Table of Contents

• What is differential earthwork volume measurement?

• Differential earthwork volume measurement by traditional methods

• What is LRTK 3D point cloud surveying?

• Accuracy comparison: Traditional methods vs LRTK

• Efficiency comparison: Traditional methods vs LRTK

• Conclusion: Differential earthwork volume measurement made simple and highly accurate with LRTK

• Frequently Asked Questions

Differential earthwork volume measurement is the measurement of the volume difference between terrains or between fill and cut. For example, it is used to quantify how much more soil must be placed to reach the design ground level at a construction site, or how much excess soil has been removed. In civil engineering, knowing earthwork volumes is essential to verify whether the as-built (completed) shape matches the plans. Traditionally, measuring and calculating these volume differences required considerable time and labor. However, in recent years, within the trends of smart construction and digital transformation (DX), methods that use 3D point cloud surveying to efficiently compute differential volumes have attracted attention. This article compares traditional differential volume measurement methods with 3D point cloud surveying using LRTK, and explains in detail the differences in accuracy and efficiency.

What is differential earthwork volume measurement?

“Differential earthwork volume” refers to the volume difference between two terrains or models. Specifically, it means measuring the volume difference between the design model and the as-built condition, or the volume change due to terrain changes between two points in time. For example, differential volume measurement is carried out to determine how many cubic meters (m3) of fill are required to reach the planned elevation for a development project, or how much excavated material was generated during tunnel excavation. In civil and construction sites, accurate knowledge of these differential volumes is important for progress management of fill/cut works and excavation, verification that the as-built shape matches the design, and for quantity-based payment and reporting.

The basic principle of differential volume measurement is simple: calculate the volume difference between a “reference model” and a “comparison model.” The reference model might be the designed ground surface or terrain data from a previous survey, and the comparison model is typically the latest as-built terrain data. By spatially overlaying the two, protruding parts or deficits relative to the reference become “excess soil” or “insufficient soil,” and calculating their volumes yields the differential earthwork volume. The results are expressed as statements such as “add X cubic meters of soil to reach the design surface” or “X cubic meters of soil were excavated,” which help adjust construction plans and verify as-built conditions.

Differential earthwork volume measurement by traditional methods

Various surveying methods have long been used to obtain differential earthwork volumes. Historically, it was common to measure heights at regular intervals using a leveling rod and level, create cross-sections, and calculate volumes from those sections. With advances in technology, more efficient surveying methods have been adopted, but several challenges remain. Below are representative traditional methods and their characteristics.

• Total station (TS) surveying: A high-precision three-dimensional survey using an electronic distance meter and prism. An operator sets up the TS, and an assistant holds the prism staff to record points. A terrain model is created from the many measured point elevations and compared with the design model. Although accuracy is high, multiple personnel are required, and covering large areas requires repeatedly relocating the instrument. If point spacing is coarse, local undulations may be missed, potentially causing errors in volume calculations.

• UAV (drone) photogrammetry: This method creates a 3D terrain model from many aerial photos taken by a drone. It can capture large areas from above in a short time and produce point clouds (from photogrammetric processing) for volume calculations. However, drone flights require specialized skills and permits, and are restricted in urban areas or no-fly zones. To obtain a high-accuracy model from photos, placement of ground control targets and GNSS positioning are essential, and data processing can take several hours or more.

• Terrestrial laser scanner (TLS): A high-performance laser scanner mounted on a tripod directly scans the ground surface to capture high-density point clouds. It produces millimeter-level detailed data and yields very high accuracy for volume calculations. However, the equipment is large and expensive and requires specialized knowledge to operate. Multiple scans from different setups are needed to cover the whole area, and post-processing to align and merge the scans is also required.

As described above, each traditional method has strengths, but common issues exist. One major problem is the heavy burden of manpower and time. Not only does the surveying itself take time, but significant effort is required to turn acquired data into drawings, compute volumes, and prepare reports. Additionally, because multiple specialized instruments and skills are required, frequent surveying can be difficult amid on-site labor shortages. Even advanced methods like drone or laser scanning have operational constraints—weather, no-fly restrictions, and safety management—so they cannot be used anywhere at any time. Against this background, demand has grown for easier and faster ways to measure differential earthwork volumes.

