitably entails some degree of error.

Advantages of soil volume calculation using the latest 3D surveying technologies

To solve these challenges, new soil volume calculation methods using 3D surveying technologies have appeared. Specifically, methods use photogrammetry by drones, 3D laser scanners, or LiDAR-equipped smartphones to obtain detailed point cloud data of the ground surface, and calculate volumes from differences between pre- and post-construction terrain models. The pre-construction ground and the post-construction (after cut and fill) ground are recorded as 3D models (digital terrain models), and the volumes excavated or filled are obtained by calculating the difference between these models. This sequence of processing can be automated by dedicated software, with complex computations executed rapidly by computers.

Point cloud data acquired by 3D surveying—collections of countless measured points—can represent terrain details at high density, dramatically improving volume calculation accuracy. Point clouds can capture fine irregularities that conventional cross-section methods cannot. For example, measuring the ground surface with a high-density point cloud at about 10 cm (3.9 in) intervals can be expected to produce much more accurate soil volume estimates than traditional methods. In practice, at a large-scale development site that had previously required four people seven days (totalling 28 person-days) to carry out soil volume measurement and calculation, switching to photogrammetry by drone enabled the same work to be completed by two people in one day (2 person-days). Moreover, the as-built quantities calculated showed an error of only about 1% compared to values obtained by traditional methods—effectively equivalent in accuracy. 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, soil volumes can be automatically calculated from the acquired point cloud using mesh (grid) methods, so once data are captured, recalculation or calculation for other areas can be done flexibly without additional surveying. For example, if design changes occur during construction, new cut and fill volumes can be obtained immediately by comparing the new and old ground surfaces on the existing point cloud model without extra surveying. This is a major advantage not available with conventional methods. In addition, data obtained through 3D surveying are easy to store and share digitally, and—as discussed later—can be used in various aspects of construction management. As a technology that excels in both accuracy and efficiency, it is rapidly spreading in the field of soil volume calculation.

How will construction management change with 3D surveying technologies?

High-precision soil volume calculation using 3D surveying technologies not only streamlines calculation tasks but also brings major changes to on-site construction management (as-built management). Traditionally, surveying results were compiled and reported after works were completed, but using 3D point cloud data makes it possible to grasp as-built quantities on the spot immediately after construction. For example, in excavation works, the designed planned volume and the actual removed soil can be compared instantly to confirm whether the work was done without surplus or shortage. Similarly, for fill works, it is possible to verify on the same day whether the placed volume matches the planned value. If there is a shortage, additional soil can be arranged promptly; if there is a surplus, disposal or reuse can be scheduled quickly. Because as-built quantity verification that used to be done days to weeks later can now be performed almost in real time, progress management and decision-making speed on site are greatly improved.

Moreover, 3D models generated from point clouds become digital materials that are easy to share among clients and construction management engineers. Instead of reporting soil volumes with paper drawings and numerical tables, visualizing and storing them as 3D data increases the persuasive power of the evidence. For example, presenting a 3D model to show “this is how it was excavated/filled” during an as-built inspection makes quantity confirmation with the client smoother. Archiving point cloud data after completion can also be 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 using 3D surveying, measurement tasks that used to rely on experienced surveyors can be performed by on-site personnel using simple procedures. The number of times staff must carry heavy surveying equipment or go onto hazardous steep slopes with poles can be reduced. Drone aerial surveys can capture areas inaccessible on foot, and smartphone surveying (discussed below) allows anyone to measure easily. 3D surveying technologies are expected to contribute to improved safety and alleviation of manpower shortages.

Emerging surveying technologies with drones and smartphones

Until recently, conducting high-precision 3D surveying required laser scanners costing hundreds of thousands of dollars or drone aerial surveys conducted by specialized operators. However, these barriers are rapidly falling. Particularly notable are drone photogrammetry and LiDAR scanning with smartphones.

Drone photogrammetry involves taking many photos from the air with a camera mounted on a drone and processing them to generate high-density point clouds and orthophotos. It can survey wide areas in a short time and safely obtain data from above even in locations where people cannot enter. Recently, drone as-built measurements have been promoted as part of the “i-Construction” initiative led by the Ministry of Land, Infrastructure, Transport and Tourism, and many field demonstrations have been conducted.

On the other hand, LiDAR scanning with smartphones and tablets is revolutionary in that it enables easy 3D surveying using familiar devices. For example, LiDAR sensors are built into the iPhone Pro series from the iPhone 12 onward and in iPad Pro models; using dedicated surveying apps, surrounding terrain and structures can be recorded as point clouds of tens of millions of points. Simply walking around the target with a smartphone in hand can complete a scan in just several tens of seconds to a few minutes. It is truly an era in which “professional-level point cloud measurement can be done with a device in your pocket.” In fact, there are already cases where site supervisors use smartphones themselves for routine soil volume measurements. The fact that detailed surveying that once required drone operation or expensive equipment installation can now be done with a single smartphone is significant, and an environment where anytime, anyone, immediately measurement is possible is taking shape.

However, point clouds obtained by smartphones, while convenient, require attention to the positional accuracy (geolocation accuracy) of the data. Ordinary smartphone GPS can have errors of several meters, and additional measures are needed to give acquired point clouds accurate coordinates. This is where the solution called LRTK introduced next comes in.

