Differential Earthwork Volume Measurement: Conventional Methods vs LRTK 3D Point Cloud Surveying — A Thorough Comparison

Table of Contents

• What is differential earthwork volume measurement?

• Differential earthwork volume measurement by conventional methods

• What is LRTK 3D point cloud surveying?

• Accuracy comparison: Conventional methods vs LRTK

• Efficiency comparison: Conventional methods vs LRTK

• Summary: Differential earthwork volume measurement made simple and high-precision with LRTK

• Frequently Asked Questions

Differential earthwork volume measurement is the measurement of the volume difference between terrains or cut-and-fill works. For example, it is used to quantify how much more fill is needed to achieve the designed ground shape at a construction site, or how much excess soil has been removed. In civil engineering, knowing the amount of earth is indispensable for confirming that the as-built (finished shape) matches the plan. Traditionally, measuring and calculating this volume difference required a lot of time and effort. However, in recent years, amid the trends of smart construction and digital transformation (DX), methods that use 3D point cloud surveying to efficiently calculate differential volumes have gained attention. This article compares conventional differential earthwork 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?

First, “differential earthwork volume” refers to the volume difference between two terrains or models. Specifically, it means measuring the difference in earth volume between a design model and the current condition, or the volume difference caused by terrain changes between two points in time. For example, differential earthwork volume measurement is conducted to determine how many cubic meters (m^3) of fill are needed to reach the planned elevation in land development, 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 and cut works, verification that the as-built shape matches the design, and quantity-based progress management (work quantity estimation).

The basic principle of differential earthwork volume measurement is simple: calculate the volume difference between a “reference model” and a “comparison model.” The reference model might be the planned ground surface at design time or the terrain data from the previous measurement, while the comparison model is the latest current terrain data. By spatially overlaying these two models, protruding parts become “excess soil” and recessed parts become “deficient soil,” and calculating their volumes yields the differential earthwork volume. The results are expressed as statements like “add X cubic meters of soil to reach the design surface” or “X cubic meters of soil were excavated,” which are useful for adjusting construction plans and verifying as-built conditions.

Differential earthwork volume measurement by conventional methods

Various surveying methods have long been used to obtain differential earthwork volumes. Historically, measuring heights at regular intervals with a leveling rod and level and creating cross-sections to compute volumes was common. With technological advances, more efficient surveying methods have been adopted, but several challenges remain. Let’s look at representative conventional methods and their characteristics.

• Total station (TS) surveying: High-precision three-dimensional surveying using an electronic distance meter and a prism. An operator sets up the TS while an assistant holds the prism rod to read measurement points. A current terrain model is created from the many measured elevation points, and the difference from the design model is calculated. Although the accuracy is high, it requires multiple personnel and frequent instrument re-setting to cover a wide area. If the measurement point spacing is coarse, local undulations can be missed, potentially causing errors in volume calculations.

• UAV (drone) photogrammetry: This method creates a 3D terrain model from numerous aerial photos taken by a drone. It can capture wide areas from the air in a short time and generate ground point clouds (point cloud data from photo analysis) for volume calculations. However, drone flights require specialist skills and permissions, and they cannot be used in urban areas or no-fly zones. Moreover, obtaining a high-accuracy model from photos requires placement of ground control targets and GNSS-based position correction, 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 obtain dense point clouds. Millimeter-level detailed data can be obtained, and the accuracy of volume calculations is very high. However, the equipment is large and expensive and requires expert handling. It is necessary to perform multiple scans from different setup positions to cover the whole area, and post-processing to merge (register) the scan datasets is required.

As described above, conventional methods each have strengths, but they share common issues. One is the heavy burden of manpower and time. Not only does the surveying itself take time, but creating drawings from acquired data, calculating volumes, and preparing reports also require considerable effort. Because multiple specialized instruments and skills are needed, frequent surveying can be difficult amid worsening on-site labor shortages. Even advanced methods such as drones and laser scanners have operational constraints—weather, no-fly restrictions, safety management—so they cannot be used anytime, anywhere. Against this background, the demand for “a simpler and faster way to measure differential earthwork volumes” has been growing.

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 approximately 125 g that is attached to an iPhone or iPad via a dedicated case. RTK stands for Real-Time Kinematic, a technology that enhances satellite positioning by adding correction information to reduce errors to a few centimeters. In other words, a smartphone equipped with LRTK becomes a surveying device capable of centimeter-level positioning accuracy. The LRTK app that runs on the smartphone includes various surveying functions, and a major feature is the 3D point cloud scanning function using the LiDAR scanner.

