Managing Earthworks with RTK: How to Measure Cut-and-Fill (Earthwork Volume) On Site

Table of Contents

• What is cut-and-fill volume?

• When and why you measure cut-and-fill volume

• Conventional methods for measuring earthwork and their challenges

• Advantages of using 3D point cloud data for earthwork measurement

• Easy cut-and-fill volume measurement with LRTK

• Benefits of using LRTK

• Summary

• FAQ

What is cut-and-fill volume?

Cut-and-fill volume refers to the *difference in soil volume* that occurs when reshaping or grading terrain during construction. Simply put, it is the amount of soil removed from the ground (cut) and the amount of soil added (fill). For example, in site leveling work, you quantify how much soil was cut from high areas (cut) and how much was added to low areas (fill). Accurately understanding the combined cut-and-fill volume allows you to quantitatively verify whether the post-construction terrain matches the design or how much it deviates from the plan.

When and why you measure cut-and-fill volume

In civil engineering and construction, measuring cut-and-fill volume is required in many situations. A representative example is as-built management. As-built management is the process of confirming whether the completed terrain and structures match the design drawings, and in public works there is an obligation to record and report the completed shape and quantities (volumes). By measuring cut-and-fill volume you can verify whether the construction result matches the design in terms of excess or deficiency, and if there is a deficiency you can decide to add fill, or if there is an excess you can correct by cutting.

Cut-and-fill volume data is also indispensable for progress management and settlement of moved soil in construction. For excavation and fill works, costs and schedules are managed according to the actual volumes excavated or placed. Volumes calculated from pre- and post-construction terrain models serve as the basis for progress reports and payment amounts in contracts. Contractors need objective earthwork measurement data to receive fair compensation for the work performed, and clients need it to confirm the work quantities are within the planned range.

Furthermore, cut-and-fill measurement is useful for progress tracking in long-term projects. In large-scale earthworks or tunneling, periodic measurement of terrain changes (excavation and fill volumes) helps determine whether work is progressing as planned. For example, calculating the latest excavation volume daily or weekly allows progress to be managed quantitatively. After disasters, measuring pre- and post-work terrain to compute differential volumes enables accurate understanding of removal quantities for debris clearing and sediment removal.

Conventional methods for measuring earthwork and their challenges

Many conventional on-site methods for measuring cut-and-fill volume relied on manual surveying and calculation. Typically, cross-section surveys are taken at set intervals before and after work, and volumes are calculated from the obtained cross-sections using the average-section method. The grid method, where the site is divided into a mesh and heights are measured at grid points to compute volume, is also used. In any case, surveying staff had to observe and record many point heights one by one using instruments such as levels (auto level) or total stations.

However, such manual-focused earthwork measurement has various challenges. The main problems include:

• Time-consuming and labor-intensive: Measuring many points requires multiple crews and long hours, posing a heavy burden on site personnel. It is also not easy to secure staff with surveying expertise, and under labor shortages it can be difficult to carry out surveys as scheduled.

• Insufficient coverage leading to oversight: The number of points that can be obtained manually is limited, making it difficult to fully cover a wide site. Data from limited cross-sections or grid points can miss subtle undulations or irregularities between them. As a result, errors may only be discovered at the as-built inspection stage when someone points out "this differs from the design," potentially requiring additional rework.

• Safety issues: Surveying steep slopes or deep excavation areas may require workers to enter hazardous locations to take measurements. Having staff stand or move on unstable soil carries risks of falls or landslides and is undesirable for safety.

• Burden of calculations and recordkeeping: Plotting field measurements on drawings to calculate sectional volumes or creating report charts and photo logs is time-consuming. Site supervisors must handle large amounts of documentation alongside construction management, and human errors such as calculation mistakes or omitted records cannot be ruled out.

Thus, conventional earthwork measurement, which can only measure discrete points and requires substantial human effort and time, has limitations in both accuracy and efficiency. In recent years, there has been demand for new measurement methods that can capture earthwork volumes more efficiently and accurately.

Advantages of using 3D point cloud data for earthwork measurement

Recently, digital technology has advanced dramatically in surveying and construction management, and methods that utilize 3D point cloud data obtained by laser scanners or photogrammetry for earthwork calculations have attracted attention. A point cloud is a collection of many points that make up terrain or structures, with accurate coordinates (X, Y, Z) assigned to each point. Unlike conventional cross-section surveys, point clouds capture the whole site in a surface-based 3D model, enabling a dramatic improvement in both the accuracy and efficiency of earthwork volume calculations.

