RTK for Concrete Layout: Anchor Bolts, Formwork, and Height Verification

Table of Contents

• Introduction

• What is RTK (Real Time Kinematic)

• Using RTK for precise anchor bolt positioning

• Improving formwork layout marking efficiency and level checks

• Using RTK for concrete height verification (leveling)

• Conclusion: What is simple surveying with LRTK

• FAQ

Introduction

When constructing concrete structures on a construction site, anchor bolt placement, formwork installation, and height verification (leveling) are extremely important tasks. Mistakes in these processes can lead to rework because components do not fit in later stages, or they can affect the quality and safety of the structure. Therefore, careful marking and surveying are performed on site, but conventional methods inevitably involve human errors and labor.

Recently, RTK positioning (Real Time Kinematic) has attracted attention as a technology to solve these problems. RTK is a high-precision positioning technique using satellites that corrects GNSS (GPS) positioning errors in real time and can achieve centimeter-level accuracy. Compared to traditional marking using tape measures or transits, using RTK can quickly and accurately determine anchor bolt and formwork positions, and is expected to greatly reduce rework due to surveying errors. This article explains, in an easy-to-understand manner for beginner to intermediate surveying and construction technicians, how RTK can be applied to anchor bolt placement, formwork, and height verification and the benefits of doing so.

What is RTK (Real Time Kinematic)

RTK (short for Real Time Kinematic) is a type of satellite positioning system that uses two receivers—a fixed receiver called a base station and a mobile receiver (carried by a worker)—to obtain high-precision position information. The error information of satellite signals received at the base station is sent to the rover via radio or communication lines, and the rover corrects its positioning results, reducing GNSS positioning errors that were previously on the order of several meters down to the order of a few centimeters. Because it can provide accuracies of about ±1～2 cm (±0.4～0.8 in) horizontally and about ±2～3 cm (±0.8～1.2 in) vertically, it offers sufficient positioning accuracy for site layout marking and as-built (post-construction) measurements on construction sites.

A major feature of RTK positioning is that results are obtained in real time. On the rover, corrected position coordinates are displayed on site, so when laying out positions on site you can immediately confirm whether your standing position matches the design coordinates. The receiver performs continuous positioning while moving, so it can trace lines and surfaces as well as points. For example, tasks such as drawing a line at a specified height on the ground can be done by measuring multiple points of height with RTK and immediately drawing a horizontal reference line.

To use RTK, you typically set up a base station with known coordinates near the site (or use public electronic reference point data) and perform positioning while communicating with the rover. In Japan, even without your own base station you can use network RTK services that deliver correction information via mobile networks, enabling RTK positioning with a single receiver. In any case, RTK can simplify the cumbersome surveying preparations that were previously required, such as batter boards and leveling water, and the equipment can be reduced to just a receiver and a pole, making it attractive for labor saving. Now let’s look concretely at how this RTK technology can be applied to actual anchor bolt installation and formwork construction.

Using RTK for precise anchor bolt positioning

An anchor bolt is a bolt embedded in concrete foundations to fix structural elements such as columns and beams. For example, in steel-frame construction, anchor bolts are installed in foundation concrete and columns are fixed to those bolts. If anchor bolt positions deviate from the design, the holes for attaching columns will not align and construction defects will occur, so accurate placement to millimeter-level precision is required.

Traditionally, anchor bolt layout was commonly done by setting up temporary wooden batter boards (known as yari-kata) and stretching strings to establish building reference lines, then measuring distances with a tape to determine positions. Skilled workers can achieve high accuracy with careful measurement, but manual marking work inevitably takes time and effort, and small misreadings or slack in the string can cause cumulative errors. While using total stations (TS) for anchor positioning is also possible, TS requires at least two people—a device operator and a prism holder—and line-of-sight must be maintained across the site. On large foundations, repeated setups are required, which is inefficient. Some technicians have had the bitter experience of having foundation positions off by several centimeters and having to cut the concrete and reinstall anchors.

RTK excels in anchor bolt positioning. With an RTK-capable positioning device, you can indicate the anchor center coordinates defined in the design directly on site. If you preload the anchor coordinate data (coordinate values) into the device, you only need to carry the rover and move around; the display will show your current position in real time with centimeter-level accuracy. The worker simply aligns their displayed position with the target point and marks the anchor location, so a single person can perform marking work efficiently.

