The Innovation in Surveying Brought by iPhone
Surveying technology in the construction industry has evolved dramatically over the past few decades. In the past, manual surveying using transit and total station equipment was the mainstream, but the introduction of total stations and GPS surveying equipment improved both efficiency and accuracy. Recently, 3D surveying using technologies such as 3D laser scanners and photogrammetry with drones has become more widespread. However, these advanced instruments are expensive and require specialized knowledge, making it true that the barrier to entry is high for small and medium-sized construction companies and on-site technicians.

Amidst this, surveying using smartphones (particularly iPhones) has gained attention as a new revolution in surveying. iPhones are equipped with high-performance cameras, LiDAR sensors, and GPS (GNSS) capabilities, and their processing power and communication functions are also excellent. As a result, they are becoming indispensable tools for improving productivity on construction sites.

Especially in Japan, with the introduction of the quasi-zenith satellite "Michibiki," the positioning accuracy of smartphones' built-in GPS has improved to an error margin of about 5 to 10 centimeters in recent years. However, the demand for "even higher precision surveying" and "easily obtaining 3D point cloud data" still persists.

This led to the introduction of technology that enables high-precision positioning by attaching an external device to the iPhone. Recently, a pocket-sized versatile surveying device called "LRTK Phone," developed by Lefixea, a startup originating from the Tokyo Institute of Technology, has been gaining quiet popularity among field technicians. By simply attaching a miniature RTK-GNSS receiver to an iPhone or iPad, it allows for positioning, point cloud scanning, marking (on-site marking of survey points), and data visualization using AR, all with centimeter-level accuracy. Furthermore, the data obtained can be instantly shared via the cloud. The price is also very reasonable compared to traditional surveying equipment, and if the concept of one smartphone per person for surveying becomes a reality, it will significantly improve productivity on construction sites.

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Challenges of 3D Surveying and the Role of RTK Technology
There have been several challenges with traditional methods in performing advanced surveying. First, traditional optical surveying (such as total stations) boasts high accuracy, but the equipment is expensive and requires skilled operators. Additionally, to survey large areas of terrain, numerous survey points need to be set up, which is time-consuming and requires significant manpower. While 3D laser scanners can obtain precise 3D data, the equipment is large, expensive, and requires specialized knowledge for data processing. In small to medium-sized civil engineering sites and infrastructure inspections, there has always been a demand for surveying and measurement to be done in a more "convenient" and "efficient" manner.

The key to solving these challenges lies in RTK technology. RTK stands for Real Time Kinematic, a positioning technology that corrects satellite positioning errors in real time to achieve high accuracy. Specifically, it involves two GNSS receivers: a reference station with known coordinates and a mobile station that moves while performing positioning. Both receivers simultaneously observe satellite signals, and by compensating for the errors contained in those signals, the position of the mobile station can be determined with centimeter-level accuracy.

In other words, it is a system that significantly reduces GPS measurement errors through real-time differential positioning. RTK positioning was originally used primarily in the surveying field, but due to its high accuracy, it has recently been applied to aerial surveying with unmanned aerial vehicles (drones), as well as to autonomous farming tractors.

In the construction industry, the importance of immediate and high-precision positioning using RTK is increasing year by year, particularly in areas such as machine guidance for heavy equipment, as-built management, and the efficiency of reference point surveying.

However, traditionally, performing RTK surveying required specialized GNSS receivers, radio equipment, and sometimes the installation of a base station or connection to network-based RTK services (such as Ntrip), making both equipment and operational costs quite high. This is where the fusion of smartphones and RTK technology is expected to provide a solution. By utilizing a smartphone, the number of specialized devices can be reduced, resulting in significant cost reductions and easier operations.