The biggest advantage of 3D point cloud surveying is that it can measure wide areas as surfaces in a short time. For example, drone photogrammetry can capture aerial images of a large site in tens of minutes and create a detailed surface model. Ground-based laser scanners can obtain extremely dense point clouds of millions of points in a short time. Because the obtained point clouds reflect fine undulations of the ground surface, using them for earthwork calculations allows high-precision volume computation that accounts for minute variations on the order of several centimeters (a few inches). Small excesses or deficiencies that manual surveys might have missed can be visualized on 3D data, greatly reducing the risk of overlooking as-built deviations.

Additionally, point cloud measurement is non-contact and safe. Operators do not need to enter hazardous areas and can scan the entire site safely from a distance. With drones or long-range laser scanners, data can be collected remotely even for steep slopes or terrain at risk of collapse where people cannot safely enter. This reduces the risk to workers while enabling efficient surveying.

Digital point cloud data is also easy to analyze and share. Dedicated software can automatically calculate fill and cut volumes from obtained point clouds at the push of a button, eliminating concerns about manual calculation errors. Results can be visualized as numeric data or color maps, making it easy to see at a glance where and how much soil needs to be added or removed. Data can be stored and shared electronically, enabling instant information sharing among stakeholders and facilitating faster decision-making.

Despite these advantages, point cloud measurement traditionally required expensive equipment and advanced expertise, posing a high barrier for small- and medium-sized sites. This led to the emergence of solutions that allow easy point cloud acquisition by combining a smartphone with RTK-GNSS. The next section introduces one such example: simple surveying with LRTK.

Easy cut-and-fill volume measurement with LRTK

LRTK is a modern surveying system consisting of a small high-precision GNSS receiver attachable to a smartphone, a dedicated app, and a cloud service. By fusing a smartphone’s built-in LiDAR scanner (an optical distance sensor) and camera with centimeter-level RTK positioning, LRTK is designed so that anyone—not only specialized surveyors—can easily measure 3D point clouds and compute earthwork volumes. A major feature is that surveying work, which previously had to be entrusted to experienced technicians, can now be performed by on-site personnel with just a smartphone.

Here is the typical procedure for measuring cut-and-fill volume using LRTK. The basic flow is as follows:

• Preparing surveying equipment: Attach the dedicated LRTK receiver (GNSS antenna) to your iPhone or iPad and connect it via Bluetooth or cable. Outdoors, turn on the device and wait several tens of seconds for satellite signals and RTK correction information to be acquired, enabling high-precision positioning. You will be able to measure positions with approximately ±2-3 cm (±0.8-1.2 in) accuracy in both horizontal and vertical directions.

• Preparing reference data: Prepare the reference surface data to compare against. If you have a 3D model of the completed design from the design drawings, load it into the LRTK app and set it as the design surface model (reference surface). If no design model exists, you can scan and save the pre-construction original ground once and use that as reference data, then re-scan the same area later to compare differences.

• Measuring the current 3D point cloud: Walk around the area you want to measure while holding the smartphone and scan the site to acquire point cloud data. Switch the LRTK app to measurement mode and activate the phone’s LiDAR; as you look around the site, a point cloud model is generated in real time. By attaching RTK-GNSS position information, each acquired point in the point cloud is assigned an accurate geographic coordinate (latitude, longitude, elevation). Move to cover the entire measurement area and, if necessary, scan from different angles to reduce blind spots. In just a few minutes you can obtain high-density point clouds of tens of thousands to hundreds of thousands of points, and a 3D model of the current terrain is reproduced on the smartphone.

• Automatic calculation of differential volumes: After scanning, calculate the differential volume between the acquired current point cloud data and the reference data in the app. The LRTK app can automatically compute fill and cut volumes at the push of a button. If you loaded the design model as the reference, simply tap the “difference calculation” button and the app will analyze the differences between the current point cloud and the design surface, instantly displaying the volumes. The excavated volume (= cut) and the volume that needs to be filled (= fill) are calculated separately, so you can immediately see how much soil should be hauled out or brought in.

• Reviewing and using results: The differential volumes can be reviewed numerically on the smartphone screen and visually with color maps. For example, display areas higher than the reference (excess fill) in red and areas lower (insufficient cut) in blue to intuitively understand where and how much soil needs to be added or removed. Using the smartphone camera overlay in AR mode, you can superimpose the design surface model onto the real terrain to instantly judge “how many more cm to cut to reach the design elevation.” Measurement results are directly saved to the cloud, making it easy to share with stakeholders later or reuse for as-built reports.

As described above, LRTK allows the entire process from surveying to volume calculation and result review to be completed on site. There is no need for special expensive equipment or complex CAD software, and intuitive tablet- or smartphone-based operation makes it accessible even to non-specialist users.

