Photogrammetry Accuracy Revolution: High-Precision Point Cloud Measurement by a Single Person with Smartphone RTK

Improving the accuracy of photogrammetry has long been a challenge in the construction industry. This article starts from that challenge and unravels the solution provided by smartphone RTK technology, which enables centimeter-level surveying (cm level accuracy, half-inch accuracy) by a single person. We explain the mechanism and benefits of this new technology, which can be called a revolution in photogrammetry accuracy, and finally introduce the easy-to-use solution LRTK.

The Potential of Photogrammetry and Its Accuracy Challenges

In construction and civil engineering sites, photogrammetry (photogrammetry) using drones and digital cameras is rapidly becoming widespread. Photogrammetry, which can generate three-dimensional point cloud models from photographs, is powerful for terrain surveys, as-built management, and construction planning reviews, and it has brought significant efficiency gains compared to traditional manual surveying. However, photogrammetry is plagued by issues of accuracy. In tasks such as verifying as-built conditions or checking against design values, even errors of several tens of centimeters can be problematic, so photogrammetry alone is often insufficient and re-measurement with instruments like total stations is not uncommon.

Several factors can make photogrammetry accuracy unstable. For example, the detail of 3D reconstruction is influenced by the resolution of the captured images (the GSD: ground sampling distance) and the number and angles of photos taken, and errors can accumulate in image analysis algorithms. But the biggest factor is the “reference for position.” Typically, point cloud data obtained by photogrammetry can capture relative shapes but require effort to achieve absolute coordinate accuracy. Traditionally, it was necessary to place multiple markers known as ground control points (GCPs) on site and survey their precise coordinates. Only by importing those GCPs as references during photo processing could the entire model be aligned to the correct coordinates. This work requires advanced surveying equipment and effort, undermining the ease of photogrammetry. A shortage of GCPs can cause large positional shifts across the entire model, making it unusable for precise measurements.

Also, the accuracy of GPS built into typical cameras and smartphones has limits. Standalone GPS usually has planar position errors on the order of about 5-10 m (16.4-32.8 ft), and using that as coordinates for photogrammetry does not produce high-precision point clouds. Reducing errors requires expensive equipment or painstaking corrections, and thus a “wall of accuracy” has existed in mastering photogrammetry.

The Emergence of Smartphone RTK: Centimeter-Level Surveying by One Person

A new technology has emerged that dramatically solves these photogrammetry accuracy issues: the approach called smartphone RTK. Smartphone RTK is a system that integrates RTK-GNSS (real-time kinematic positioning) capability into a smartphone to enable high-accuracy positioning. What used to require dedicated surveying GPS receivers for centimeter-level positioning can now be achieved by combining a smartphone with a small device.

The RTK method uses correction information from a reference station to reduce satellite positioning errors to a few centimeters. Previously, you had to install your own base station at each survey site or subscribe to an expensive network RTK service. Recently, however, RTK solutions that are easy to use for smartphones have begun to appear. In smartphone RTK, a compact GNSS receiver with an integrated antenna attached to the phone acts as the rover and receives correction data in real time via the internet or satellite broadcast. This allows centimeter-level positioning (cm level accuracy, half-inch accuracy) comparable to dedicated equipment even with a smartphone.

Notable is the ease and the possibility of solo operation. Smartphone RTK receivers are small and light enough to fit in a pocket, weighing only a few hundred grams. There is no need to carry tripods or heavy equipment; upon arrival at site you can power on the smartphone and receiver and start surveying immediately. Because it can be carried and operated by one person, surveying work that previously required two people can be completed by one person. This removes the need for the skilled operation of a total station or for placing targets at height using heavy machinery, improving safety as well.