LiDAR on the iPhone (light detection and ranging) became available on iPhone 12 Pro and later models. A smartphone alone can scan surrounding three-dimensional shapes and obtain point cloud data, but with only the built-in sensors, slight positional drift and distortion can accumulate when walking over large areas, causing accuracy degradation. By combining with LRTK, the smartphone’s position can be continuously corrected to cm level accuracy (half-inch accuracy) during scanning. Because LRTK continuously provides high-precision position coordinates to the smartphone in real time, point clouds do not become distorted no matter how much the user moves, from office floors to expansive construction sites. The resulting point cloud data are tagged with global coordinates (latitude, longitude, altitude), so no post-measurement alignment is required. The measurer simply walks the site holding an iPhone + LRTK and waves it around to quickly obtain a high-density 3D model of the current terrain.

The as-built point cloud data obtained by LRTK 3D point cloud surveying are uploaded to the dedicated LRTK Cloud (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. To calculate differential earthwork volumes, simply upload the design model (3D design data) to the cloud and overlay it with the current point cloud. For example, if you prepare a design ground model, the cloud will automatically compute the volume difference with the current point cloud. Cut and fill volumes for a user-specified area on the point cloud can be calculated with one click, so you can immediately see “where soil is sufficient and where there is a surplus.” Another useful LRTK Cloud feature 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, while places lower than the design and lacking fill are shown in red, making it intuitive to see which points are how many centimeters above/below the reference. The required soil volume is also calculated from the difference in real time, so specific numbers such as “add X cubic meters of soil to reach the planned elevation” are instantly available. Thus, using LRTK point cloud surveying enables a workflow of scanning on-site → computation in the cloud to instantly grasp differential earthwork volumes.

Accuracy comparison: Conventional methods vs LRTK

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

First, conventional methods: total stations and leveling measurements have very high point-by-point measurement accuracy on the order of millimeters to a few centimeters. However, the data obtained are discrete point sets, and because a terrain model must be created by interpolating between measured points, there is an accuracy limit that depends on the density and placement of measurement points. For example, if points are measured on a 5 m (16.4 ft) spacing grid, small bumps or depressions between those points will not be captured, and such unrecorded undulations can be a source of error in volume calculations. To measure an entire site in detail would require increasing the number of measurement points, but there are practical limits to doing this manually. On the other hand, drone photogrammetry and TLS acquire data as surfaces, yielding point clouds that cover the ground without gaps. However, with photogrammetry, accuracy depends on flight altitude, image analysis accuracy, and the measurement accuracy of ground control points (GCPs). Aerial photogrammetry can sometimes produce vertical errors on the order of several centimeters to several tens of centimeters, so strict accuracy control is required for precise differential volume calculations. Terrestrial laser scanners offer very high accuracy, but as noted, their large, complex equipment may prevent fine scanning of every required area, potentially causing data omissions.

In contrast, point cloud surveying with LRTK can obtain dense point clouds over wide areas, enabling volume calculations that capture fine terrain irregularities. Because the point cloud itself represents the ground surface almost continuously, interpolation errors are greatly reduced. In addition, LRTK positioning accuracy is extremely high—about ±1–2 cm (±0.4–0.8 in) horizontally and about ±3 cm (±1.2 in) vertically. This level is comparable to the Geospatial Information Authority of Japan’s electronic reference points and first-order GNSS surveying equipment, and some comparisons between LRTK-equipped smartphones and surveying GNSS instruments have reported only millimeter-level differences at the same points. In other words, each point in the smartphone-acquired point cloud stays within a few centimeters of error. This level of accuracy, which was difficult to achieve with smartphone-only scans, has been realized with LRTK. As a result, differential volumes computed from LRTK point clouds can be expected to be on par with results obtained from traditional surveying equipment. Indeed, compared to coarse manual surveying, the much more detailed terrain capture afforded by LRTK improves the accuracy of volume calculations.

Furthermore, an important factor for ensuring accuracy with LRTK point cloud surveying is the ability to verify data quality in real time during measurement. Because the point cloud is displayed on the smartphone screen continuously during scanning, you can immediately judge whether any areas were missed. If part of the area was not captured, you can re-scan on the spot to obtain complete data. This avoids the conventional situation of returning to the office only to realize “we didn’t measure that point,” and ensures you end up with a complete point cloud dataset. Overall, LRTK 3D point cloud surveying provides a foundation that allows differential volumes to be calculated with higher accuracy compared to conventional methods.

Efficiency comparison: Conventional methods vs LRTK

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

• Shorter work time: Conventional site surveying for earth volume measurement could take half a day to a full day in some cases. For example, after fill or excavation is completed by heavy equipment, survey staff might walk around the site to take measurements, return to the office to perform calculations, and provide results the next day or later. With LRTK, the measurement itself can be completed over a wide area in about several tens of minutes. Walking around the site with an iPhone in hand for scanning creates a 3D model directly. Since upload to the cloud and calculations are automated, differential earthwork volume results can be obtained the same day. In actual LRTK deployment sites, the time for volume calculation has been shortened from “several hours → several minutes,” and as-built verification work that used to take a full day has been completed within the same day.