What is LRTK 3D point cloud surveying?

A new solution that has emerged is 3D point cloud surveying using LRTK. LRTK is a pocket-sized high-precision positioning device developed by a venture company from Tokyo Institute of Technology. It is an ultra-compact RTK-GNSS receiver weighing about 125 g, designed to be attached to an iPhone or iPad via a dedicated case. “RTK” stands for real-time kinematic, a technique that adds correction information to satellite positioning to improve positioning accuracy to the centimeter level. In other words, a smartphone equipped with LRTK becomes a surveying instrument capable of positioning with centimeter-level accuracy (half-inch accuracy). The LRTK app that runs on the smartphone includes various surveying functions; in addition to high-precision single-point positioning, a key feature is a 3D point cloud scanning function that uses the LiDAR scanner.

The iPhone LiDAR (light detection and ranging) point cloud scanning capability became available starting with the iPhone 12 Pro. While a smartphone alone can scan surrounding 3D shapes and obtain point cloud data, the built-in sensors alone previously accumulated small positional errors and distortions, causing accuracy to degrade when walking large areas to scan. By combining with LRTK, the smartphone’s position can be continuously corrected with cm level accuracy (half-inch accuracy) during scanning. Because LRTK continuously provides high-precision coordinates to the smartphone in real time, whether scanning an office floor or the expansive grounds of a construction site, the point cloud will not suffer distortions no matter how much the user moves. The resulting point cloud data is tagged with global coordinates (latitude, longitude, height), eliminating the need for post-survey alignment. The surveyor simply walks the site holding the iPhone+LRTK and scans, enabling the rapid acquisition of a high-density 3D model of the as-built terrain.

The as-built point cloud data obtained with LRTK 3D point cloud surveying is uploaded to the dedicated LRTK Cloud (a web platform) for use. On the cloud, the acquired point cloud can be displayed in a 3D viewer and distance, area, and volume can be measured directly. For differential volume calculations, you only need to upload the design model (3D design data) to the cloud and overlay it with the as-built point cloud. For example, if you prepare the design ground model, the cloud can automatically compute the volume differences with the as-built point cloud. The fill/cut volumes for a user-specified area on the point cloud can be calculated with a single click, so “where soil is sufficient and where there is excess” becomes immediately apparent. Another convenient feature of LRTK Cloud is heatmap visualization of differences between the point cloud and the design model. Areas finished to design elevation can be shown in blue or green, and areas lower than design and lacking fill can be shown in red, allowing intuitive understanding of which points are how many centimeters higher or lower than the reference. In addition, the required soil volume is calculated in real time from the differences, providing concrete numbers like “add X cubic meters to reach the planned elevation.” Thus, using LRTK point cloud surveying enables on-site scanning → cloud computation workflow to instantly grasp differential earthwork volumes.

Accuracy comparison: Traditional methods vs LRTK

Let’s compare traditional methods and LRTK 3D point cloud surveying in terms of accuracy for differential earthwork volume measurement.

First, traditional methods: total stations and leveling surveys provide very high point measurement accuracy—on the order of a few millimeters to a few centimeters. However, the resulting data are discrete point sets, and constructing a terrain model requires interpolation between survey points, so accuracy is limited by point density and distribution. For example, if points are measured on a grid at 5 m (16.4 ft) intervals, small mounds or depressions between points will be missed, and such unobserved undulations can introduce errors in volume calculations. Covering an entire site in detail requires increasing the number of measured points, but there are practical limits to doing this manually. On the other hand, drone photogrammetry and TLS acquire surface data as continuous surfaces, producing point clouds that cover the ground without gaps. However, photogrammetric accuracy depends on flight altitude, image processing quality, and the measurement accuracy of ground control points (GCPs). Aerial photogrammetry can exhibit vertical errors on the order of a few centimeters to a few tens of centimeters, so strict accuracy control is necessary to obtain reliable differential volumes. Terrestrial laser scanners are extremely accurate, but as noted earlier, their large-scale setup can prevent exhaustive scanning of all required locations, potentially causing data gaps.