Benefits of using LRTK

Finally, here are the main benefits of using LRTK on site.

• Easy operation that anyone can use: Operation follows the smartphone screen, so technicians without surveying expertise can start using it immediately. Complex instrument setup or volume calculation knowledge is unnecessary, and intuitive operation delivers accurate results, enabling everyone on site to participate in earthwork management.

• Reduced personnel and costs: Surveying and as-built verification that once required multiple people can be completed by one person with LRTK. There is less need to outsource to surveying companies or have heavy machinery standing by, reducing labor costs and opportunity loss. Initial investment can be lower than purchasing specialized expensive equipment, and it can be introduced easily when needed.

• Improved safety: Non-contact smartphone measurement eliminates the need to enter hazardous areas, improving on-site safety. For example, as-built measurement on steep slopes can be performed by scanning from a distance without workers climbing the slope, reducing the risk of falls or collapse.

• High-precision unified coordinate data: Because RTK-GNSS records all point cloud data in a common coordinate system, volumes can be accurately compared even for distant locations. The positioning accuracy meets the precision standards required for as-built management, enabling reliable earthwork data.

• Ease of data use and reporting: Surveying results are accumulated as digital data in the cloud. It is easy to calculate differences against past measurements or to use the data directly for electronic deliverables. The effort to assemble drawings and photos for reports is reduced, contributing to efficient preparation of as-built documentation.

Summary

Advances in earthwork measurement using RTK have dramatically improved the efficiency and sophistication of on-site earthwork management (cut-and-fill management). Introducing new smartphone × RTK technologies like LRTK allows volume calculations that once took days to be completed the same day, enabling immediate feedback into construction management. This directly contributes to shorter schedules and improved quality, benefiting both clients and contractors. Measuring cut-and-fill volumes with a smartphone is becoming the new standard.

If you currently have issues with the efficiency or accuracy of earthwork management or as-built measurement, consider trying simple surveying with LRTK. Incorporating the latest digital technology on site can yield substantial labor savings and accuracy improvements that overturn conventional practices.

FAQ

Q: Why do we measure cut-and-fill volume? A: The purpose of measuring cut-and-fill volume is to accurately grasp the change in terrain caused by construction (excavation volume and fill volume). This allows verification that construction is carried out according to the design and proper cost settlement based on work quantities. Cut-and-fill volume data serve as solid evidence for as-built management and are important for quality assurance and contractual documentation.

Q: What data are needed to calculate cut-and-fill volume? A: To calculate cut-and-fill volume, you need two terrain datasets (models) to compare. Generally, a “reference surface” and a “current surface” model are prepared and their volume difference is calculated. The reference surface can be a completed design model from drawings or pre-construction original ground data. The current surface refers to measured data after (or during) construction. For example, in excavation work you compare pre-excavation and post-excavation terrain models to determine how much soil was actually removed.

Q: What preparations and equipment are needed to use LRTK? A: To use LRTK you need an iPhone or iPad equipped with a LiDAR scanner (e.g., iPhone 12 Pro or later), the LRTK high-precision GNSS receiver unit, and the dedicated app. Install the app on your smartphone, attach and connect the LRTK unit, and then you can walk the site with the phone to perform point cloud measurement. Because GNSS correction information is obtained via the internet, a cellular connection (4G/5G) at the site is required.

Q: How reliable is the measurement accuracy? A: LRTK uses RTK-GNSS to achieve approximately ±2-3 cm (±0.8-1.2 in) accuracy in horizontal position and a few centimeters (a few inches) in height. This is far more accurate than standalone GPS and meets the precision standards required for as-built management. However, accuracy is affected by satellite reception and the surrounding environment, so it is important to use it in open areas and allow sufficient stabilization time. Point clouds obtained by the smartphone’s built-in LiDAR can capture shapes at close range with accuracy within a few centimeters (a few inches). With proper measurement practice, you can achieve practical accuracy suitable for earthwork volume calculations.

Q: Which is better: drone photogrammetry or smartphone + LRTK point cloud surveying? A: Drone photogrammetry and smartphone + LRTK point cloud surveying have different strengths. Drones can quickly capture aerial images over large areas and are suitable for measuring inaccessible forests or large-scale earthworks. LRTK is easy to use on site and provides results immediately without waiting for image processing. It can also be used in urban areas with strict flight regulations or in indoor spaces, and is less affected by weather. Additionally, LRTK’s GNSS function can be used for reference point surveying for drone photogrammetry—combining reference points measured with LRTK and drone photogrammetry models allows high-precision correction. Using drones and LRTK according to site scale and conditions can achieve more efficient and accurate earthwork management.