Photogrammetry vs RTK Surveying: Differences in Accuracy and Efficiency Achieved by Introducing Smartphone LRTK

In the construction industry, surveying technology is undergoing major changes amid the shift to ICT construction and on-site DX (digital transformation). Among these, photogrammetry and RTK surveying are representative technologies that streamline and advance on-site measurement tasks. However, these two methods differ in terms of accuracy, work efficiency, and barriers to adoption. This article compares photogrammetry and RTK surveying from multiple perspectives and explains how the gap can be bridged by utilizing smartphone LRTK (an RTK positioning system compatible with smartphones). Construction and civil engineers, surveying companies, and local government officials interested in ICT construction may find this useful as a hint for promoting on-site DX.

What is photogrammetry? Its characteristics and applications

Photogrammetry is a method that measures three-dimensional shapes and dimensions based on multiple photographic images of an object or terrain taken from various angles. In English it is called photogrammetry, and in recent years aerial photogrammetry using drones (UAVs) has become particularly widespread. The basic principle of photogrammetry is to analyze the parallax of feature points between images taken from different positions and determine the three-dimensional coordinates of each point using triangulation. By reconstructing a large number of points through computation, you can generate point cloud data (a collection of many coordinate points) and orthophotos (composite overhead imagery) of terrain and structures.

The appeal of photogrammetry lies in its ability to grasp wide areas quickly. For example, by flying a drone you can capture a steep slope or a vast development site at once from the air—places where people cannot safely enter—allowing you to acquire current-condition data efficiently and safely. The resulting point clouds and 3D models are visually intuitive, making it easier to share the overall terrain changes or construction progress among stakeholders. Because photogrammetry is based on color images of the subject, the acquired 3D data also contains color information, which yields outputs that are easy to understand visually.

On the other hand, photogrammetry has issues and constraints to be aware of. First, photo capture and data processing require a certain level of knowledge and effort. In the case of drone aerial photography, flight planning and compliance with aviation laws (such as flight permission applications) are required, and after shooting you must process hundreds of photos with dedicated software (image analysis and point cloud generation). Processing requires a high-performance PC or cloud service, so obtaining immediate results on-site is difficult; analysis is usually done back at the office and takes several hours to several days. Regarding measurement accuracy, photogrammetry alone makes it difficult to achieve absolute coordinate precision; it is common to survey reference points (ground control points) in advance and use their coordinates to correct the entire model. With sufficiently high-resolution photos and appropriate control points, the accuracy of the resulting point cloud can be improved to on the order of several centimeters, but without placing control points the entire model can shift by tens of centimeters to more than 1 m (3.3 ft). Additionally, photogrammetry is affected by the shooting environment: drone flights can be difficult in rain or strong winds, and images become unclear at dusk or nighttime when sunlight or light levels are insufficient—therefore there are constraints due to weather and time of day. Thus, photogrammetry is highly useful but has the characteristic that “while you can visualize wide areas, ensuring absolute accuracy and immediacy requires extra measures.”

What is RTK surveying? Its characteristics and applications

RTK surveying uses a satellite positioning error correction technology called real-time kinematic (RTK). Standard GNSS positioning such as GPS can have errors of several meters due to satellite signal errors, but RTK uses two or more GNSS receivers to correct those errors in real time, achieving centimeter-level high-precision positioning (centimeter-level (cm level accuracy (half-inch accuracy))). Typical RTK surveying operates one receiver as a base station installed at a known coordinate and another as a rover that moves while measuring. By comparing satellite data received simultaneously at both stations and sending error information obtained at the base station to the rover via radio, the rover can compute its precise position in real time. In conventional RTK surveying with a ground base station, if operated within a few kilometers, horizontal accuracy of about 1–2 cm (0.4–0.8 in) and vertical accuracy of about 3–5 cm (1.2–2.0 in) can be achieved.

Recently, network RTK that eliminates the need to place a dedicated base station on-site has become widespread. This method integrates nationwide base station data such as the Geospatial Information Authority of Japan’s continuously operating reference station network and sets virtual reference stations near users to distribute correction information; this is called the VRS (Virtual Reference Station) method. Surveyors can carry only a rover (one high-precision GNSS receiver) and connect to the correction service via the internet to obtain accuracy equivalent to having a base station nearby. Network RTK has made centimeter-class surveying (cm level accuracy (half-inch accuracy)) feasible with one receiver per person. However, this method requires a subscription to a monthly correction service and a communication line (mobile internet) to receive correction information.