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The Mechanism of RTK Surveying and Point Cloud Scanning Using iPhone

The key point when using an iPhone for RTK surveying is combining the iPhone's sensor technology with RTK-GNSS. Recent iPhones (e.g., iPhone 12 Pro and later) are equipped with a compact LiDAR (Light Detection and Ranging) sensor, which allows for the acquisition of surrounding environments as point cloud data. LiDAR-based point cloud scanning involves emitting infrared laser light onto objects and measuring the distance to the reflection, enabling the collection of 3D coordinates for numerous points in a short time. Traditionally, dedicated 3D laser scanners were required for point cloud measurement, but now, with just a smartphone, this is possible (within a certain range), which is a groundbreaking development.

However, there are limitations to the point cloud and position data that can be obtained using the smartphone alone. The GPS accuracy of standalone positioning may have errors of several meters, and point clouds acquired by the smartphone's LiDAR are only relative data in a local coordinate system, which cannot be directly linked to survey reference coordinates. This is where the combination of an RTK-GNSS receiver proves effective. By attaching an external device like the aforementioned LRTK to the iPhone and obtaining high-precision real-time positioning using the RTK method while performing LiDAR scanning, it becomes possible to assign absolute coordinates (public coordinates) to the acquired point cloud data.

As a result, photos taken with the iPhone and point cloud data acquired will be tagged with centimeter-level location information, and data obtained at different times or with different devices can be accurately overlaid on the same coordinate system. This is the moment when a smartphone transforms into a high-precision 3D surveying instrument.

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So, how is RTK surveying and point cloud scanning conducted using an iPhone? Here is an example of a typical procedure.

1. Attaching and Starting the RTK Device:
   Attach the dedicated RTK-GNSS receiver (e.g., LRTK device) to the iPhone. This device connects wirelessly via Bluetooth, so no complicated cables are needed. Turn on the device and launch the dedicated app on the iPhone.

2. Initialization of High-Precision Positioning:
   Start RTK positioning on the app. For network-based RTK, connect to the Ntrip service, which receives reference station data (correction information) via the mobile network, and corrects the positioning in real time. In Japan, you can use the Geospatial Information Authority of Japan’s electronic reference point network or private correction services. Additionally, devices like the LRTK, which support CLAS (Centimeter-Level Augmentation Service) provided by the quasi-zenith satellite Michibiki, can receive correction signals directly from satellites even in areas without communication coverage, maintaining high-precision positioning. Within a few dozen seconds to a few minutes, RTK achieves the "Fix" solution (a fixed high-precision state), allowing positioning with errors within a few centimeters both horizontally and vertically.

3. Executing Point Cloud Scanning:
   Use the scanning function within the app or a corresponding point cloud acquisition app to scan the target you wish to survey. For example, if you want to measure the volume of fill or excavation terrain, walk through the target area while scanning the surroundings with the iPhone’s camera and LiDAR. The high-precision position, corrected sequentially by RTK, and the numerous points obtained by LiDAR are combined to instantly generate high-precision 3D point cloud data. You can check the point cloud data on the dedicated app or cloud service and perform volume calculations or shape analysis. In fact, according to official information from LRTK, it is easy to calculate the volume of fill, such as by simply scanning the fill area and immediately obtaining the volume calculation results. Complex terrain surveying and as-built management can be efficiently handled with just an iPhone.

4. Recording Survey Points (Single-Point Positioning):
   If necessary, record the coordinates of specific points. Just like with traditional GNSS surveying equipment, by pressing a button on the app at the point you wish to measure, the latitude, longitude, and elevation of that point can be saved with centimeter-level accuracy. This allows you to easily survey benchmark points, boundary stakes, and structure placement locations that were previously recorded manually, all using the smartphone.

