Checking Grade and Elevation with RTK: Efficiently Inspecting Slopes and Elevations

Table of Contents

• What is RTK? Why high-precision positioning is needed for grading and slope checks

• How introducing RTK changes grading work (labor saving and efficiency)

• How to efficiently check slope and elevation with RTK

• Points to watch when using RTK surveying

• Simple surveying with LRTK

• FAQ

What is RTK? Why high-precision positioning is needed for grading and slope checks

When grading land or checking slopes on construction sites, high positioning accuracy is required. Even an error of only a few centimeters can lead to poor drainage gradients or unevenness in structures, so it is important to accurately measure and manage finished heights and slopes. However, conventional GPS positioning can sometimes have errors on the order of several meters, making it inadequate for grading or slope-checking purposes. This is where RTK positioning comes in. RTK (Real Time Kinematic) uses relative positioning with two GNSS receivers—a base station and a rover—to cancel error factors and achieve centimeter-level high-precision positioning in real time. Under typical conditions, horizontal positions can be kept to about 2–3 cm (0.8–1.2 in) and vertical directions to about 3–5 cm (1.2–2.0 in), meeting the accuracy required for civil surveying and construction management. With the high-precision positioning provided by RTK, you can efficiently check on-site whether the graded surface height meets the specified level and whether slopes are as designed.

The advantages of RTK surveying are not limited to accuracy. Because observation data are obtained in real time, results can be confirmed on site immediately, and additional measurements or on-the-spot corrections can be made if necessary. This responsiveness was difficult to achieve with traditional surveying instruments (optical levels or total stations), and as smart construction and on-site DX (digital transformation) progress, RTK is attracting attention as a technology key to productivity improvement. GNSS-based positioning is recommended as part of ICT construction in the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* initiative, and high-precision surveying with RTK is now being adopted across many sites.

How introducing RTK changes grading work (labor saving and efficiency)

Introducing RTK significantly changes how on-site grading work and height management are performed. Traditionally, it was common to rely on benchmarks such as string lines (chouhari) or numerous stakes with height marks and to progress grading with multiple people using a level (optical leveling instrument). This required labor and manpower, and on large sites, moving between measurement points and resetting equipment took time. By using RTK, these grading surveying processes can be greatly streamlined.

The main benefits are as follows:

• Enables single-person operation: Survey tasks that previously required two or more people can be completed by one person with RTK. Without deploying many survey crew members separate from heavy equipment operators, a single person can handle on-site height checks and staking.

• Measure large areas in short time: RTK-GNSS surveying can obtain absolute coordinates directly through corrections from a base station, allowing positioning of distant points without sequential line-of-sight requirements. On large grading sites, simply walking with the rover antenna lets you measure ground heights at many locations, greatly reducing work time.

• Confirm results in real time: You can immediately check measured point elevations and positions numerically on site, enabling instant judgments such as “how many centimeters of fill are needed to reach the design height” or “is this out of the specified slope?” Additional measurements or real-time correction instructions to heavy equipment operators can be given as needed.

• No need to re-set instruments frequently: With a total station, you need to move and re-establish the instrument and check backsights whenever the line of sight changes, but with RTK, once the base station is set up (or a network RTK is used), you can continuously measure just by walking the rover around the site. The number of times equipment must be reset is reduced, simplifying complex setup procedures.

• Measurement without relying on expert skills: Traditional stringing, level reading, and TS operation required skilled techniques, but RTK surveying equipment displays position and height digitally, so once basic operation is learned, less experienced personnel can achieve a certain level of accuracy. Combined with user-friendly guidance software, measurement becomes even closer to “anyone can do it.”

As described above, introducing RTK can achieve labor saving and efficiency in surveying tasks associated with grading, while also providing high-precision data. Even when leveling a large development site, RTK enables you to check heights across the entire area in a short time, preventing overlooked unevenness and improving construction quality. In recent years, with a serious labor shortage in the construction industry, single-person surveying with RTK is a potential savior. Against the backdrop of DX and *i-Construction*, GNSS-based surveying methods like this are expected to spread further.

How to efficiently check slope and elevation with RTK

So how do you actually use RTK to check the graded surface’s slope and elevation? Here are the methods and key points.

● Measure many points to understand the terrain: In as-built surveys used to confirm whether finished shapes match the design, it is necessary to measure many points across the surface to verify heights and slopes. Using an RTK-GNSS receiver, a single person can carry the antenna around a large site and simply press a button to record coordinates (latitude, longitude, elevation) at each location. For example, where finish heights on a development site were traditionally checked at intervals of tens of meters with leveling, RTK makes it easy to increase measurement points into a fine grid. Plotting the many measured points obtained allows detailed understanding of subtle surface undulations. Slope can be calculated from elevation differences between multiple points, or visually confirmed with contour lines or color maps; digital data enables analyses unique to surveying. RTK surveying, which can measure wide areas in detail and in short time, is a tool that can significantly improve quality control accuracy compared to manual methods.

