Mobile RTK for Earthworks: How to Measure Cut and Fill on Site

Table of Contents

• What is RTK surveying? What mobile RTK can do

• Importance of cut and fill surveying at earthworks sites

• Traditional cut-and-fill surveying methods and challenges

• Specific methods for measuring cut and fill with mobile RTK

• Benefits of introducing mobile RTK

• Precautions and tips for mobile RTK surveying

• The future where mobile RTK changes site work

• Achieving high-precision cut-and-fill measurement with simple surveying using LRTK

• FAQ

At civil engineering sites, cutting the ground (“cut”) and building it up (“fill”) are routine operations. Accurately measuring the amount of soil moved in these operations (cut and fill volumes) is critically important for construction planning and cost control. Traditionally, surveyors painstakingly measured with batter boards and levels, but recently a technology called mobile RTK has made it easy to measure cuts and fills directly on site. This article explains RTK surveying basics, how to measure cut and fill on site using mobile RTK, and the benefits and caveats. Let’s take a look at how earthwork sites are changing with the use of these latest technologies.

What is RTK surveying? What mobile RTK can do

RTK (real-time kinematic) surveying is a surveying method that applies real-time error correction to satellite positioning data (such as GPS) to achieve centimeter-level (inch-level) accuracy. Standalone GPS typically yields errors of several meters (several ft), but RTK cancels out those errors by using relative positioning to a reference receiver, enabling high precision. This makes it possible to obtain position coordinates with the accuracy required for topographic surveys and civil works (on the order of a few centimeters (a few in) both horizontally and vertically). In recent years, network RTK, which distributes correction data via cellular networks, has become more common, enabling RTK surveying on site without having to install a physical base station.

With the advent of network RTK, a worker carrying a surveying device can move around site alone while obtaining high-precision positioning in real time. In particular, compact GNSS receivers that can pair with smartphones or tablets have been developed, promoting the use of convenient on-site RTK surveying known as mobile RTK. Positioning data can be collected on survey apps for smartphones and differences from design values can be displayed on the screen, greatly reducing the need to compare paper drawings and instruments as before. Using mobile RTK makes on-site surveying faster and simpler.

Importance of cut and fill surveying at earthworks sites

In earthworks (site formation, foundation excavation, etc.), cutting and filling are combined to bring the ground to the specified elevations. Accurately knowing how much soil has been cut or filled (cut and fill volumes) is indispensable for efficient and proper project management. Quantifying soil volumes is directly linked to construction planning and cost estimation. If the cut-and-fill estimates are wrong, you may incur extra disposal costs from excessive excavation or face delays in material procurement due to insufficient fill. Conversely, accurate soil quantity management enables balanced construction and smooth schedule control.

On site, the crew needs to continually check the difference between the design elevations and the actual ground surface. If you don’t monitor whether you’ve excavated too much or whether the fill has reached the designed level, rework due to out-of-specification results can occur. Frequent measurement of cut and fill conditions helps prevent such rework and leads to more efficient execution. Although frequent on-site measurements are required, conventional methods are time-consuming and labor-intensive, so in practice they are not always performed as often as needed. Mobile RTK promises to solve that problem by enabling swift on-site surveying.

Traditional cut-and-fill surveying methods and challenges

Traditionally, measuring cut and fill volumes at earthwork sites required considerable labor and time. A representative method is elevation measurement using an auto level or total station, followed by calculating volumes using the average cross-section method. The average cross-section method measures cross-sectional shapes at regular intervals on site, computes cut and fill areas for each section, and multiplies the average area by the distance between sections to get volumes. While the calculations are simple, this method cannot reflect fine surface irregularities between measurement points, which can lead to estimation errors. On large sites the number of sections increases, and surveying alone can take days to weeks.

As-built measurements after heavy-equipment work were also a heavy burden when performed manually. Surveying staff had to walk the site thoroughly, set rods, and check elevations at many points, which was challenging from both labor and safety standpoints. Moreover, processing the measurements into drawings or CAD to calculate volumes took time, making it difficult to know soil quantities immediately on site. In short, confirming cut and fill accurately with conventional methods required a huge amount of effort and time, making frequent measurement impractical.

Specific methods for measuring cut and fill with mobile RTK

Below is an example procedure for measuring cut and fill on site using mobile RTK. Compared with conventional methods, the setup is simpler and measurement is faster.

• Prepare equipment and reference: Prepare an RTK-capable GNSS receiver and a smartphone, and mount them on a pole if necessary. When using network RTK, connect the receiver to the Internet via a SIM card or tethering to receive correction data from the reference station. Before starting, verify the receiver’s position at a known point on site and confirm coordinate system settings (for example, setting the plane rectangular coordinate system zone number) and that antenna height is entered correctly.

