Top-Down Construction Method × Smart Technology — LRTK Opens Up Next-Generation Construction Management

As construction site digitization accelerates, traditional construction methods are also creating new value through integration with smart technologies. A representative example is the combination of the "reverse construction method" used in underground works and the latest surveying technologies. The reverse construction method is a technique in which, for buildings with underground floors, the conventional sequence is deliberately reversed and the aboveground and underground parts are built concurrently; it has been adopted in many large-scale projects because of benefits such as shortened construction schedules and stable construction on soft ground. However, because of its complexity, it is also a highly demanding method that requires precise construction management and surveying.

On the other hand, in recent years next-generation surveying solutions that leverage smart technologies such as RTK (real-time kinematic) positioning, AR (augmented reality), and cloud integration have emerged. Among them, LRTK is attracting attention. Composed of a smartphone, a compact high-precision GNSS receiver, and a dedicated app, LRTK is a surveying DX service that combines centimeter-level RTK positioning accuracy (half-inch accuracy), the ease of use of operating on a smartphone, intuitive AR coordinate guidance, and data sharing via cloud synchronization. This ease of use—allowing site construction managers and surveyors to operate with one smartphone per person—combined with these features proves powerful in advanced construction sites such as those using the reverse construction method.

In this article, primarily aimed at construction managers and surveyors at general contractors, we explain the transformation that the fusion of "reverse construction method × smart technologies" brings to construction management. We summarize the outline of the reverse construction method and its conventional challenges, and take a concrete look at how smart surveying technologies, including LRTK, overcome those challenges. We also consider the potential for improving on-site operational efficiency and accuracy, as well as the prospects for construction management DX (digital transformation) looking to the future.

What is the top-down construction method? Benefits and challenges in construction management

First, let's outline the basics of the "top-down construction method." The top-down construction method is a specialized construction technique used for buildings with basement floors, characterized by constructing underground structures in the opposite order to the usual. In conventional underground construction (the bottom-up method), the site is excavated and all below-ground portions are completed before proceeding with the above-ground construction. In contrast, in the top-down method, the above-ground portion of the structure (particularly the first-floor slab) is constructed first, and that structure is used as support while excavation and structural work for the underground levels proceed in parallel. For example, after casting the first-floor slab, the slab supports the surrounding retaining walls while basement level 1 is excavated and constructed, then basement level 2, and so on — in this way, the basement floors are created sequentially from top to bottom.

This construction method has several advantages.

• Reduced construction period: Because above-ground work and underground work can be carried out simultaneously and in parallel, the overall construction period can be greatly shortened. Especially in large urban projects, the effects of earlier opening and cost reduction due to a shortened construction period are significant.

• Stable underground construction: Because the retaining walls can be supported by the superstructure while excavating, the stability of the underground space is improved. Compared with conventional methods, ground loosening and settlement of surrounding ground can be suppressed, allowing safe deep excavation even in weak ground.

• Reduction of temporary construction costs: Since the superstructure (floor slabs, etc.) also serves as the bracing for the retaining walls, a portion of the large number of struts and shoring materials that were traditionally required can be omitted. This can reduce temporary construction costs and the amount of work in some cases.

On the other hand, the top-down construction method also has management issues and precautions.

• Constraints on the construction space: Work on the underground floors is carried out in the confined space beneath the upper floors, so the working environment becomes cramped. Access routes for heavy machinery and material handling are restricted, and there are cases where securing installation space for surveying equipment is also difficult.

• High-precision surveying and setting out: Because the above-ground and underground structures must be precisely joined later, greater-than-normal accuracy is required for the setting out of columns and walls (sumi-dashi). Tasks to transfer reference points between upper and lower floors increase during construction, and even minor surveying errors carry the risk of causing misalignments in later stages or rework.

• Complex process management: In addition to parallel work above and below ground, the fact that the structures support each other means that planning construction schedules and site management require a high level of judgment. Miscommunication between work teams or differences in interpretation of the design drawings can lead to serious troubles.

• Requirement for advanced construction techniques: Because the underground structure is built while using the upper structure as support, advanced construction techniques different from conventional methods are required, such as treatment of concrete construction joints and planning of temporary shoring. Thorough construction planning and technical reviews are required on site, and ensuring that workers are fully informed is also important.