Because anchors can be placed according to the drawings even without an experienced surveyor, rework due to construction errors can be greatly reduced. On sites that have adopted RTK, feedback includes statements like “We can now lay out anchor positions according to the design coordinates, and rework such as cutting the foundation and re-embedding bolts has almost been eliminated.”

Improving formwork layout marking efficiency and level checks

Formwork is temporary structure used to shape concrete so it does not flow away during placement. The shape and dimensions of foundations and walls are determined by formwork positions, so accurate placement and leveling are important. If formwork placement is off, the finished concrete structure may exceed design dimensions or exhibit tilt or steps.

In conventional formwork layout, chalk lines are drawn on the ground or slab aligned with the building grid, and formwork is assembled along those lines. For straight sections, strings are stretched to create straight lines, and right angles are established using rulers or surveying tools to measure offsets from batter boards. Although skilled workers can achieve accuracy with this method, long strings stretched over wide areas may bend from slight slack, and curved or complex formwork shapes require repeated measurement and marking, which is very time-consuming. Moreover, if chalk lines or strings shift or break during work, points may be lost and require re-measurement.

Using RTK significantly improves the efficiency of formwork layout. For example, by measuring formwork reference points such as foundation corners and column positions with an RTK receiver, you can immediately check whether each point is at the design coordinate. If a point is off, it is identified in real time and the formwork can be corrected on the spot. Tasks that previously required two people for horizontal and vertical leveling can be done by one person carrying a pole and measuring heights at each point to confirm they are at the specified elevation. Height information is automatically corrected by the receiver, allowing the top height of the formwork around the perimeter to be made uniform within a few centimeters of error.

RTK layout marking eliminates the need to calculate distances and angles between measured points one by one. The receiver’s screen displays the “deviation from the design line” numerically, so you simply move the formwork into position while checking that value. For curved or diagonal formwork sections, RTK can display complex shapes as coordinates, enabling accurate line layout without relying on craftsmen’s intuition. The result is reduced work time, reduced manpower, and improved as-built accuracy.

Using RTK for concrete height verification (leveling)

Height (level) control is also a key quality assurance point in concrete work. Verifying whether foundation and slab top elevations match the design and are uniformly flat is always performed during post-construction inspection. Traditionally, optical auto-levels and leveling staffs were used to perform leveling surveys. Extending levels from a benchmark of known elevation to each measuring point typically requires two people who must shuttle back and forth to transfer elevation values. On large placement areas, the number of measurement points is high and confirming small elevation differences requires considerable effort. Human errors such as misreading the staff or recording numbers incorrectly can also occur.

Using RTK positioning greatly increases efficiency for height verification. RTK receivers can calculate altitude (elevation) coordinates in real time, so once a reference point with known elevation is set, you can measure the elevation of any point with the push of a button. For example, after concrete placement you can walk across the slab and measure top elevations with RTK to immediately identify deviations from design elevations. Instead of reading and recording levels point by point and comparing them later, RTK displays values like “design diff +◯◯ cm” on the receiver screen, allowing you to decide on corrective actions on the spot.

Although vertical accuracy is somewhat inferior to horizontal accuracy, under good conditions it stays within about 2～3 cm (0.8～1.2 in). This precision is sufficient for general civil and building works; for critical structures requiring millimeter-level settlement monitoring, conventional leveling can be used in conjunction for confirmation. The advantage of RTK height checks is that you can grasp level conditions over a wide area in a short time. You can immediately see how flat the top was placed and where elevation differences exist, so it can be used for immediate screeding or bump adjustments. Measurement results are also stored as digital data, making it easy to compile as-built documentation later or share via the cloud for review by all stakeholders.

Conclusion: What is simple surveying with LRTK

As we have seen, RTK technology makes anchor bolt and formwork positioning and level control in concrete works significantly easier and more accurate. Getting the job right the first time prevents cost increases due to rework and directly contributes to shorter schedules and improved quality. With initiatives promoted by the Ministry of Land, Infrastructure, Transport and Tourism such as *i-Construction*, digital surveying technologies including RTK are increasingly being adopted on sites.

However, some may hesitate at the thought that RTK surveying requires expensive specialized equipment and expertise.

One notable solution is a simple RTK surveying system that combines a smartphone with a small GNSS receiver. A representative example is the LRTK series. By attaching a pocket-sized RTK-GNSS receiver to a smartphone and using a dedicated app to receive network RTK and the QZSS (Michibiki) augmentation signals (CLAS), the system enables real-time high-precision positioning.