• Set up design data: From site design drawings or formation plans, identify the target ground elevations (design elevations) and the finished surface model. If possible, import that design data into the surveying app. For example, loading the planned ground surface model or breaklines into the app allows automatic calculation of differences between measurements and design elevations, which is convenient. Set the reference surface (design surface) so it can be referenced on the device in advance.

• On-site measurement: Once the GNSS receiver (rover) has a FIX solution, measure points across the site. Place the pole tip at the point to be measured and press the measurement button in the survey app on the smartphone to record the 3D coordinates (latitude, longitude, elevation) of that point. Measuring points in a grid pattern to some extent is effective for understanding elevation differences from the design. In areas with large undulations, reduce the spacing between points, and concentrate points around boundaries between cut and fill. If necessary, take multiple measurements at key locations to increase data reliability.

• Check differences and calculate volumes: Compare measured elevations with the design surface elevations to check cut and fill amounts (elevation differences) on site. Many RTK-enabled apps can automatically display the “difference from design elevation” for each survey point, making it easy to see at a glance which locations are how many centimeters higher (overfilled) or lower (over-excavated) than the design. In addition to point-by-point differences, many collected points can be used to automatically calculate fill and cut volumes. Generating a mesh model in dedicated software or within the app enables immediate on-site calculation of total soil volumes after measurement.

• Real-time feedback: Immediately feed the cut-and-fill data back into construction. For example, “Excavate 20 cm (7.9 in) more to reach the design elevation” or “This area has a deficit of 5 cubic meters of fill” can be communicated to machine operators or site supervisors so they can decide on additional excavation or fill procurement. Being able to grasp differences between current conditions and the design in real time prevents rework and excessive excavation, enabling efficient construction.

Benefits of introducing mobile RTK

Introducing mobile RTK surveying on site provides many benefits, including:

• High positioning accuracy: RTK reduces site surveying errors that used to be on the order of several tens of centimeters (several tens of inches) to a few centimeters (a few in). This makes it possible to reliably identify differences from design values and improves quality control accuracy.

• Improved work efficiency: Surveying work that previously required several people and a long time can be completed by one person in a short time. Since data processing can be completed immediately, there is no more waiting for “survey → bring data back to the office and compute.”

• Reduced labor costs and effort: Streamlining surveying reduces the burden on crews and allows sites with labor shortages to cope. It cuts down on cumbersome tasks like setting batter boards and creating cross-section drawings, reducing worker burden and labor costs.

• Real-time on-site decisions: Because cut-and-fill conditions can be understood on site, quick decision-making is possible. Adjustments to machine operation or immediate ordering of additional soil can be made based on soil surpluses or shortages, enabling waste-free construction management.

• Improved quality and safety: Frequent measurements help keep the finished surface within acceptable limits and prevent rework due to out-of-specification results. Also, reducing the number of times people must enter hazardous areas for surveying improves safety management.

Precautions and tips for mobile RTK surveying

Mobile RTK is convenient, but there are points to keep in mind to achieve high accuracy. Below are the main precautions and tips.

• Choose measurement environments with good visibility: GNSS positioning works best in open-sky environments. In locations with many tall buildings or trees, satellite signals may be blocked or reflected (multipath), increasing errors, so measure in places with as clear a view as possible.

• Ensure a stable communication environment: When using network RTK, stable Internet communication on site is required. In mountainous or out-of-service areas, correction data may not be receivable and required accuracy may not be achievable. Also, continuously verify that the GNSS receiver is connected to the base network and that the RTK solution remains in a Fix state.

• Verify coordinate systems and reference points: Care is needed to match measurements to the coordinate system used in design drawings (for example, public coordinate systems or site-local coordinates). Compare RTK results with known reference points in advance and, if necessary, perform localization adjustments for each site to correct for shifts relative to the design coordinates.

• Handle equipment carefully: Mistakes in entering antenna height or a tilted pole can cause measurement errors. Enter antenna height correctly and keep the pole vertical using the bubble level during measurement. Also monitor receiver and smartphone battery levels, and prepare spare power sources for long surveying sessions.

The future where mobile RTK changes site work

Surveying technologies that use mobile RTK and smartphones are expected to further transform site work. Centimeter-level surveying that once required expensive dedicated equipment and specialists is becoming possible with affordable devices and intuitive operation. A “one person, one device surveying” era is emerging where each site worker can capture and immediately share survey data, aligning with the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* (ICT construction) initiative. As DX (digital transformation) progresses across the construction industry, new construction management methods that enable real-time control of as-built quantities and quality are beginning to spread.