● Pass/fail determination by comparison with design values: Elevation data obtained with RTK can be compared with preprepared design elevation standards or model data to evaluate construction quality. Comparing the measured elevation at each point with the design elevation allows you to check whether fills or cuts are within the specified range and whether the slope angles of roadbeds or floors match the instructions. Many RTK-capable surveying apps and software automatically calculate and display differences from the design when measurement points are imported (for example, “+3 cm high” or “-2 cm low”). This streamlines inspection work and allows immediate identification of problem areas on site. For example, when checking a road’s longitudinal gradient, measure the start, end, and intermediate points with RTK and compare them with the planned longitudinal line on the spot. If irregularities are found in the gradient, you can measure surrounding areas to define the extent or immediately request correction from the construction crew.

● Specific methods for measuring elevation differences and slopes: A concrete example of slope checking with RTK is to survey the upper and lower ends of a slope with GNSS and calculate the average gradient from the elevation difference and horizontal distance. If you know the height difference between two points, gradient (%) = height difference / horizontal distance × 100. Of course, actual terrain is continuous, so it is important to measure not only one location but also changes along the way. With RTK, you can take multiple points at equal intervals along a slope to create a profile (cross section) and closely examine gradient changes and unevenness. Tasks that traditionally required setting up a level and repositioning staff to check elevation differences can be done by continuously acquiring points while walking with RTK, making it easier to obtain consistent slope data.

As described, as-built surveying and slope checks with RTK are speedy and comprehensive, enabling the collection of detailed data in a short time that would be difficult by manual methods. As a result, post-construction quality inspections can reduce human error and omissions, allowing more reliable verification.

Points to watch when using RTK surveying

RTK surveying is convenient and highly accurate, but there are points and limitations to be aware of when introducing and operating it. Below are items to consider when using RTK on site.

• Dependence on positioning environment: RTK accuracy is greatly affected by the reception status of satellite signals. High precision is obtained in open sky environments where many satellites are visible, but in narrow urban sites surrounded by tall buildings or in forests, signal blockage and multipath (reflections) tend to increase errors. In fact, while suburban open sites may keep errors to 2–3 cm (0.8–1.2 in), cases of temporary deviations of more than 10 cm have been reported in building-lined areas. Therefore, do not overtrust RTK in locations with poor sky visibility; depending on circumstances, it is necessary to flexibly use traditional total stations or levels for cross-checking.

• Vertical accuracy and height references: GNSS positioning generally has lower accuracy in the vertical direction than in the horizontal. Even with RTK, several centimeters of vertical error may remain, so be careful in situations requiring millimeter-level strict height control. For inspections that require strict absolute height values, such as verifying concrete pavement thickness, it is effective to compare RTK-measured elevations with height control points obtained by leveling in advance, or to recheck important heights with an optical level. The reliability of RTK height data increases if you compare them with known elevation points or confirm stable values by multiple measurements.

• Initialization and ensuring sufficient satellites: RTK achieves full accuracy only after obtaining a fixed solution (Fix) from phase measurements of satellites. Just after starting positioning or when satellite geometry is poor, it may take time to obtain this fixed solution, and even after Fix is achieved it can revert to a float solution due to some disturbance. It is advisable to predict GNSS satellite geometry (GDOP, etc.) before work and choose time windows favorable for positioning. Also, during times of few visible satellites or when ionospheric effects are large, it may be necessary to wait rather than force measurements. Using equipment that supports multi-frequency and multi-constellation tracking is also effective for stable positioning.

• Dependence on equipment and communication: When using network RTK (described below), the communication line is critical. In mountainous areas or around underground structures, when mobile communication is out of range, correction data cannot be received and RTK positioning cannot be maintained. Equipment failures or battery depletion also interrupt work. Prepare backup batteries, arrange to transmit corrections with your own base station and radio when out of cellular coverage, or have a fallback option to post-processing (post-processing kinematic) if needed.

• Reliability of results and submission: When submitting surveying data obtained by RTK as official deliverables or having them inspected by third parties, be prepared to explain GNSS-specific processing and error characteristics. For example, cadastral surveys or other legally effective surveys may require supporting documents (observation logs, accuracy verification results) even for points observed with RTK. Surveying regulations also recommend accuracy control with known points and retention of observation logs for GNSS surveys. Even if RTK-only surveying is sufficient for on-site checks, it is prudent to keep records of verification by total station or similar instruments for official purposes. While RTK significantly improves work efficiency, it is important not to leave everything to RTK; perform traditional double checks at key points to ensure accuracy and reliability.

With proper attention to the above points and appropriate operation, RTK surveying becomes a very powerful tool. Understand environmental conditions and selectively use traditional methods as needed to maximize RTK benefits while controlling risks.