Complex device settings are automated in the app; tap the coordinate of the point you want to measure and a guide appears to help you lay out the position. AR (augmented reality) features can project design positions and elevations onto the smartphone screen for confirmation. The receiver also supports a tilt compensation function that corrects the pole’s tilt using an internal inclination sensor, so the tip position is measured correctly even if the pole is somewhat tilted.

Simple surveying with LRTK is attracting attention on sites as “RTK anyone can use.” Compared to dedicated equipment, it has lower introduction costs and intuitive smartphone operation makes positioning and measurement accessible even to non-experts. In practice, tasks that used to be outsourced to external surveying specialists are increasingly being completed quickly by in-house staff, leading to dramatic productivity improvements.

It may be a good idea to start using such easy RTK solutions on a small scale. If you can experience the benefits, you can expand them across the site to expect significant efficiency gains and error reduction.

If you are interested in details about LRTK, please also visit the [LRTK official site](https://www.lrtk.lefixea.com) to get a glimpse of a new surveying style on site. The LRTK series also includes devices that support drone photogrammetry and 3D LiDAR measurement, providing comprehensive support for digital measurement on site.

FAQ

Q1. What level of accuracy can be obtained with RTK surveying? A. Typical RTK-GNSS surveying provides about ±1～2 cm (±0.4～0.8 in) horizontally and about ±2～3 cm (±0.8～1.2 in) vertically. However, this assumes good satellite reception conditions. The more open the sky, the easier it is to maintain a high-precision “Fix” solution; in contrast, in urban canyons or under trees satellite signals can be blocked and accuracy may decrease.

Q2. Can RTK be used in environments with many obstructions or indoors? A. RTK is fundamentally a technology that performs best outdoors with a clear view of the sky. In areas with many tall buildings in urban centers or inside forests, satellites may not be sufficiently received or multipath reflections can make positions unstable. In such obstructed environments, you may need to move to a location where positioning is possible or use RTK-obtained reference points and a total station for detailed measurements. Indoors or underground where satellites cannot be received directly, RTK is generally not usable.

Q3. Can RTK measure height (elevation) as well? A. Yes, RTK-GNSS receivers can measure vertical coordinates (Z coordinate) as well as horizontal positions. However, vertical measurements are more susceptible to errors from the ionosphere and other factors and are somewhat less accurate than horizontal measurements. With corrections based on high-precision reference points, centimeter-level vertical accuracy can be expected, but for critical structural settlement monitoring requiring millimeter-level precision, it is reassuring to also confirm with conventional leveling.

Q4. What equipment and preparations are needed to introduce RTK? A. The basic set includes an RTK-capable GNSS receiver (rover) and a base station that provides the reference signal. The base station is a fixed station installed on a known coordinate point and can be provided by yourself or you can use a network RTK service that distributes corrections by communication. The rover needs a communication method (radio modem or internet connection) to receive correction information from the base station. It is also recommended to perform calibration (localization) with known points in advance to match the site’s coordinate system. Systems like LRTK can start RTK positioning immediately by using correction data delivered over the Internet or Japan’s satellite augmentation information without preparing a base station.

Q5. What is LRTK? A. LRTK is the name of a positioning system that combines a smartphone with an ultra-compact RTK-GNSS receiver. It consists of a receiver attachable to a smartphone and a dedicated app, and uses network RTK and QZSS augmentation signals to realize real-time centimeter-level positioning. Because it can directly receive Japan’s Michibiki signals, high-precision positioning is possible even in mountainous areas without mobile coverage. It is much more user-friendly than conventional fixed GPS surveying units and is drawing attention as a new solution that lowers the barrier for on-site positioning and surveying tasks.

Q6. How far from the RTK base station can you perform positioning? A. Generally, RTK is considered to provide high-precision positioning within an area of about 10 km. As distance increases, correcting ionospheric and tropospheric errors becomes more difficult, which can delay obtaining a Fix solution or reduce accuracy from a few centimeters to several tens of centimeters. Network RTK services that cover wide areas set virtual reference points nearby to provide corrections and can extend coverage to some extent, but real-time positioning may become unstable if the distance exceeds several tens of kilometers. To maintain positioning accuracy, it is desirable to operate as close to the base station as possible.