5. Stake-out:
   iPhone + RTK also demonstrates its power in stake-out work, which involves reproducing the coordinate values from design drawings or BIM data on-site. For example, when marking the position of a building column, by inputting the design coordinates into the app in advance, the iPhone will display the difference between the current position and the target position in real time. The operator holds the smartphone mounted on an optional monopod (pole) and moves to the designated position according to the guidance displayed, which is where the marking should be made. Vertical offsets (such as the height from the pole tip to the survey point) can also be easily corrected within the app, enabling anyone to easily perform stake-out work. This transforms the traditional transit-based stake-out, which was typically done by a two-person team, into a task that can be completed by one person, fully digitized. Furthermore, with the use of AR functionality, it becomes easy to visualize the design model overlaid on the real-world view while checking the position. Since there is no positional drift with RTK, the AR display aligns perfectly with the physical location, and it won't shift even if there is slight movement.

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As mentioned above, RTK surveying using an iPhone is characterized by its flexibility, allowing for everything from point cloud scanning and single-point measurements to stake-out, all in one. Additionally, the operation is intuitive through a smartphone app, making it easy for those unfamiliar with specialized equipment to handle it after a short period of training. The data can be uploaded to the cloud on-site and shared immediately, allowing for the verification of drawings and models between the site and the office, as well as the instant sharing of information with stakeholders, thus benefiting from the advantages of ICT in construction.

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Use Cases in the Construction Industry
So, how is this new surveying method using iPhone × RTK being applied in the construction industry? Here, we will introduce several examples and potential scenarios, ranging from major general contractors to small and medium-sized civil engineering firms and infrastructure maintenance.

First, large general contractors have started introducing it as part of tools for improving productivity on-site. In one site, each construction manager carries an iPhone with LRTK, allowing them to quickly measure as-built conditions or perform stake-out tasks as needed. Tasks that were previously outsourced to specialized surveying teams are now being handled independently by each department, resulting in reduced waiting times. It is pointed out that if the "one smartphone per person for surveying" concept is fully integrated into site operations, productivity across the entire site will significantly improve. A major advantage is that construction will no longer be interrupted due to waiting for surveying, allowing for faster execution of work.

For small and medium-sized civil engineering contractors, RTK surveying with an iPhone is a cost-effective solution. For example, even in small land development projects or sewer construction, there is a need to assess the current terrain in 3D, calculate earthworks, or record the location of underground utilities. Normally, this would require outsourcing to surveying companies or renting expensive equipment, but with an iPhone and LRTK, it can be easily handled in-house. In a blog from a civil engineering company, it was noted that when using LRTK on a site in a mountainous area, it was praised for being "lightweight, compact, and very easy to use, even for surveying in remote areas."

In mountainous areas or remote locations, just transporting large equipment can be a struggle, but with the iPhone surveying device, which fits in your pocket, mobility is on a completely different level. Additionally, you can record daily construction progress with point cloud data and use it for as-built management or volume calculation. Information that could only be recorded in two dimensions in photos can be saved in three dimensions with point clouds, allowing for flexible analysis such as creating cross-sections or calculating volumes later on.

The iPhone surveying system is also making an impact in the field of infrastructure maintenance. For example, in infrastructure inspections for organizations like JR or highways, tasks such as measuring displacement of roadbeds and structures or recording abnormal areas are part of daily patrols. Traditionally, these tasks were done using visual inspection and camera photos, and in some cases, separate 3D scanner surveys were conducted. However, with the use of iPhone and RTK, both routine inspections and detailed measurements can be performed simultaneously. As a real-world example, during the Noto Peninsula earthquake in 2024, an iPhone equipped with LRTK was used to measure and record the damage.

By measuring the height of utility poles that sank due to ground liquefaction or the depth of cracks in the road on-site, and immediately sharing the data as high-precision, location-tagged photos to a cloud-based system, relevant authorities were able to quickly access the information. In this way, the role of a high-precision surveying device for each person is significant, even in disaster sites and infrastructure inspections, and it is expected that the scope of its use will continue to expand in the future.

Furthermore, in the future, there is potential for applications such as visualizing aging infrastructure on-site using AR technology or simulating repair plans in real-time. RTK surveying with the iPhone is not just a "low-cost surveying device," but is becoming a platform for advancing construction DX.