In the future, integration of high-precision RTK positioning with smartphone LiDAR and AR (augmented reality) technologies will likely advance. For example, overlaying the design model on the current ground through a tablet to visually check discrepancies on site may become possible. As-built quantity checks that were previously done after construction could be digitally recorded and shared immediately after construction, allowing next-day decisions to be made on the same day. Mobile RTK will play an increasingly important role as the foundational technology supporting such next-generation site operations.

Achieving high-precision cut-and-fill measurement with simple surveying using LRTK

An example solution that maximizes the advantages of mobile RTK described above is LRTK. LRTK is a new device that turns a smartphone into a high-precision surveying instrument by attaching a dedicated small antenna to an iPhone or similar device. It supports network RTK and can correct smartphone GPS errors in real time to a few centimeters (a few in). LRTK vastly improves smartphone positioning accuracy that used to be on the order of meters (ft), giving high-precision coordinates to each acquired point.

Using LRTK, you can link high-precision position coordinates to 3D point cloud data obtained by the smartphone’s built-in LiDAR scanner or camera on the spot. For example, scanning terrain with an LRTK-compatible smartphone app will immediately calculate fill and cut volumes from the acquired point cloud and display the results on the phone screen. Processes that formerly required drone flights or laser scanners and PC-based point cloud analysis for volume calculation can now be completed immediately after on-site scanning with LRTK. Acquired data are automatically saved to the cloud, making it easy to review details on office PCs or share with the team. Even non-specialist personnel can routinely perform surveys and volume checks, dramatically improving construction management accuracy and speed. On-site decisions such as “decide today whether soil disposal is required” or “confirm fill volume and order additional material immediately” become possible within the same day.

LRTK is attracting attention as a tool that enables simple surveying allowing anyone to perform high-precision measurements easily. The emergence of a revolutionary solution that lets you survey with just a smartphone has greatly lowered the barriers to surveying at earthwork sites. Break from traditional assumptions and try RTK surveying and point cloud measurement with a smartphone—you may gain a new experience that improves site productivity and enables precise construction management.

FAQ

Q: What do I need to use mobile RTK surveying? A: You need an RTK-capable GNSS receiver and a smartphone or tablet to connect to it. When using network RTK, prepare a SIM card or mobile router for communication. In addition, registration to the Geospatial Information Authority of Japan’s electronic reference station network (CORS) or a commercial correction data service may be required. For first-time implementation, prepare three things: the complete equipment set, a communication environment, and a correction data service contract.

Q: What level of accuracy can RTK surveying achieve? A: In general RTK surveying, under favorable conditions horizontal positioning errors of about 2-3 cm (0.8-1.2 in) and vertical accuracy of about 3-5 cm (1.2-2.0 in) can be obtained. This is comparable to elevation differences measured by an auto level. However, accuracy can degrade depending on the surrounding environment and satellite reception conditions. Always perform error checks and manage important measurement points with margin.

Q: Which is better: drone photogrammetry or mobile RTK? A: Each has advantages and disadvantages depending on the application. Drone photogrammetry can survey large areas in a short time and allows measurements without putting people into hazardous locations. However, it requires flight permissions, is weather-dependent, and post-processing takes time. Mobile RTK is limited to areas people can reach, but its convenience of direct on-site measurement and instant results is attractive. Mobile RTK is suitable for small sites or routine as-built checks, while drones are effective for mapping vast areas; they should be used according to the task.

Q: Can you really survey with just a smartphone? A: Yes. Recent smartphones (for example, higher-end iPhones) have built-in LiDAR scanners and, with dedicated apps, can capture surrounding 3D point clouds. For small-scale fill volume measurements, a smartphone alone can sometimes provide sufficient accuracy. However, smartphone GPS accuracy is limited, so for strict positioning it is desirable to combine with RTK-capable external devices. Using an RTK device that attaches to a smartphone, like LRTK, enables centimeter-level surveying with a phone.

Q: Can someone with no surveying experience use it? A: Mobile RTK is designed for relatively simple operation. Many products feature intuitive app interfaces where you can follow on-screen prompts to perform surveys without needing to understand specialized terminology. However, understanding basic surveying principles and precautions will help you use the system more accurately and safely. For first-time users, attending a brief training or reading the user manual and doing some trial measurements to get a feel for the system before real use is recommended.

Q: What is LRTK? A: LRTK is the name of a high-precision GNSS surveying device that attaches to a smartphone. It supports network RTK and corrects smartphone position information in real time to centimeter-level accuracy. LRTK consists of a dedicated antenna module and an app, and is designed so anyone can easily perform high-precision positioning and 3D measurement. It is attracting attention as a new solution combining the functionality of traditional surveying instruments with the convenience of smartphones.