The Accuracy Revolution in Photogrammetry Brought by Smartphone RTK

With the dramatic improvement in position accuracy provided by smartphone RTK, the world of photogrammetry changes entirely. The previously cumbersome process of aligning to absolute coordinates is nearly automated, and accurate 3D models can be obtained from captured photos almost immediately. Specifically, by importing the capture coordinates (latitude, longitude, height) of each photo obtained with smartphone RTK into photogrammetry software, these positions act equivalently to the GCPs that used to be placed separately. Because all images are tagged with high-precision position data, software misalignments can be minimized, and the resulting point clouds and orthophotos are from the start aligned to the site coordinate system.

This accuracy revolution lays the groundwork for applying photogrammetry to centimeter-order accuracy checks and as-built management, which were previously considered difficult with photogrammetry. For example, with smartphone RTK photogrammetry you can directly confirm pavement thickness or excavation depth from point cloud data, or check the installation accuracy of structures on site. The need for repeated additional measurements to account for errors is reduced, streamlining quality control on site.

Furthermore, this technology accelerates on-site DX (digital transformation). Precision surveying that used to be left to specialists can now be performed quickly by on-site personnel themselves, promoting real-time data utilization. In cloud-connected systems, point clouds and photos captured can be shared the same day so stakeholders can immediately grasp the situation, speeding up construction management decisions. Smartphone RTK contributes not only to improved photogrammetry accuracy but also to faster on-site decision-making processes.

Use Cases for High-Precision Point Cloud Data

High-precision 3D point cloud data acquired with smartphone RTK can be used in various construction and civil engineering scenarios. Here are some specific use cases.

• As-built management: Record post-construction terrain and structures as point clouds and verify as-built conditions against design data. With high-precision point clouds, you can accurately measure embankment and cut volumes or structural dimensions, helping prevent rework and ensure quality.

• Progress management: Regular photogrammetry of a site to generate point clouds lets you visualize construction progress in 3D. Whereas progress was traditionally communicated with planar photos or reports, overlaying point cloud models enables intuitive understanding. You can immediately calculate quantities for progress payments and schedule control by comparing with past point cloud data.

• Construction planning and review: Use the acquired point cloud data for simulations and planning. For example, detailed 3D site information helps consider routes for heavy equipment delivery or layouts for temporary yards. Overlaying design models (BIM/CIM data) on the point cloud makes clash checks and sharing completed-image visuals with stakeholders easier before construction.

• Maintenance and disaster response: High-precision photogrammetry data are effective for regular infrastructure inspections and rapid damage assessment after disasters. Point clouds can capture cracks and deformations, or quickly record the conditions of damaged areas for recovery planning. Because one person can survey large areas, initial response speed is also increased.

These uses allow point clouds to be integrated into formal survey deliverables such as drawings and reports, not just for situational awareness. The Ministry of Land, Infrastructure, Transport and Tourism is promoting use of 3D data as part of "i-Construction," and high-precision point clouds obtained from sites will become an increasingly important foundation. Their utilization value will continue to grow.

Comparison with Traditional Methods: Efficiency and Accuracy Benefits

How does photogrammetry using smartphone RTK compare with conventional surveying methods? Below are comparisons with some representative methods.

• Total station surveying: Highly accurate instruments can measure down to millimeter levels, but setup and tasks like reversing sights and placing backsights are complex and usually require two or more people. Regular calibration and maintenance are necessary to maintain accuracy, and sending the equipment to the manufacturer for inspection adds overhead. A total station measures single-point coordinates at a time, so many points must be measured to understand the whole terrain. With smartphone RTK photogrammetry, one person can rapidly acquire wide-area 3D data, including local details, with no omissions. The accuracy at the centimeter level (cm level accuracy, half-inch accuracy) is sufficient for construction management, and where needed, a total station can be used selectively for precision checks.

• Conventional RTK-GNSS surveying: Traditional RTK surveying using a base station and rover can be performed by one person, but equipment costs are high and setting up the base station and communications takes time and expertise. RTK records positions point by point, so it is not well suited for modeling whole terrains. Smartphone RTK can be operated easily with inexpensive devices and a smartphone app, capturing data as a point cloud so you don’t have to connect individual points to create terrain maps.