As described above, the top-down construction method offers significant benefits but requires precise management and technical skill. In particular, the importance of surveying and layout work is high, and there are many situations where construction managers and surveyors can showcase their skills. So how can smart technologies play a role on sites using the top-down method? In the next chapter, we will look at the solutions that smart surveying technologies can provide, using LRTK as an example.

Worksite Transformation Brought by Smart Surveying Technology LRTK

Now, let's look at what specific solutions the smart surveying tool LRTK can provide for complex sites using the top-down construction method. LRTK is a surveying system that combines high-precision GNSS with a smartphone app and provides powerful support for construction management through the features introduced below.

Enhance construction accuracy with RTK positioning precision

One of LRTK's greatest strengths is centimeter-level (half-inch-level) high-precision positioning via RTK positioning. By attaching a dedicated compact GNSS receiver to a smartphone and correcting satellite positioning data using real-time kinematic techniques, GPS errors that are normally on the order of several meters (several ft) are reduced to just a few centimeters or less (a few in or less). Because it can measure not only horizontal position but also elevation (height) with high precision, it proves invaluable for managing all kinds of three-dimensional coordinates that are critical on top-down construction sites, such as slab height management for basement levels and verification of structure installation positions.

Traditionally, verifying millimeter-level accuracy (mm, 0.04 in) required detailed measurements with a total station and careful height checks with a level. However, by using LRTK, site staff can instantly determine their current position coordinates with a smartphone in hand and compare them to the planned values on the design drawings. For example, when setting out underground columns, using the global coordinates (World Geodetic System) calculated by LRTK allows immediate confirmation of whether there is any offset in the column center between the aboveground and underground floors. Because even slight positional deviations can be detected early, this helps prevent rework and improves structural accuracy.

Furthermore, LRTK also supports Japan's "Michibiki" satellite high-precision positioning service (CLAS), enabling stable positioning even in urban areas with unstable radio environments or in situations where the sky is temporarily blocked by underground structures. Even in underground locations that are typically difficult to survey with the top-down construction method, LRTK can continue to provide seamless and accurate positioning, preserving the reliability of survey data. The reassurance of always having high-precision location information will be an extremely powerful asset for construction managers.

Anyone can survey instantly with the ease of smartphone operation

Because LRTK can be operated with a smartphone, its convenience brings significant benefits on site. By attaching the dedicated terminal (a compact GNSS unit weighing approximately 165 g) to your smartphone and simply launching the app, you're ready for high-precision surveying. No complicated installation or advanced equipment configuration is required, so you can go to the site and start measuring immediately. This is especially helpful on sites using the reverse construction method, where tasks must be carried out efficiently within a limited time.

In the past, surveyors would transport and set up total stations, and teams often carried out positioning work. However, with LRTK, construction managers and junior staff can perform surveys quickly on their own. Because the device is lightweight and small enough to fit in a pocket, it can be carried at all times and taken out immediately when needed. For example, if you suddenly need to check the elevation at a particular point or want to check the positional relationship between the plans and the site on the spot, with LRTK you can handle it yourselves without waiting for a surveying specialist. By creating an environment in which anyone on site can obtain survey data, this leads to reduced waiting times and faster decision-making, and will allow more flexibility in tight schedule management such as reverse construction methods.

Moreover, the fact that the user interface is unified as a smartphone app also makes it easier to learn. With an intuitive UI design and an easy-to-understand workflow, even those without deep IT or surveying equipment expertise can become proficient in a short time. If surveying tasks that previously had to rely on veterans can be shared among the whole team, it will be easier to maintain efficiency even on sites suffering from labor shortages. If the spread of LRTK realizes "one-device-per-person" surveying DX, it will directly lead to raising productivity across the entire site.

Intuitive construction checks and positioning using AR coordinate guidance

The AR (Augmented Reality) features provided by LRTK are transforming how onsite positioning tasks and construction verification are carried out. Instead of the traditional layout marking and position checks done with plan drawings, tape measures, and surveying instruments, these tasks are intuitively guided through the smartphone screen. For example, because the LRTK app displays design reference points and the end positions of structures as markers on the camera feed, workers can see at a glance in the real scene "where to drive piles" and "where to construct the next wall." In underground-floor construction using the reverse-construction method, marking locations that cannot be directly seen from the upper floors is a challenge, but by using AR coordinate guidance you can project virtual layout lines and stakes onto the field of view, enabling smooth positioning even in confined spaces.