Simple surveying with LRTK

To fully leverage RTK technology on site, having easy-to-use equipment and systems is also important. Until not long ago, RTK surveying required specialized and expensive equipment and complicated setup. Today, however, solutions like LRTK make it possible for anyone to easily introduce RTK surveying.

LRTK is a solution that combines a small high-performance GNSS antenna that attaches to a smartphone or tablet with dedicated apps and cloud services. This allows your smartphone to function as a high-precision GNSS receiver without the need for large surveying equipment, and perform centimeter-class surveying with intuitive app operations. For example, with LRTK, personnel with limited surveying experience can handle simple high-precision positioning through to advanced staking tasks. Traditional two-person staking operations can now be guided reliably by one person through AR functions linked to LRTK, creating new use cases such as accurate single-person guidance.

The emergence of LRTK series products has greatly lowered the adoption barrier for RTK-only surveying for small and medium-sized contractors and local governments. The latest LRTK solutions are compatible with the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction*, and are attracting attention as tools that dramatically improve on-site productivity and surveying accuracy. If you are concerned about what to equip and how to operate to survey alone with RTK, LRTK provides integrated devices, apps, and cloud environments for smooth introduction. Designed with cost and ease of operation in mind, LRTK is accessible even to companies without in-house surveying departments.

As RTK becomes more widespread, surveying work will shift from “craft skill of veterans” to “digital tasks that anyone can perform.” Leading this change is the simple surveying enabled by LRTK, which has the potential to redefine on-site norms. Consider using LRTK to easily practice high-precision and efficient RTK surveying and create a revolution in your construction management.

FAQ

Q: How accurate is RTK surveying? Is RTK alone really sufficient? A: Under good conditions, RTK-GNSS surveying can achieve approximately 2–3 cm (0.8–1.2 in) horizontally and about 3–4 cm (1.2–1.6 in) vertically. This accuracy largely meets the control precision required in civil engineering work such as road construction and land development, and in many cases RTK-alone surveying is sufficient. However, in places with poor satellite reception—such as between buildings or inside forests—temporary errors on the order of tens of centimeters can occur. Depending on environmental conditions, supplementing RTK with traditional methods should be considered. The important point is to double-check critical areas by another method after RTK measurements to operate with greater confidence.

Q: If we have RTK, are total stations and levels unnecessary? A: RTK is a very useful tool, but it does not completely replace total stations (TS) or optical levels. Each has different strengths. RTK is advantageous for efficiently measuring many distant points and for positioning where line-of-sight is not possible, but for detail surveys requiring millimeter accuracy or for highly precise height measurements in situations where direct line of sight is available, TS and levels can deliver higher precision. Therefore, it is ideal to use RTK and other surveying instruments appropriately according to site conditions. Use RTK to greatly improve efficiency in areas it can cover, and use TS or levels for final fine adjustments (accurate center marking, setting reference heights, etc.) to leverage the strengths of both.

Q: What is required to use network RTK? A: To use network RTK (such as VRS), in addition to a rover GNSS receiver, you need an environment to receive correction data via the Internet. Specifically, the rover must be able to communicate via mobile data (either using a receiver with built-in SIM or connected to a smartphone), and you must subscribe to a service that provides correction information. In Japan, there are paid GNSS correction services by private operators and free correction information using the Geospatial Information Authority’s continuously operating reference stations, and connecting to such services enables RTK positioning without installing your own base station. Note that in remote mountain areas where there is no communication coverage, network RTK cannot be used; in such cases, set up a local base station with radio correction transmission in advance or switch to post-processing later.

Q: I am considering introducing RTK surveying for the first time. Can beginners use it easily? A: Modern RTK equipment and software are user-friendly, and basic operations are not extremely difficult. Systems like LRTK allow surveying to be completed simply by following on-screen guidance on a smartphone app, making them accessible even to those with little technical knowledge. However, there are several tips to get the best accuracy from RTK. Avoid shielding the antenna with obstacles and extend it as high as possible, choose times when satellite geometry is favorable, and confirm that positioning has stabilized before recording. Practice first in open areas with good satellite reception and take multiple measurements of the same point to check variability. If you have questions, consult the manufacturer or provider’s support. With proper learning and experience, beginners can master RTK surveying on site in a relatively short time.

Q: What is “simple surveying with LRTK”? A: LRTK is a solution that combines a small RTK-GNSS receiver that can be used with smart devices and a dedicated app, enabling centimeter-level surveying (simple surveying) without specialized heavy equipment or complicated settings. For example, attaching an LRTK receiver to a smartphone and walking the site automatically records high-precision position data, and data can be managed and shared in the cloud. Tasks that were previously performed by veteran surveyors, such as staking and as-built management, can be supported with LRTK’s intuitive AR navigation so that less-experienced technicians can perform them accurately. In short, “simple surveying with LRTK” means “a new surveying method that achieves RTK-level accuracy with easier equipment and operation,” and its effectiveness is beginning to be demonstrated at many sites.