Positioning tasks on construction sites, such as stake driving and layout marking, are important steps for building structures accurately. However, up until now these tasks have required surveying instruments like total stations and skilled technicians, consuming a lot of manpower and time. If precise staking positions could be achieved using only a smartphone, how much would efficiency and accuracy improve? This article explains a new staking method that leverages RTK-GNSS-enabled smartphones. Starting with the challenges of staking using conventional surveying instruments, it provides a detailed overview of how this cutting-edge technology works, how to use it, and the benefits it brings to the field. Finally, it also touches on how the smartphone surveying system "LRTK" makes this possible.

Challenges of Setting Out Stakes with Conventional Surveying Instruments

In traditional staking and layout marking, surveying teams use surveying instruments such as transits and total stations to set out positions. They first establish reference points and then, from those, perform distance and angle measurements based on the coordinates on the drawings, marking the locations where stakes will be driven. This method has a proven track record, but it also faces several significant challenges.

Dependence on manpower and skilled techniques: Skilled surveyors and technicians capable of operating surveying instruments are required, and the work tends to be person-dependent. Basic operations are typically carried out by at least 2 people, with one person operating the instrument and another holding the staff (leveling rod) or prism to indicate stake positions. Because the process relies heavily on experienced personnel, it is becoming difficult to cope in the construction industry as labor shortages worsen. In addition, much of the work depends on technicians' intuition and experience, which can lead to variation in accuracy and methods between workers.

Time and labor burden: Setting up a total station, angle-setting by back-sighting and fore-sighting, repeatedly moving and re-measuring—using conventional methods, it takes time to determine the stake position for a single point. On large sites, hundreds of points may need to be set out, and with traditional methods this can become a long-duration task that delays other work. Even if batter boards (installation of reference stakes and ledger boards) are installed in advance, if the reference shifts rework can occur, posing a risk of affecting the overall schedule.

Accuracy and Human Error: Surveying with optical instruments boasts high accuracy, but errors can still occur due to human operator mistakes or misreadings. For example, if the prism is positioned incorrectly or a mistake is made when copying down calculated figures, the position for driving stakes may be off by a few centimeters (a few in). Because stake position deviations affect the overall quality of a structure, once a mistake is made, rework or corrections can incur significant costs. From a quality management perspective, there was also the challenge that methods relying on manual work make error detection difficult.

Heavy Labor and Safety: Carrying and installing surveying equipment and long periods of standing place a significant burden on field workers. Surveying in harsh environments—particularly under scorching heat or in cold regions—wears down physical stamina and increases safety risks. When setting up tripods on slopes or conducting surveying work at height, there is also the risk of falls and trips. In such situations, even experienced workers are prone to near-misses and are forced to remain tense each time they work.

As described above, conventional pile staking surveys have challenges in terms of personnel, time, accuracy, and safety, and have been a factor impeding productivity improvements at construction sites. What is therefore expected is a "no surveying instrument required" pile staking method that uses the latest technologies. What makes this possible is positioning with a smartphone equipped with RTK-GNSS.

How RTK-GNSS Smartphones Achieve Precise Positioning

Recent RTK-GNSS-compatible smartphones achieve centimeter-level (inch-level) positioning accuracy that until now could only be obtained with specialized surveying instruments. GNSS (Global Navigation Satellite System) determines position by receiving signals from multiple satellites including GPS, but the GPS built into typical smartphones can have errors of several meters (several ft). By contrast, by using a technique called RTK (Real Time Kinematic), it is possible to reduce errors to several centimeters (several in).

In RTK-GNSS systems, correction information from a base station (reference point) is received in real time by a smartphone to cancel out satellite positioning errors. Specifically, a smartphone receives data distributed from the Geospatial Information Authority of Japan’s network of Continuously Operating Reference Stations and from private GNSS correction services, and applies it to its own GPS positioning. In Japan, high-precision positioning services using "Michibiki" (the Quasi-Zenith Satellite System) are also in place, and with compatible equipment correction information can be obtained even at sites where Internet connectivity is difficult. With such RTK corrections, position coordinates obtained on a smartphone are accurate to approximately horizontal ±1–2 cm (±0.4–0.8 in) and vertical ±3–4 cm (±1.2–1.6 in). The fine adjustments of batter boards and the precise alignments using mason’s lines that were required by conventional methods also become unnecessary if coordinates obtained by GNSS are available.

The important point is that the positions obtained with an RTK-GNSS smartphone directly correspond to public coordinate systems (the World Geodetic System or the plane rectangular coordinate system). This makes the coordinate values on design drawings and the coordinates acquired in the field match as geographic coordinates. In other words, you can reproduce the design-stage data on site as-is. For example, if the positions of building columns or foundations are specified as coordinates on the drawings, you can identify the corresponding points in the field simply by entering those values into the smartphone. Conversely, it is also easy to record the coordinates of important points surveyed in the field and feed them back into the design drawings or a BIM model.

The advent of RTK-GNSS smartphones has made precise positioning—which until now required expensive surveying equipment and specialized expertise—possible with a palm-sized handheld device. So how can this smartphone-based precise positioning be applied to actual stake-setting work?