The merits of RTK surveying lie in its high positioning accuracy and immediacy. Using GNSS receivers, you can obtain the kind of accuracy that a skilled surveyor would achieve with a total station, in real time, significantly reducing the work of establishing control points and post-processing. RTK is powerful in situations that do not tolerate centimeter-level errors—setting out coordinate points (staking and marking), fixed-point measurements for as-built control, confirming property boundaries, etc. Vertical measurements that are unreliable with standalone GPS can also be handled reliably with RTK, making it useful for verifying embankment and excavation heights and for aligning structural installations. GNSS surveying is in principle possible at night or in light rain (as long as satellite signals are receivable), so it offers the flexibility to measure around the clock depending on site needs.

RTK surveying also has caveats. First, surveying is impossible where GNSS cannot be used: dense forests, tunnels, underground spaces, and building shadows block satellite signals, preventing RTK positioning (in such locations, conventional total stations and the like are still required). Accurate measurements also require stabilization time for each measurement, such as keeping the rover stationary for several seconds, so continuous precision recording while moving is difficult (although the latest IMU-equipped GNSS devices improve dynamic measurement, typical operations measure point-by-point). In terms of data volume, each RTK observation yields a single coordinate, so many points must be captured to record a wide area in detail; this differs fundamentally from photogrammetry, which measures areas at once. Moreover, the cost of adoption has been a significant barrier: high-precision GNSS receivers and radio equipment were very expensive, historically requiring initial investments on the order of millions of yen, and transporting and setting them up required specialized know-how. Thus, RTK surveying is “excellent in accuracy and immediacy but constrained by dedicated equipment and environmental conditions.”

Accuracy comparison between photogrammetry and RTK surveying

When choosing a surveying method, the accuracy difference is the first concern. Both photogrammetry and RTK surveying provide practically sufficient accuracy, but their natures differ.

• Photogrammetry accuracy: The accuracy of point cloud data and models obtained by photogrammetry is strongly affected by shooting conditions and control point preparation. If you shoot high-resolution photos with sufficient overlap and place control points with known coordinates appropriately on-site, the relative accuracy (point dispersion) of the generated point cloud can be improved to the order of a few centimeters. In particular, RTK-equipped drones have emerged recently, and applying positioning corrections to aerial images themselves can ensure accuracy even with fewer control points in some cases. Nevertheless, absolute accuracy (the correctness of coordinates in a global coordinate system) often still relies on conventional ground surveying corrections; without effort, errors of several tens of centimeters can occur. Vertical accuracy tends to be poorer than horizontal, and aerial photogrammetry can leave height representation errors on the order of several tens of centimeters in some cases.

• RTK surveying accuracy: RTK surveying, by design, yields positioning results that fall directly within centimeter-level error ranges. GNSS that would be off by 5–10 m when used alone can be corrected by RTK to about 1–2 cm horizontally and 3–5 cm (1.2–2.0 in) vertically. However, this accuracy is only achieved under good reception conditions and when a fixed solution (Fix) is obtained. In environments where satellites cannot be sufficiently tracked, RTK solutions become unstable and measurements may be impossible. Still, in typical outdoor sites, RTK’s precision is comparable to traditional first-class surveying for obtaining point coordinates. If photogrammetry is a method that reproduces the shape of an area with acceptable accuracy, RTK surveying is a method that measures each point accurately and reliably; for high-accuracy control point surveying and boundary confirmation, RTK is generally superior.

Work efficiency comparison: time for data acquisition and processing

Next, compare photogrammetry and RTK surveying from the perspective of work efficiency. On-site measurement time and data processing effort directly affect cost and schedule.