In contrast, point cloud surveying using LRTK can capture wide areas with high-density point clouds, enabling volume calculations that capture fine surface undulations. Because the point cloud itself represents the ground surface almost continuously, interpolation errors are greatly reduced. LRTK’s positioning accuracy is very high—about ±1–2 cm (±0.4–0.8 in) horizontally and around ±3 cm (±1.2 in) vertically. This level is comparable to national control points and first-class GNSS surveying equipment, and reports indicate that coordinates measured at the same point with an LRTK-equipped smartphone and surveying GNSS equipment differed by only a few millimeters in some comparisons. In other words, each point in the smartphone-acquired point cloud falls within a few centimeters of true position. This accuracy, difficult to achieve with smartphone-only scanning, is realized with LRTK. As a result, differential volumes computed from LRTK point clouds can be expected to match the accuracy of results obtained with conventional surveying instruments. Compared to coarse manual measurements, the much more detailed terrain capture improves the accuracy of volume computations.

Additionally, LRTK point cloud surveying allows quality checks in real time during measurement, which is important for ensuring accuracy. The point cloud is displayed sequentially on the smartphone screen as you scan, so you can immediately see if any areas were missed. If any coverage gaps exist, you can re-scan those spots on the spot to obtain complete data. Unlike the traditional workflow where one might only realize in the office that a survey point was missing, LRTK enables the acquisition of complete point cloud data in the field. Overall, LRTK 3D point cloud surveying provides a foundation for calculating differential earthwork volumes with high accuracy compared to traditional methods.

Efficiency comparison: Traditional methods vs LRTK

Next, compare traditional methods and LRTK point cloud surveying in terms of efficiency. There are large differences in required measurement time, personnel, and speed of data processing.

• Reduced work time: Traditionally, field surveying for earthwork volume measurement could take from half a day to a full day, depending on the situation. For example, after heavy machinery finishes fill/excavation, surveyors would walk the site to measure various points, then return to the office to perform calculations and deliver results the following day or later. With LRTK, the measurement itself can be completed in tens of minutes for wide areas. Walking the site with an iPhone in hand and scanning produces a 3D model directly. Uploading to the cloud and computation are automated, so the differential volume results can be obtained the same day. On sites that adopted LRTK, time for volume calculation has been reported to shrink from hours to minutes, and tasks that used to take a full day for as-built checks were sometimes completed on the same day.

• Reduced personnel requirements: Traditional surveying often required multiple people. For TS surveying, at least an operator and a staff holder are needed, and sometimes additional survey assistants. Drone surveys also require a minimum of one to two people for operation and safety. In contrast, LRTK point cloud surveying can be performed by a single person. A single operator carrying a smartphone and a small antenna can acquire data by walking the site. The impact of enabling one-person as-built management with one smartphone per person is significant where labor shortages are severe; measurements can be performed without pulling other workers away from their tasks, improving overall efficiency.

• Immediate processing and feedback: Traditionally, measured data were taken back to the office and processed on PCs with specialized software to compute volumes and prepare drawings and reports. This could sometimes take a full day or more for reconciliation with design data and report creation. With LRTK, these processes are automated and simplified on the cloud. Once the point cloud and design model are uploaded, volume differences are displayed instantly, and results can be shared via the cloud with stakeholders. Being able to check results on site and make immediate decisions dramatically shortens the feedback cycle. This enables timely instructions for rework or additional fill, improving not only operational efficiency but also quality management efficiency.

• Work environment and safety: The work environment affects surveying efficiency. Traditionally, measuring steep slopes often required personnel to climb hazardous slopes to measure distances and angles. With LRTK point cloud scanning, the entire slope can be measured remotely from a safe location such as the slope base. Areas with poor footing or where heavy machinery is operating can be scanned from a distance without interrupting work, enabling both safety and efficiency. LRTK is also effective in urban areas or indoor spaces where drones cannot be used. It is less susceptible to weather conditions and provides the flexibility to “measure whenever you need to,” which is a major advantage.