Streamlining Pile Driving Work with AR Navigation

When smartphones can provide centimeter-level (cm) position information (half-inch accuracy), staking and position-setting procedures change significantly. The key is smartphone app-based coordinate guidance and AR display. Traditionally, the surveyor would call out adjustments like "move 5 mm (0.20 in) east" or "1 cm (0.4 in) north" while fine-tuning the stake's position, but with an RTK-GNSS smartphone, you simply follow the instructions on the smartphone screen and move yourself.

Specifically, when you enter the target coordinates where a stake should be driven into the dedicated surveying app, the smartphone calculates in real time the difference between the current position and the target position. On the screen, a guide such as "○○ cm (○○ in) east, ○○ cm (○○ in) north to the target point" is displayed, and the user holds the smartphone and moves little by little to find the point where that difference becomes zero. On the smartphone's screen, AR (augmented reality) technology displays a marker indicating the target point overlaid on the real-world image. Like a treasure hunt, by simply following this marker you can arrive at the precise location to drive the stake.

The advantage of AR navigation is that, thanks to intuitive guidance, anyone can accurately set out positions. Even without the ability to interpret complex drawings or specialist surveying knowledge, users only need to move by following the arrows and markers on the smartphone screen, so even inexperienced workers can carry out highly accurate stake placement. Also, because the app detects the user's position in real time, the next instructions are updated immediately when they move. This means that, without having to check with the surveyor each time, one person can quickly complete the positioning tasks.

By further leveraging AR display, you can virtually draw a mark such as "this is the pile-driving position" directly onto the live site image seen through a smartphone camera, allowing you to visualize the finished result even before marking the ground as was traditionally done. By overlaying AR models not only of the location where the pile will be driven but also of the structures planned to be installed around it (for example, columns or walls), you can intuitively verify whether the location is correct. This dramatically streamlines fine-tuning and verification of positions and helps prevent rework caused by mistakes in advance.

Thus, by combining coordinate guidance from an RTK-GNSS smartphone with AR navigation, staking work is becoming possible in an era of no surveying instruments required and no need for multiple people. On-site workers will only need to walk around the site with a smartphone in hand, and their workload and mental stress will be greatly reduced.

Response to reverse construction methods and special site environments

The piling layout method using an RTK-GNSS smartphone is highly effective for positioning in special situations that were difficult with conventional methods. For example, in the reverse construction method (top-down construction) used for underground structures, upper floor slabs are constructed at ground level while lower floors are excavated, so the building is erected from top to bottom, opposite the usual sequence. In this case, it is not possible to survey foundation positions directly from the ground surface, and it is necessary to mark the positions of columns and piles on already cast concrete slabs. Traditionally, complicated measurements to derive lower-floor positions from the upper-floor reference, or laser alignment in confined spaces, were required. However, with an RTK-GNSS smartphone, absolute coordinates can be obtained by satellite positioning even on the upper floor slab, allowing foundation positions shown on design drawings to be plotted directly onto the slab. When marking directly on concrete, the smartphone’s AR display also ensures you do not miss the exact points.

Similarly, it becomes easier to handle stake-driving and layout work on floors where concrete has already been poured or on pavement. For example, when laying out anchor positions on a concrete floor, the traditional method required using a square, a plumb bob, and chalk for marking. With an RTK-GNSS smartphone, you can identify positions by digitally drawing invisible reference lines on the concrete surface. Instead of driving physical stakes into hard concrete, you can mark the positions indicated by the phone with spray paint or drill holes to mark them. Relying on the virtual lines and points shown by AR makes it possible to achieve accurate layout marking without drawing layout lines on the substrate.

Also, smartphone positioning demonstrates greater effectiveness than conventional methods even on steep or highly terraced terrain. On slopes where it is difficult to set up a tripod or on sites with large elevation differences, securing line of sight with a total station used to be a major challenge. With GNSS positioning, all you need is a location with an open view of the sky, and surveying is possible on moderately sloped ground as long as a person can access it. Because you can check positions while moving with a smartphone in hand, there is no longer a need to carry equipment and work for long periods in places where there is a risk of falling or sliding, improving safety by eliminating the need to carry equipment and perform long-duration work and enhancing safety. On steep slopes, you can complete positioning quickly and withdraw to safety, making safety-first operations easy to carry out.

Furthermore, GNSS-based visualization of pile-driving positions is effective even in nighttime or low-visibility environments. Even on dark sites or in foggy conditions, as long as satellites can be acquired, the target position will be displayed on the smartphone screen. This allows layout marking work, which was previously limited to daytime, to be carried out at night as well, contributing to shorter construction schedules and greater flexibility in project workflows.

