A New Norm for Top-Down Construction! Instantly Locating Pile Coordinates with LRTK Smartphone AR Navigation

The top-down construction method, increasingly adopted for urban underground works, demands advanced skills and precise surveying for site management. While top-down construction — carrying out surface and subsurface works in parallel in confined spaces — dramatically improves construction efficiency, it is a challenging method in which errors in pile locations or coordinate setting are unacceptable. Recently, however, an innovative solution has emerged in the form of smartphone AR navigation combined with RTK technology, called “LRTK.” The task of pinpointing pile coordinates, once entrusted to veteran surveyors, is becoming a new norm that can be accomplished instantly with a smartphone in hand. This article explains in detail the basics of top-down construction, urban use cases, and the limitations of traditional surveying methods, and discusses the revolutionary speed and accuracy improvements in pile coordinate identification brought by LRTK smartphone AR navigation. It includes information useful to construction management professionals and concludes by touching on the benefits of simplified surveying using LRTK.

What Is Top-Down Construction: An Innovative Method of Building Underground Structures from Top to Bottom

First, let’s review the basics of the top-down construction method. Top-down construction refers to the method of constructing underground structures by progressing from the ground floor downward. In conventional underground construction (the “bottom-up method”), the entire site is excavated and the underground frame is built upward from the lowest foundation level. By contrast, top-down construction initially builds part of the above-ground structure (for example, the first-floor beams or slabs) and uses those as temporary struts (waler/beams of the retaining system) while repeating excavation and concrete frame construction sequentially from the first basement, second basement, and so on, proceeding from top to bottom. The term “top-down” comes from the idea of reversing the usual construction sequence.

This method makes it possible to carry out underground and above-ground works in parallel. As a result, even for large buildings overall construction time can be greatly shortened, which is a major advantage for urban development projects seeking early opening and early revenue. By constructing the first-floor slab early and using it as a temporary cover that doubles as a working platform and upper-structure support, safety during excavation is improved and the stability of surrounding ground can be maintained. Because the floor slab serves as a temporary lid, noise and dust dispersion during excavation can be suppressed, allowing environmentally considerate construction even in densely populated urban areas. For these reasons, top-down construction is gaining attention as a new urban standard for underground construction that can achieve both “shorter construction periods” and “safety and environmental considerations,” and is being adopted in many urban projects.

Expansion of Top-Down Construction in Urban Areas: Use Cases and Challenges Faced

There have been many reports of the adoption of top-down construction in urban high-rise and redevelopment sites. For example, when building a high-rise with deep underground spaces on a limited urban site, top-down construction enables parallel progress of above-ground and underground works even for large-scale structures such as a 48-story building with five basement levels, minimizing impacts on surrounding roads while shortening the construction period. Top-down construction is also effective in cases where interference with the surrounding environment should be minimized, such as buildings over subway stations or the construction of underground parking lots on narrow plots. In practice, there are examples where the early-constructed slab (top-down slab) served as the waler for the retaining wall and supported earth pressure during excavation, reducing the need for temporary support and enhancing safety, as well as examples where simultaneous progress of upper-frame construction and underground excavation achieved nearly six months of schedule compression.

However, top-down construction also presents unique technical challenges. The biggest challenge is ensuring precise surveying control and pile-driving (layout) accuracy. In top-down construction, the underground columns (top-down columns) and foundation piles constructed early in the sequence become part of the permanent structure that supports the columns and beams of subsequent basement levels. Therefore, each pile must meet strict requirements for position and vertical accuracy on the order of a few millimeters (a few hundredths of an inch), and positional deviations can directly lead to distortion of the upper structure or defects in the frame. Additionally, because surveying must be performed within spaces covered by the first-floor slab or intermediate slabs, coordinate setting must be done in confined environments with poor lines of sight. Unlike bottom-up methods where the site is fully open and extensive batter boards and reference lines can be installed or a total station can be set up from an overhead vantage, such leisurely surveying techniques are often impractical. Urban top-down sites are often adjacent to high-rise buildings, making it difficult to secure setup locations and surveying lines for conventional optical surveying equipment.