Easy high-precision measurement with LRTK for anyone

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., Ltd.) is an RTK positioning device integrated with a smartphone; by attaching a dedicated small antenna to an iPhone or the like, network RTK enables centimeter-level accuracy (cm level 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 meter-level errors—to the level of a few centimeters. Using LRTK, it is possible to link high-precision position coordinates in real time to point cloud data acquired by a smartphone’s built-in LiDAR or camera. Therefore, high-precision 3D surveying that previously required a drone plus a GNSS base station or an expensive laser scanner can be realized with a single smartphone.

When using LRTK, the workflow from on-site point cloud scanning to soil volume calculation can be completed almost in real time. For example, when scanning cut or fill terrain with an LRTK-compatible smartphone app, the acquired 3D point cloud is automatically used to calculate volumes, and the results are displayed on the smartphone screen immediately. Since the point cloud data are tagged with high-precision position information from the start, differences from a reference surface (design surface) can be accurately calculated on site. This greatly shortens the process that used to require post-processing and volume calculation on a computer, allowing site crews to know as-built quantities immediately after scanning on site. Decisions such as “determine if spoil removal is required today” or “confirm backfill volume and place an additional order immediately” can be made the same day, dramatically increasing the agility of construction management.

LRTK also has cloud integration features, allowing on-site data to be shared and stored instantly. Point clouds and high-precision photos (with geotags) acquired by an LRTK smartphone app are automatically uploaded to the cloud, making it easy to check details on office PCs or share data with other team members. By saving multiple point clouds over time, construction progress can be understood as changes in 3D models. By enabling site personnel to routinely perform surveying tasks that were previously outsourced to specialized departments or survey companies, the accuracy and speed of construction management are expected to improve significantly. If easy high-precision surveying devices like LRTK become widespread, a literal “one-person-one-survey device” era may arrive. Recording all as-built quantity data on site digitally and using it immediately for the next decision—that new form of construction management is becoming a reality. Why not try point cloud scanning with a smartphone or soil volume measurement using LRTK at your site?

FAQ

Q: What are cut and fill? A: Cut is the work of excavating and removing soil from high ground, and fill is the work of placing soil to raise low ground. These terms are used for works such as cutting down hills to create flat land or filling valleys to raise ground levels. The series of processes in which excess soil is cut and used to fill deficient areas in roadworks or land development is also called “cut-and-fill works.”

Q: How are cut-and-fill volumes calculated? A: Traditionally, the average cross-section method has been commonly used. Cross-sections measured at regular intervals are created, the average area of adjacent cross-sections is multiplied by the distance between them to get the volume for each segment, and the total soil volume is obtained. The latest methods calculate cut and fill volumes by computing the volume difference between pre- and post-construction ground surface models obtained from 3D terrain data acquired by drones or 3D scanners. The 3D surveying method can calculate volumes more accurately and efficiently than conventional methods.

Q: What is 3D surveying? A: 3D surveying is a surveying method that acquires many points in space digitally to create three-dimensional models, rather than measuring terrain with points and lines as in the past. For example, scanning terrain with a laser scanner or generating point cloud data by analyzing drone aerial photos records surface undulations as high-density point sets (point clouds). This allows terrain and structures to be reproduced as 3D models and volumes and distances to be measured with high precision.

Q: What are the advantages of using drones? A: Drone photogrammetry has the advantage of surveying wide areas in a short time, including dangerous locations where people cannot enter. Aerial photography efficiently acquires terrain data, significantly reducing working time even for large-scale cut-and-fill works. Compared to conventional methods that require many ground survey points, 3D models allow detailed volume calculations and improved accuracy. However, drones have their own constraints such as flight permissions and weather conditions.

Q: Can you really survey with a smartphone? A: Yes. Recent smartphones (for example, high-end iPhone models) have LiDAR scanners built in, and dedicated apps can measure surrounding 3D point clouds. Although expensive equipment was previously required, a smartphone alone can now perform sufficiently accurate measurements for small-scale fill volume estimation. However, because smartphone GPS accuracy is limited, when precise positioning is required it is recommended to use RTK-capable devices (such as products like LRTK) in combination to correct accuracy.

Q: What is LRTK? A: LRTK is the name of a high-precision positioning device that attaches to a smartphone. It utilizes Real Time Kinematic (RTK) satellite-positioning correction technology to enhance smartphone GPS to centimeter-level accuracy. This allows point cloud data and photos captured with a smartphone to be geotagged with accurate position information, enabling soil volume measurement and as-built management at accuracies comparable to professional surveying equipment. The fact that anyone can easily achieve high-precision 3D surveying using LRTK without specialized equipment or advanced skills is groundbreaking.

Q: Is there a cost to introducing 3D surveying technologies? A: Costs vary by technology, but the barriers to entry have dropped significantly compared to the past. Drones and laser scanners incur purchase or outsourcing costs, but for small sites it is possible to start 3D surveying at low cost by combining a smartphone with an inexpensive RTK device. In addition, more software is available as cloud services, allowing operational costs to be controlled by using them only when needed. Many companies start by trialing 3D surveying on a portion of a site to evaluate effectiveness before full deployment.