Use of Point Cloud Surveying for As-Built Management and Cloud Integration

RTK-GNSS smartphones can be applied not only to positioning for pile driving but also to point cloud surveying and as-built management. Point cloud surveying is a method of capturing three-dimensional shapes by measuring the surface of an object or terrain as a collection of many points (point cloud data). Even without a dedicated 3D laser scanner, by using a smartphone to rapidly position and record multiple points, you can obtain simple point cloud data. In addition, some recent smartphones are equipped with LiDAR (light detection and ranging) scanners, and by combining these with RTK positioning, creating high-precision 3D models is becoming feasible.

When pile-driving work is linked with point cloud surveying, it becomes easy to visualize the difference between the construction plan and the as-built (actual finished condition). For example, after pile installation is complete you can use a smartphone to capture the measured coordinates and elevations of each pile head as point cloud data, and by comparing them with the design values you can create a heat map that shows deviations as a color distribution. Because you can see at a glance which points are displaced from their design positions and which locations exceed allowable tolerances, you can quickly identify areas that require corrective action.

In earthwork, too, by taking the difference between the point cloud of the current ground obtained via smartphone surveying and the designed ground model, you can automatically calculate the volumes of fill or excavation excesses and shortages and present them to the site as a color-coded map. This enables the determination of as-built conditions, which traditionally relied on the experience of foremen and surveyors, to be made quantitatively and instantly.

Also, data obtained with an RTK-GNSS smartphone can be shared instantly between the field and the office by being linked with a cloud service. If you upload measurement data to the cloud with a single tap from the surveying app on the smartphone, staff in distant offices and clients can also check progress and results in real time. There is no need to record notes in a paper field notebook and bring them back, and a major benefit is that it also prevents duplicate data entry and communication errors. Furthermore, on the cloud you can view accumulated point cloud data and coordinate information on browser-based maps and 3D views, and stakeholders can exchange comments. This digitally links the entire process from staking out to as-built inspection, dramatically improving the quality and speed of construction management.

Effects of Introducing Smartphone-based Pile Driving: Time Savings, Error Reduction, and Labor Savings

The renewal of stake-setting and surveying workflows using RTK-GNSS smartphones also has a tremendous impact on-site. First and foremost, it brings a significant reduction in work time. It eliminates the hassle of repeatedly setting up a total station at each survey point and of surveyors running around to secure line of sight, allowing a single worker to lay out points one after another. Depending on the case, there are examples where stake location surveying that used to take a two-person team half a day was completed by one person in a matter of hours with a smartphone. This leads to a shortening of the overall construction schedule and creates the advantage of being able to move on to the next stage with more margin.

Reducing mistakes is another major benefit. Because the system guides based on coordinate data, positional errors caused by human estimation or misunderstandings are less likely to occur. As long as the entered coordinates are correct, the target point will fall within a few centimeters (a few inches) of the intended location. This reduces the likelihood of major rework such as re-driving stakes or readjusting structures. Furthermore, because survey results from the field are shared instantly via the cloud, supervisors and designers can check early and provide feedback. If there are problems, corrective measures can be taken the same day, which also reduces the cost of rework.

Moreover, the effects of labor-saving and manpower reduction cannot be overlooked as a countermeasure to labor shortages in the construction industry. Because smartphone-based positioning can be operated intuitively, not only surveyors with special qualifications but also site supervisors and general workers can handle it. It changes the situation from "surveying must be left to specialists" to one where anyone on site can collect and use survey data. As a result, it becomes possible to reduce the number of survey specialists required at a site or reassign them to other tasks. Reducing workers' burdens through labor-saving also contributes to improving working conditions. If the number of times heavy equipment must be carried decreases and long periods of confinement are eased, physical and mental fatigue will be reduced, and workers will be able to work with greater safety awareness.

Moreover, adopting the latest smartphone surveying technologies itself becomes a catalyst for promotion of DX (digital transformation) on construction sites, and contributes to increasing a company's added value. By digitizing traditional surveying methods that relied on paper and manual work, it can appeal to younger engineers and have positive effects on recruitment and training.

Conclusion: Make stakeout operations smarter with smartphone surveying "LRTK"

We introduced RTK-GNSS smartphone technology that brings innovation to pile-driving and layout marking work—what do you think? An era when anyone can perform precise pile driving without surveying equipment is just around the corner. One solution that brings this kind of smartphone surveying to the field is LRTK from Refixia, a venture company originating at Tokyo Institute of Technology. LRTK consists of a pocket-sized RTK-GNSS receiver that can be attached to an iPhone and a dedicated app, enabling staking out, point-cloud measurement, and AR navigation with positioning accuracy comparable to conventional Class-1 surveying instruments. By simply attaching a small device weighing about 165 g, your smartphone will instantly transform into a versatile surveying instrument.

By leveraging LRTK, you can turn all the features discussed in this article—stake position setting using only a smartphone, AR-based guidance, and cloud sharing—into reality. Use has already begun at construction sites, and there are reports that the "one-person-one-smartphone surveying device" approach has dramatically streamlined work processes. If you face challenges at your site in shortening the time for staking and layout marking, preventing human error, or resolving labor shortages, consider introducing a smartphone surveying system. The latest RTK-GNSS smartphone technologies, including LRTK, will smartly evolve staking operations and raise on-site productivity and safety to a higher level.