Furthermore, in top-down construction where surface and subsurface works progress simultaneously, there is pressure to avoid schedule losses caused by re-surveying or rework. A single surveying point mistake can easily propagate to other processes, so stringent quality control systems are implemented to eliminate surveying errors as much as possible. On large sites, multiple survey teams may double-check the same pile positions and confirm that differences between measurements are within only a few millimeters (a few hundredths of an inch) before proceeding to pile driving. To maximize the benefits of top-down construction, it is crucial to introduce reliable and efficient solutions for surveying and pile-driving guidance.

Traditional Coordinate Verification and Surveying Methods and Their Limitations

Pile layout (commonly called “marking out” or “top-down surveying”) in civil engineering and building construction has long been considered a craft. Traditionally, surveyors would first use coordinate values on the design drawings, then measure offsets from known on-site reference points with tape measures or optical surveying instruments and mark the ground or structure to indicate pile locations. In open sites, batter boards showing reference axes would be installed around the perimeter, and distances measured from their intersection to determine positions. However, in constrained spaces like top-down sites, it is often impossible to set up extensive batter boards, requiring frequent re-measurement using existing structures or temporary materials as references.

Surveying using optical distance meters like total stations (TS) is indispensable for high-precision pile layout. Even so, securing lines of sight for TS setup and performing reference-point setting such as instrument points and back-sight points require time and effort. Because obstructions vary by basement level, instrument relocation and recalculations from known points are sometimes necessary for each survey. Additionally, TS surveying typically involves a two-person operation — a surveyor operating the instrument and another person holding the prism at the target point. Deploying personnel in narrow scaffolding or excavation pits poses safety challenges and reduces operational efficiency.

There are also limits to the reliability of manual marking. Marks made with marking stakes or paint can be erased as construction progresses or displaced by heavy equipment, requiring the survey team to be called back for remarking and affecting overall site progress. Human error is unavoidable in manual operations; small mistakes such as slack in a tape measure, misreading, or transcription errors in point calculations can lead to significant positional deviations. In short, traditional coordinate verification and surveying methods face a triple limitation of “time-consuming,” “labor-intensive,” and “prone to mistakes,” and these weaknesses are especially pronounced in demanding top-down construction environments.

How LRTK Technology and Smartphone AR Navigation Change Pile Coordinate Identification

To solve these surveying and coordinate-setting challenges, the combination of LRTK (a smartphone system compatible with high-precision RTK positioning) and AR navigation has emerged. LRTK is attracting attention as a next-generation technology that realizes RTK positioning, which formerly required stationary or large equipment, using a single smartphone. RTK (Real Time Kinematic) is a method of high-precision GNSS positioning that achieves centimeter-level accuracy by simultaneously receiving satellite signals at a base station and a rover and correcting errors in real time. In LRTK, a compact GNSS receiver compatible with RTK is attached to an iPhone, iPad, or similar device, transforming the smartphone into a high-precision surveying instrument. By using network-based RTK correction information and CLAS augmentation signals provided by Japan’s quasi-zenith satellite system “Michibiki,” the system can determine its position with astonishing accuracy — from just a few centimeters (a few inches) down to, in some cases, a few millimeters (a few hundredths of an inch).

LRTK’s strength is not limited to hardware simplicity. Its true value is realized through AR (augmented reality) navigation functions running on the smartphone. The dedicated app overlays target points and directional guidance from design drawings onto the camera view in real time, allowing users to be guided to pile positions simply by looking at the screen. For example, selecting the pile center coordinates uploaded to the cloud and tapping “Start Navigation” will display an arrow and distance to the target on the smartphone screen. Site personnel need only walk in the arrow’s direction; as they approach the target, the displayed distance shrinks. Near the destination, the arrow provides fine directional adjustments, and by performing the final tweaks as indicated, the user can reach the target coordinates with an error of only a few centimeters (a few inches). Even without advanced surveying knowledge, simply “following the on-screen guidance” allows accurate placement at the pile location — a revolutionary departure from the traditional reliance on the intuition and experience of seasoned workers.

