RTK for Earthwork Management: On-site Cut-and-Fill Measurement

Table of Contents

• Introduction: The importance and challenges of surveying in earthwork management

• What is RTK (Real-Time Kinematic)?

• Benefits of using RTK for earthwork management

• Methods for measuring cut and fill volumes using RTK

• RTK use cases (road construction, residential land development, river works)

• Conclusion: Promoting simple surveying with LRTK

• FAQ

Introduction: The importance and challenges of surveying in earthwork management

In civil works such as road construction, residential land development, and river works, cutting (excavation) and filling (embankment) operations are performed routinely. In earthwork management, surveying to verify that the site ground achieves the designed elevations and forms is extremely important. If cut volumes are insufficient and the ground remains too high, or if fill is insufficient and the ground is too low, not only will the cross section differ from the design, but structural problems may also arise. Therefore, on-site cut-and-fill (earthwork) measurements are used to determine how much soil should be removed or placed to reach the prescribed elevation, and to manage schedule and quality.

However, conventional surveying methods have challenges. For example, when using a level (auto level) or total station, leveling and line-of-sight checks are required for each survey point, and work is typically performed by a two-person team. Because elevation can only be checked at limited points, comprehensively grasping the ground over a wide area requires considerable effort and time. When calculating earth volumes from survey results, data often need to be taken back to the office and computed with CAD software, making it difficult to make immediate on-site decisions. With manpower shortages and pressure to shorten construction periods, more efficient and higher-precision surveying methods have been sought. In fact, the Ministry of Land, Infrastructure, Transport and Tourism has supported the on-site use of RTK-GNSS by formulating management guidelines for as-built verification in conjunction with the promotion of ICT construction.

What is RTK (Real-Time Kinematic)?

RTK surveying has attracted attention in recent years. RTK (Real-Time Kinematic) refers to a technology that corrects GNSS positioning errors in real time and can measure positions with centimeter-level accuracy. Normally, positions obtained from a general GPS receiver can be off by several meters due to atmospheric and satellite orbit errors. RTK places a GNSS receiver at a reference station (base) with known coordinates and computes corrections by comparing it with a second mobile receiver (rover). Correction data are transmitted from the base to the rover via radio or the internet (cellular network), and the rover applies real-time corrections to achieve centimeter-level positioning that ordinary GPS cannot attain.

RTK positioning accuracy is generally about 2–3 cm (0.8–1.2 in) horizontally and about 3–5 cm (1.2–2.0 in) vertically. This meets the precision required in surveying tasks for roads and development sites and is practically a high degree of accuracy. However, high-precision positioning requires a clear environment where satellite signals can be stably received. Immediately after starting positioning, the solution may be in a somewhat larger-error state called a Float solution, but if the receiver remains still for several tens of seconds and acquires satellites, it converges to a high-precision Fix solution (integer-fixed solution). Once Fix is obtained, coordinates can be acquired at each point with centimeter-level accuracy even while moving. Conversely, if the sky view is blocked, the number of visible satellites drops or signals are disturbed, the solution can revert to Float and accuracy degrades. Therefore, the rule in RTK surveying is to “record after achieving Fix.”

Benefits of using RTK for earthwork management

Incorporating RTK’s high-precision positioning into earthwork management dramatically improves on-site work efficiency and management accuracy. First, there are benefits in streamlining and reducing survey labor. A single person can carry a GNSS rover and walk the site to instantly measure the elevation and coordinates at each location, eliminating the need for multiple people to set up equipment and take detailed readings. Because there is no limit on the number of measurement points, it is possible to measure broadly and in a planar fashion to understand ground irregularities and slopes. This enables detection of localized elevation differences that may have previously been overlooked, improving quality control accuracy.

Next, RTK excels in immediacy and data sharing. Using RTK-GNSS equipment and compatible apps, measured elevations can be displayed numerically on the spot, and differences from the design surface can be color-coded. For example, if design data are preloaded into the device, you can get real-time cut-and-fill guidance at each measured point, such as “+◯ cm (△△ in) of fill required relative to the design elevation.” Because results are available immediately after surveying, operators can instruct heavy equipment operators on site or decide the same day whether additional fill is required. Tasks such as as-built verification and progress quantity calculation, which used to wait for the survey team’s data processing and be done the next day or later, can be completed on-site the same day with RTK surveying and cloud analysis, speeding up and improving efficiency across the entire project.

Improved safety is another benefit. RTK enables one-person surveying, reducing exposure time to hazards in high-traffic road sites or difficult footing areas by allowing quick measurements with minimal personnel. When measuring large areas, RTK is advantageous over drone surveying because it does not require permission for overhead photography and can be performed safely from the ground (it is also less affected by strong winds or rain). Thus, RTK contributes to balancing productivity and safety.