• UAV (drone) photogrammetry: Drones can capture large areas from above in a short time, but are subject to aviation regulations, battery flight time limits, and sometimes require expensive RTK-equipped drones and many GCPs. Ensuring accuracy often still requires some ground surveying. Smartphone RTK is limited to areas accessible by foot but has no flight-permission hurdles and can be used in obstacle-rich urban areas or indoors. Ground-based smartphone photos also capture details and vertical faces that aerial photos might miss. Combining drone and ground smartphone captures as needed enables more precise and gap-free surveying.

• Terrestrial laser scanner measurement: Fixed or mobile laser scanners can deliver millimeter accuracy and dense point clouds but are expensive and require specialized handling. Large models take time to set up and data processing and alignment are laborious. Methods using smartphone RTK can obtain sufficiently dense point clouds for many purposes without special equipment. If combined with built-in LiDAR sensors on devices like iPhones, scanning can be automated and point clouds acquired in real time. Cost-wise, this approach can require only a fraction of the investment of a dedicated scanner.

From these comparisons, smartphone RTK photogrammetry stands out for its balance of ease and accuracy. Traditional instruments still have their place where absolute accuracy and detail are critical, but for routine as-built checks and small-site surveys, smartphone RTK can cover most needs. Above all, the revolutionary point is the ease of being able to measure solo whenever needed.

Recommendation for Easy Surveying with LRTK

So how should you introduce smartphone RTK that brings about this photogrammetry accuracy revolution? One answer is to use the smartphone RTK system LRTK. LRTK is an ultra-compact RTK positioning device that works with the iPhone, appearing as a solution that transforms a smartphone into a versatile surveying instrument.

Attach LRTK to a smartphone and centimeter-precision positioning (cm level accuracy, half-inch accuracy) starts immediately within the dedicated app. The receiver, about 165 g in weight and smartphone-sized, runs for over 6 hours on its internal battery and provides position information to the phone via Bluetooth. It supports CLAS, the centimeter-level augmentation service provided by Japan’s quasi-zenith satellite system “Michibiki,” enabling stable high-precision positioning even in mountain areas or other places outside mobile coverage.

The LRTK app integrates various functions to support on-site surveying. For example, its 3D point cloud scanning using the iPhone camera and LiDAR lets you obtain high-precision point clouds with absolute coordinates simply by walking slowly around the site. Captured data are automatically uploaded to the cloud, allowing office staff to review the 3D model remotely. You can measure distances, areas, and volumes on the measured point cloud, or overlay design CAD data to detect differences, all on site.

Moreover, LRTK covers not only photogrammetry but also solo stakeout (layout) and AR-based as-built inspection. Using the coordinate navigation function, you can specify reference points or structure coordinates from the survey design and have the smartphone guide you to those positions on site. Tasks like stakeout, previously done by a surveyor and a worker in pairs, can now be performed solo and accurately with LRTK. It also has a function to overlay point clouds and 3D design models and display them in AR on site, enabling immediate verification of finishes. This helps early detection of construction errors and prevents rework, and it can also aid in presenting completion images to clients.

In this way, LRTK is a powerful all-in-one tool that supports surveying and measurement tasks from photogrammetry to drawing creation, stakeout, and inspection. Its operation is simple, and with minimal training it can be used on site right away. Users who introduced LRTK have reported results such as “I could scan a large slope alone after a 5-minute briefing” and “we completed as-built surveying in-house in a short time instead of outsourcing.” There are also cases where local governments adopted smartphone RTK surveying for disaster investigations, speeding recovery work and reducing costs.

The smartphone RTK technology that can be called a photogrammetry accuracy revolution will greatly change surveying styles in the construction industry. By using easily introduced systems like LRTK, you can extract maximum information with a minimum number of people. In an era of labor shortages and work-style reforms, the option to measure even as a single person will be a strong ally on site. Adopt the new measurement method of photogrammetry plus smartphone RTK and experience its benefits on your sites.