As described above, differential earthwork volume measurement using LRTK brings dramatic efficiency improvements compared with traditional methods. Shorter measurement times, reduced personnel costs, real-time processing for quick decision-making, and better safety together raise overall operational efficiency significantly. This contributes to shorter schedules and reduced overtime, improving productivity across the site.

Conclusion: Differential earthwork volume measurement made simple and highly accurate with LRTK

We compared traditional methods and LRTK from the perspectives of accuracy and efficiency. While traditional surveying methods have established reliability, they require significant manpower and time and lack flexibility. In contrast, 3D point cloud surveying with LRTK is revolutionary in that it allows anyone to perform high-precision surveying easily with a familiar device—a smartphone. One person can rapidly capture as-built conditions over wide areas and instantly compute differential volumes, changing the way progress management and quality checks are performed on site. In terms of accuracy, LRTK achieves positioning precision comparable to professional surveying equipment, enabling detailed analysis from point cloud data.

In short, differential earthwork volume measurement has been dramatically simplified and enhanced by the advent of LRTK. Complex measurements can now be completed with a smartphone in hand, and the data can be shared and utilized immediately. As the comparison with traditional methods shows, LRTK 3D point cloud surveying is a next-generation solution that supports the field with overwhelming efficiency and sufficient accuracy. If you are struggling with the workload of earthwork management or as-built measurements, consider trying LRTK-based simplified surveying. Adopting cutting-edge technology can improve site productivity and safety, enabling projects to progress with unprecedented speed.

Frequently Asked Questions

Q: What is differential earthwork volume measurement? A: Differential earthwork volume measurement is the process of measuring the difference in soil volume between a reference terrain dataset and a comparison terrain dataset. For example, comparing the ideal ground surface in design drawings with the current terrain to calculate how much soil needs to be added or removed is differential volume measurement. It is widely used for as-built management and quantifying excavated material in civil works.

Q: What traditional methods are used to measure differential volumes? A: Traditional methods mainly use surveying instruments. Specifically, three-dimensional surveys using total stations to obtain cross-sections and calculate volumes, periodic height measurements using levels and staffs to derive volumes from sections, and more recently, creating 3D models from drone aerial photos. Each method can achieve a certain level of accuracy, but they require manpower and time, and it can be difficult to densely cover large areas.

Q: What are the differences between drone surveying and LRTK point cloud surveying? A: Drone surveying photographs large areas from the air to create terrain models, which allows one-time coverage of large surfaces. However, drones cannot be used in no-fly zones or in strong winds, and pilot qualifications and permit applications are required. Photo data processing also takes time. LRTK point cloud surveying measures from the ground by walking, and can be performed anywhere, including urban areas and indoor spaces where drones are restricted. LRTK offers real-time point cloud acquisition and on-site result checking, so it has superior responsiveness. In short, LRTK can handle environments where drones are unusable, and it reduces data processing lag.

Q: Is surveying with a smartphone really accurate enough? A: Yes—when combined with LRTK, smartphone surveying achieves sufficient accuracy. LRTK adds correction information to satellite positioning and enables smartphone positioning with centimeter-level accuracy (half-inch accuracy). This provides high reliability for acquired point clouds. Coordinates measured with an LRTK-equipped iPhone have been shown to closely match values measured with conventional high-precision GNSS equipment. Moreover, the high density of point clouds averages out local errors, ensuring adequate accuracy for volume calculations.

Q: What do I need to use LRTK point cloud surveying, and is it difficult to use? A: You need an LiDAR-equipped iPhone/iPad, the compact LRTK mounting device, and the LRTK app. The operation is intuitive and not difficult. Attach the LRTK device to the smartphone, start the app, and walk the area you want to measure. The app is available in Japanese, and starting/stopping positioning and scanning are done with simple button operations. Acquired data syncs automatically to the cloud, so there is no need for complex PC software processing. The system is designed so those without specialized surveying knowledge can become proficient in a short learning period, allowing for easy introduction and use.