Methods for measuring cut and fill volumes using RTK

How do you actually measure cut and fill volumes on-site using RTK? The basic workflow is as follows.

• Survey equipment preparation: RTK surveying requires a high-precision GNSS receiver (rover) and a base station that provides correction information. For the base station, you can set up your own reference receiver at a known point near the site or use a network RTK service (reference station data distribution) provided by private companies or the Geospatial Information Authority of Japan. Electronic reference stations are set up across Japan, and by receiving correction data for a virtual reference station (VRS) via services such as Ntrip, RTK positioning can be achieved without placing a physical base station. Choose the appropriate method based on the site’s communication environment and area. On the rover side, mount the GNSS antenna on a pole or monopod and place the antenna tip at the point you want to measure. Before positioning, set the coordinate system to be used (plane rectangular coordinate system or geodetic system) and, if necessary, perform on-site calibration (localization) at known points.

• Measuring the existing ground: Once equipment is ready and RTK has acquired a Fix solution, measure the site ground elevations. In cut-and-fill measurement, volume differences are typically obtained by comparing ground shapes before and after construction. For example, for road excavation, measure the pre-excavation ground elevations with RTK across the area and save them as point cloud data, then measure the same area after excavation. The same applies to fill: measure the ground before and after filling. The spacing between measurement points depends on site size and required accuracy, but if the area can be walked, taking points every few meters (several meters (several feet)) at fine intervals is advisable. Recent RTK-compatible apps plot the path you walked and acquired points in real time on the smartphone screen, helping to ensure no points are missed.

• Volume calculation and confirmation: Acquired ground survey data can be uploaded to a tablet or the cloud for analysis on the spot. By automatically calculating the volume difference from the two surveys, excavation and fill volumes can be determined. Traditionally, survey data had to be taken back to the office for earthwork calculation, which could take days for results. With RTK surveying combined with cloud analysis, quantities can be understood immediately after measurement. Results can be displayed not only numerically but also as color-coded maps in a 3D viewer. Areas with excessive fill or insufficient fill relative to the design surface become obvious at a glance, allowing site managers to take immediate corrective action. Performing such checks regularly in large-scale earthmoving projects prevents rework and excessive excavation or filling, reducing costs.

RTK use cases (road construction, residential land development, river works)

Cut-and-fill measurement using RTK is increasingly used in various civil engineering sites. Here are representative cases.

Road construction sites: In new road construction, cuts in hilly areas and fills in valleys form the longitudinal gradient. RTK allows high-precision management from checking the formation elevation (roadbed height) to shaping slopes. For example, in cut sections, you can frequently measure remaining soil relative to design height and instantly calculate the amount to be hauled out by dump trucks. In fill sections, you can check planarly whether each layer is placed at the specified compaction thickness, contributing to quality uniformity. Road cross-section shapes that were previously measured only at several extracted sections can now be obtained as continuous terrain data with RTK, enabling detailed verification of curves and gradients against the design.

Residential land development sites: In residential land development, the entire site is cut and filled to create a flat building platform, and the larger the site, the more difficult height leveling becomes. Introducing RTK surveying enables a single person to efficiently verify as-built conditions even on extensive development areas. For example, an operator can check the finished height immediately after grading a section with heavy equipment by carrying a pole-type RTK receiver. Results are shared to the cloud on the spot, so offsite construction managers can instantly verify elevations. This prevents overlooking issues such as “fill is thinner than planned in a certain location” and reduces later rework. RTK survey data are also used to estimate restoration fill volumes in cases where rain causes soil erosion.

River work sites: RTK is powerful in river and erosion-control works for tasks such as embankment filling and riverbed excavation (dredging). Even in wide riverbeds where access is difficult, RTK GNSS surveying makes it possible to measure from a distance regardless of line-of-sight (with techniques such as extending the survey rod if needed). For embankment fills, regularly measure the shoulder and crest elevations with RTK to confirm as-built conditions and monitor long-term settlement. In riverbed excavation, measure terrain before and after dredging with RTK to calculate dredged volumes and inform appropriate disposal planning. Surveying near water can be hazardous, but by mounting RTK equipment on a pole you can measure remotely and obtain cross-section shapes across the river width from a safe position. RTK surveying is applicable to any outdoor environment with changing terrain and contributes to more efficient quantity management and quality inspection.