LRTK’s AR display can also visually indicate the target location itself. If direct ground marking is not possible, a virtual pile (an “AR pile”) can be placed on the smartphone screen to indicate the position, enabling coordinate checks from safe locations even on concrete-covered floors or in hazardous areas that are difficult to access. Combined with photogrammetry, coordinates of remote or steep locations that cannot be physically approached can be obtained by photographing the site and then projecting a virtual pile for later location identification on site. This is an innovative feature that enables pile guidance in situations that were previously impossible.

Additionally, LRTK systems feature cloud integration for data utilization. Uploading 3D design data such as BIM/CIM to the cloud makes it easy to automatically compare point-cloud data acquired on site and AR-project the as-designed model onto the actual terrain. Because GNSS tracks the user’s position with high precision in real time, the AR-projected model remains fixed in place even as the user moves. This nonshifting AR projection is useful for client inspections of as-built conditions and pre-construction checks, and strongly supports digital transformation (DX) on site.

Dramatic Improvements in Speed and Accuracy with Smartphone AR Navigation

With LRTK smartphone AR navigation, the speed and accuracy of identifying pile coordinates improve dramatically. Regarding work speed, significant efficiency gains have been reported compared to conventional methods. In one comparative example, an AR piling system using GNSS (essentially an LRTK-like setup) reduced the time required for point layout to about one-sixth of that needed for conventional optical surveying. This is because time spent setting up instruments and securing lines of sight is eliminated, and a single operator can move and guide points sequentially. Tasks that formerly required two people and half a day for layout can be completed by one person within a few hours using LRTK in many cases. Increasing the number of pile locations processed per day directly shortens the overall construction schedule and allows earlier progression of subsequent tasks.

On the accuracy side, LRTK AR navigation surpasses traditional methods. RTK positioning itself delivers centimeter-level (half-inch accuracy) precision, and AR visual guidance minimizes human error. Previously, surveyors measured coordinates and marked the ground, and machine operators or workers would rely on these marks for construction. With LRTK, people are directly navigated to target points in the digital data, eliminating interpretation or transmission errors. For example, following on-screen instructions to position a pile driver at the designated location enables pile installation that matches design coordinates. Human errors such as misreading marks or confusing construction locations are greatly reduced because AR provides consistent guidance visible to anyone. Moreover, LRTK automatically records all coordinate data and navigation histories digitally during positioning and guidance. Arrival errors and guidance times for each point are digitized, allowing construction managers to use them as quality control records and making root-cause analysis straightforward if errors occur. This also frees managers from the burden of verifying handwritten field notes, providing significant benefits for quality assurance.

Labor reduction and improved safety are additional important benefits. As noted above, LRTK can allow surveying and marking tasks that previously required two or more people to be completed by one person. Reducing personnel lowers labor costs and, more importantly, reduces safety risks by keeping fewer workers in areas where heavy equipment is operating. There is less need to descend into dangerous excavation bottoms or climb to heights for surveying, and when AR allows remote guidance from a distance, physical burdens and fall/slide risks for workers are avoided.

In sum, the speed, accuracy, and labor-saving effects of LRTK smartphone AR navigation represent a revolutionary impact not only for top-down construction but for all piling and surveying operations.

LRTK Use Cases on Urban Construction Sites: How Top-Down Construction Changes

So how can LRTK and smartphone AR navigation be used on urban construction sites? Below we imagine specific changes after LRTK introduction using top-down construction sites as examples.