• Reduced personnel requirements: Conventional methods typically require multiple people. TS surveying requires an operator and a person to hold the staff, and sometimes additional assistants. Drone surveying needs at least one or two people for safe operation and piloting. By contrast, LRTK point cloud surveying can basically be done by a single person. A person carrying a smartphone and a small antenna can acquire data by moving around the site alone. In sites with severe labor shortages, the impact of “one-person, one-smartphone for as-built management” is significant, allowing measurements to be taken without stopping other workers and improving overall efficiency.

• Immediate processing and feedback: Traditionally, on-site measurements were brought back to the office and input into dedicated software on a PC to compute volumes and produce drawings and reports, which could take a full day or more for matching with design data and report preparation. LRTK automates and simplifies these processes on the cloud. Once point clouds and the design model are uploaded, volume differences are displayed instantly, and results can be shared with stakeholders via the cloud. Being able to confirm results on-site and make judgments for subsequent work dramatically shortens the feedback cycle. This enables timely prevention of rework and prompt instructions for additional fill, improving not only operational efficiency but also quality management.

• Work environment and safety: The efficiency of surveying is also affected by site conditions. Conventional methods sometimes required people to climb steep slopes to measure distances and angles—a dangerous task. LRTK point cloud scanning allows remote measurement of an entire slope from a safe location at the foot of the slope. You can scan from the periphery even in areas with poor footing or where heavy machinery is operating, achieving 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, and its flexibility to “measure when you want” is a major advantage.

As described above, differential earthwork volume measurement using LRTK yields dramatic efficiency improvements compared with conventional methods. Time required for measurement is shortened, personnel costs are reduced, real-time processing enables faster decision-making, and safety is improved—resulting in a substantial overall boost in operational efficiency. This contributes to shorter construction schedules and reduced overtime, enhancing overall site productivity.

Summary: Differential earthwork volume measurement made simple and high-precision with LRTK

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

In short, differential earthwork volume measurement has been dramatically simplified and advanced by the advent of LRTK. Complex surveys can now be completed with a smartphone, and the data are immediately shareable and usable. As the comparison with conventional methods shows, LRTK 3D point cloud surveying is a next-generation solution that supports sites with overwhelming efficiency and sufficient accuracy. If you are struggling with workload in earth quantity management or as-built measurement, consider trying LRTK-based easy surveying. Adopting cutting-edge technology can improve on-site productivity and safety and allow your project to proceed with unprecedented speed.

Frequently Asked Questions

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

Q: What conventional methods have been used to measure differential earthwork volumes? A: Conventional methods mainly involve surveying instruments. Specifically, these include total station three-dimensional surveying to obtain terrain cross-sections and compute volumes, periodic height measurements with a level and staff to derive cross-sections for volume calculation, and more recently, creating 3D models from drone aerial photos. Each method provides a certain level of accuracy but has issues such as manpower and time requirements and difficulty in obtaining high-density coverage over wide areas.

Q: What is the difference between drone surveying and LRTK point cloud surveying? A: Drone surveying photographs from the air to build terrain models and can cover large areas at once. However, drones cannot be used in no-fly zones or in strong winds, and operators need qualifications and permit applications. Processing photo data also takes time. LRTK point cloud surveying is performed by a person walking and measuring from the ground and can be done regardless of location, making it suitable for urban areas or indoor spaces where drone use is restricted. LRTK acquires point clouds in real time and allows on-site confirmation of results, so it excels in responsiveness. In summary, LRTK can handle environments where drones cannot be used and has less 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 to realize positioning accuracy on the order of several centimeters even with a smartphone. This ensures high reliability of the acquired point cloud data. Coordinates measured with an LRTK-equipped iPhone have been found to closely match values measured with conventional high-precision GNSS equipment. Moreover, the high density of point clouds helps average out local errors, ensuring adequate accuracy for volume calculations.

Q: What is required to use LRTK point cloud surveying, and is it difficult to use? A: You need an LiDAR-equipped iPhone/iPad, the small LRTK attachment device, and the LRTK app. The operation is intuitive and not difficult. Attach the LRTK device to your smartphone, launch the app, and walk around the area you want to measure. The app is available in Japanese, and starting/stopping positioning and scanning operations are done with simple buttons. Acquired data are automatically synced to the cloud, so you don’t need complex PC software for processing. The system is designed so that even those without surveying expertise can become proficient after a short learning period, making introduction straightforward.