Conclusion: Promoting simple surveying with LRTK

RTK’s utility is transforming earthwork management sites. However, conventional high-precision GNSS surveying equipment has been expensive and required operator training, which hindered widespread adoption. Enter LRTK, a new RTK solution that uses smartphones. LRTK is a smartphone RTK system that attaches a pocket-sized receiver to an iPhone or iPad and, combined with a dedicated app, enables anyone to easily achieve centimeter-level positioning. By simply attaching a receiver weighing approximately 125 g to a smartphone, the era in which each person can carry their own high-precision surveying instrument has become reality.

LRTK supports network RTK corrections from base stations and also supports the Quasi-Zenith Satellite System (QZSS) “Michibiki” centimeter-class augmentation service (CLAS). This makes high-precision positioning possible even at sites in mountainous or out-of-cell areas as long as the sky above is open, allowing surveying in environments that were previously impractical. In addition, by integrating with the smartphone’s camera and sensors, it can perform photogrammetry and 3D scanning functions, handling not just single-point positioning but also as-built management using point cloud data and AR-based surveying assistance in a single device. Measured data are uploaded to the cloud instantly and shared with stakeholders in real time. These advantages make LRTK an attractive digital-transformation tool for sites in line with the Ministry of Land, Infrastructure, Transport and Tourism’s “i-Construction” initiative.

LRTK greatly contributes to productivity improvement and labor reduction in all surveying tasks, including cut-and-fill measurement for earthworks. If you want easier and more accurate on-site surveying, consider simple surveying with smartphone-based LRTK. Embrace the latest technology and evolve your site management methods to the next level.

FAQ

Q1. Are there recommended environmental conditions for RTK positioning? A1. Yes. For stable RTK-GNSS surveying, an open outdoor environment with a clear sky overhead is desirable. In locations with many buildings or trees blocking the sky, the number of visible satellites decreases and multipath interference from signal reflections increases, making it harder to obtain a Fix solution. To achieve high-precision results, measure in locations with as much surrounding visibility as possible and avoid nearby obstacles when erecting the antenna. Conversely, even in mountainous areas outside of cellular coverage, it is possible to perform RTK positioning by directly receiving Michibiki’s CLAS signal if the sky is open (LRTK supports CLAS). RTK positioning is difficult in indoor environments with extremely poor sky visibility, such as tunnels, but in most outdoor settings partial obstruction can be mitigated by receiving multiple satellite systems (GPS, GLONASS, Galileo, QZSS, etc.).

Q2. Is there a difference in positioning accuracy between smartphone RTK (LRTK) and conventional RTK equipment? A2. If a Fix solution can be obtained under favorable reception conditions, smartphone RTK can achieve accuracy comparable to conventional dedicated GNSS equipment. In actual comparisons of LRTK and expensive surveying instruments at the same point, measurement results have sometimes differed by only a few millimeters. Both satisfy typical RTK nominal accuracies such as horizontal ±2–3 cm (±0.8–1.2 in) and vertical ±4–5 cm (±1.6–2.0 in). However, smartphone RTK must also maintain an appropriate Fix state. If signal conditions deteriorate and the solution temporarily reverts to Float, errors can increase similarly to conventional equipment (to the order of tens of centimeters). Therefore, when measuring precise points, it is important to verify that the current solution is Fix before recording. Under proper conditions, smartphone RTK delivers comparable accuracy and is fully applicable to everyday civil surveying tasks.

Q3. If the site has no internet or radio communication, is RTK surveying impossible? A3. You do not have to abandon surveying entirely if there is no communication. One option is to set up a local base station as a known point on-site and communicate with the rover via radio to perform RTK. In Japan, it is also possible to perform high-precision positioning by directly receiving CLAS augmentation signals from Michibiki (a compatible receiver is required, and LRTK supports this by default). Thus, even without network connectivity, real-time positioning can be achieved using satellite augmentation information. However, if you are deep in the mountains where satellites themselves are scarcely visible, GNSS may be difficult regardless of RTK, and in such cases consider terrestrial surveying methods (total station, etc.).

Q4. What do “Fix” and “Float” commonly heard in RTK surveying mean? A4. In RTK, Fix refers to a resolved integer-fixed solution in which unknown integer biases that cause positioning errors have been solved, yielding centimeter-level accuracy. Float refers to a provisional solution that is not yet stable and may include errors on the order of tens of centimeters. Positioning typically starts in a Float state and switches to Fix once enough satellites are captured and the computations converge. In the field, the rule is to “record points after Fix is obtained,” and RTK devices and apps indicate whether the current solution is Fix or Float. If the solution remains Float for a long time, try moving to a location with better sky view, stabilize the antenna, or extend measurement time for averaging to obtain a Fix. Once a Fix solution is achieved, accuracy tends to be maintained even while moving, so securing a Fix at the start is key to high-precision positioning.