Case 1: Underground Work for a High-Rise in the City Center In a redevelopment project in Tokyo involving a high-rise office building with three basement levels of parking, the top-down method was adopted. Under conventional practice, survey teams would repeatedly set pile centers in narrow basement pits and workers would erect steel columns (top-down columns) based on marked positions. After introducing LRTK, construction managers could simply walk the site with a smartphone and sequentially identify pile centers. Receiving satellite signals through openings in the first-floor slab or gaps in the underground frame, and following the arrows displayed on the app, users could be guided to target coordinates even in heavily covered underground spaces. At column installation points, the smartphone screen displayed both the column center position and the column orientation in AR, enabling workers to set column bases in the correct position while simultaneously confirming the column’s rotation angle. This made it smooth to precisely align column positions and angles along curved underground roads — which had been difficult — and greatly reduced the days required for surveying and erection.

Case 2: Pile Foundation Work on a Narrow Plot On a project building a new structure between existing buildings where there was scarcely any space to occupy the road, there was no room to install batter boards before construction. Using LRTK’s coordinate navigation function, foundation pile coordinates pre-registered to the cloud were used to guide pile driving on site. Because piling yards were typically occupied by heavy equipment and difficult for personnel to enter, arrangements were made so that the pile driver operator could check the smartphone screen from the cab, allowing the operator to perform fine adjustments by following the AR navigation. When a ground guide held a smartphone for guidance, a wide field of view was not necessary; they only needed to stand at the instructed location. As a result, a process that formerly required three steps — “survey team sets pile center → pile driver positioned based on the mark → survey team rechecks” — could be completed in a single guided operation. This reduced wasted waiting time due to logistical changes typical of narrow sites and made full use of limited working hours.

Case 3: As-Built Inspection and Feedback for Underground Structures LRTK is also useful for as-built inspections of underground frames constructed by top-down methods. For example, after the structure was built to the second basement level, a construction manager could re-measure the positions of major columns and walls with LRTK and immediately share those coordinates to the cloud. Deviations from design values are color-coded on the app in real time, allowing all stakeholders to identify locations that require correction on the spot. Traditionally, the process was survey team measurement → office drawing comparison → feedback, creating time lags. With LRTK, stakeholders can look at the smartphone screen together on site and immediately confirm “this column position matches the design” or “this one is 5 mm (0.20 in) to the south.” Because top-down construction tends to accumulate positional errors between ground and underground works, such rapid as-built confirmation and feedback enable early correction and a PDCA cycle that reflects corrections in subsequent processes. As a result, as-built accuracy at completion improves and the need for rework or additional work decreases.

As shown above, introducing LRTK fundamentally reforms many processes related to top-down construction. Surveying and pile guidance, previously specialized and dependent on individuals, are standardized with digital tools, improving overall site productivity and reliability.

The New Norm in Top-Down Construction: Shift to Digital Surveying and the Future Opened by LRTK

With the spread of top-down construction, the surveying and pile layout technologies that support it are entering a new stage. As exemplified by the Ministry of Land, Infrastructure, Transport and Tourism’s “i-Construction” initiative, construction sites are being pushed toward productivity improvements through full utilization of ICT and digital technologies [i-Construction - Ministry of Land, Infrastructure, Transport and Tourism](https://www.mlit.go.jp/tec/i-construction/index.html). The LRTK smartphone AR navigation solution aligns with this trend and can be considered a key technology that brings digital transformation (DX) to top-down construction sites. With concerns over the retirement of experienced surveyors and a shortage of skilled personnel, intuitive AR surveying that anyone can use has the potential to establish new norms in future construction management.

Finally, we touch on the benefits of simplified surveying using LRTK. LRTK enables more than just “high-precision pile guidance.” Routine site surveying, as-built control, and verification of buried utilities — tasks that previously required dedicated instruments and expert knowledge — can be performed simply with a single smartphone. For example, tasks such as boundary confirmation surveys or earthwork volume control can be measured quickly by the site manager and immediately shared to the cloud using LRTK. The benefit of being able to perform sufficiently accurate surveying without mobilizing a specialist survey team is invaluable. LRTK breaks the conventional wisdom that “surveying is difficult and time-consuming” and makes high-precision positioning accessible to anyone. By adopting this new technology, those involved in top-down construction can achieve safer and more efficient site management. Make LRTK smartphone AR navigation — now the new norm for top-down construction — your ally and experience its power on your next project.