Furthermore, LRTK’s AR functionality goes beyond single-point guidance and also enables the superimposed display of construction imagery using 3D models. By AR-displaying design 3D data such as BIM/CIM uploaded to the cloud on a smartphone aligned to the site coordinate system, the design model and the as-built conditions can be compared and reviewed on the spot. For example, before casting concrete on an underground floor, the completed 3D model can be overlaid and projected on site to confirm in advance the positional relationships of the structural frame and openings. Because the post-completion appearance—which was difficult to grasp from paper drawings—can be shared on site, the risk of rework due to misunderstandings is reduced. In meetings with clients and stakeholders, showing a concrete vision of the finished product via AR also makes consensus-building easier and can shorten the time spent on explanations.

Thus, using AR coordinate guidance to bridge the gap between the field and the design becomes the key to smoothly advancing the complex construction involved in top-down construction methods. By visualizing the aspects that once relied on intuition and experience with digital data, the burden on construction managers is reduced and quality assurance becomes more reliable. LRTK’s AR features can be regarded as a dependable support that anticipates the future of smart construction.

Seamless information sharing and record-keeping through cloud synchronization

LRTK is equipped with a system that can immediately sync to the cloud any data collected on site. Measured coordinates, photos taken, point cloud data, and notes can be uploaded from a smartphone with a single tap, allowing team members in the office or at other locations to view the latest information from the very moment it was measured in the field. On sites using top-down construction methods, it is important for multiple stakeholders—such as the above-ground crew and the underground crew, or construction and design teams—to grasp progress in real time. Cloud synchronization bridges the information gap between the field and the office, creating an environment where everyone can make decisions while sharing the same data.

For example, if the position of a column on basement level 1 is measured and immediately synchronized to the cloud, the designer can check those coordinates from the office and instantly verify any discrepancies with the plan. If there is a problem, feedback can be received right away, allowing corrections to be made on the spot. Also, because accumulated survey points and point clouds can be viewed on maps or drawings in the LRTK cloud, inspection of as-built (finished) conditions and the preparation of reporting materials are streamlined. Survey records that site supervisors used to compile manually after work are automatically stored digitally, helping to prevent record omissions and reduce the burden of document preparation.

LRTK Cloud also makes data sharing with external stakeholders simple. If you want to show specific measurement data or point-cloud models to partner companies or clients, you can select the relevant data in the cloud and issue a shareable URL, and the recipient can immediately view it in a 3D viewer from their browser. Because there is no need to install dedicated software or go through cumbersome exchanges, communication costs across the entire project are reduced. By leveraging the cloud in this way, the time lag in information transmission in complex construction such as the top-down construction method is eliminated, enabling speedy construction management that integrates the site and the office.

Summary: Construction management enters the next generation with the top-down construction method × LRTK

We have examined the benefits that the integration of the top-down construction method and smart surveying technologies brings. By combining advanced technologies like LRTK with the top-down method—which achieves shorter schedules and more stable execution—dramatic improvements can be expected across accuracy, efficiency, and communication. Elements that previously relied on craftsmen’s intuition and experience can be replaced by data-driven management, reducing mistakes and easing operational burdens. With the site and the office seamlessly connected, overall project visibility improves, and it is worth noting that all stakeholders can approach construction with a shared sense of unity.

In fact, as the shift to digital construction progresses both domestically and internationally, smart surveying systems like LRTK are becoming the new standard in construction management. The benefits of their adoption have been reported not only for advanced works such as the top-down construction method but also for general civil engineering and building projects. As keywords like "on-site DX" and "i-Construction" suggest, with the entire construction industry steering toward digital transformation, there is no reason not to take advantage of these tools.

If you have the opportunity to work with the reverse construction method at your site, please consider leveraging LRTK. Because LRTK can be started easily with a smartphone, the barrier to adoption is low even for first-timers. By introducing smart technology to the site even in small steps, you should be able to experience its convenience and effectiveness. Why not bring to your project the next-generation construction management that the combination of reverse construction methods and smart technology is pioneering?