• Photogrammetry efficiency: Photogrammetry excels in rapid fieldwork and wide-area data acquisition. For example, a drone photogrammetry operator can photograph tens of thousands of square meters in a short time, covering an area that would take several days by conventional ground surveying in tens of minutes. Dangerous cliff faces and hazardous spots that are difficult to survey on foot can be recorded from the air, improving safety efficiency. However, subsequent data processing takes substantial time: generating point clouds and models from captured photos can take several hours or more on a high-performance PC. Photogrammetry is unsuitable when immediate on-site confirmation is needed—the lead time to wait for analysis results occurs. Also, acquired data can be enormous (point clouds numbering tens of millions of points), so data organization and utilization require time and skill. Thus, photogrammetry’s efficiency characteristic is “fieldwork is speedy, but office work time is required to obtain results.”

• RTK surveying efficiency: RTK surveying delivers limited information per observation but has outstanding immediacy, with results available on-site. Measuring points is a repetitive process of setting the rover at prescribed positions and observing for several seconds. While it takes longer than photogrammetry to measure detailed wide-area terrain, each observed point coordinate can be checked immediately on-site, so waiting for data processing is virtually unnecessary. For example, if you need to check heights at several locations on a development site, RTK allows you to measure elevations on the spot and immediately judge deviations from design values. Since complex post-processing is unnecessary, RTK has strong capability for on-site validation and additional measurement. However, mapping surfaces requires many point measurements, which imposes limits in terms of manual measurement. When converting measurements into drawings or calculating earthwork volumes, you may need to derive cross-sections point by point rather than work automatically from a point cloud. Overall, RTK surveying’s efficiency characteristic is “precise data are obtained in real time, but detailed wide-area mapping requires time.”

Comparison of adoption barriers: cost, equipment, and skills

When introducing new technology on-site, you must also compare the barriers in terms of cost, required equipment, and skill acquisition. Let’s look at the burdens of adopting and operating photogrammetry and RTK surveying.

• Photogrammetry adoption barriers: To start photogrammetry, you need high-quality cameras and drones as shooting equipment. Although commercial drones and compatible software have become cheaper over time, purchasing a drone, spare batteries, a high-performance PC, and cloud processing services still requires a substantial initial investment. Operators must have drone piloting skills and knowledge of aviation law, and acquiring qualifications or permissions requires effort. In addition, operating analysis software and processing point cloud data requires expertise and takes time to master. In other words, photogrammetry is often performed by specialized staff or outsourced rather than “anyone can do it immediately,” and accumulating know-how in-house requires effort. Operationally, you must also handle drone storage and maintenance, software license renewals, and monitoring legal regulations. However, once the system is in place, significant efficiency gains are obtainable for wide-area as-built control and pre/post-construction terrain comparisons, so the investment can be justified.

• RTK surveying adoption barriers: Traditionally, the biggest obstacle to RTK adoption was the high initial cost. High-precision GNSS receivers themselves are expensive, and a two-unit set of base and rover plus communication equipment could require investments on the order of millions to tens of millions of yen. Therefore, adoption was not easy except for large surveying companies or public projects. The equipment’s high value and precision also required careful handling and management on-site, and operation and setup required specialized skills. With the spread of network RTK, only one receiver is now necessary, but the receiver still needs to be purchased as a dedicated device, and monthly correction service fees (tens of thousands of yen depending on region) and communication costs apply. Thus, RTK surveying has historically been high-performance but costly, requiring ongoing running costs and specialized knowledge. Additionally, result utilization often follows conventional CAD drawing workflows and is less automated or digitized than photogrammetry. Recently, however, usability has improved with surveying apps that run on tablets and general-purpose devices.

Operability and on-site responsiveness comparison

Finally, compare day-to-day operability and on-site responsiveness. How flexibly a technology adapts to changing environmental conditions and on-site demands is an important viewpoint.

• Photogrammetry on-site responsiveness: Photogrammetry excels at comprehensively recording site conditions. From pre-construction terrain to as-built during construction and completed structures, keeping a time-series 3D record allows you to verify dimensions and calculate changes later as needed. In particular, as-built inspections benefit from photogrammetric point clouds, which allow detailed verification during on-site inspections and help align understanding between client and contractor. However, as noted above, photogrammetry lacks immediacy and is not suitable when on-the-spot results are required. Drone flights are also affected by battery life and weather, making ad hoc immediate measurement difficult (sudden flights without a flight plan carry risks). Still, if you perform photogrammetry weekly or at key milestones, you can analyze site-wide changes retrospectively, making it highly effective for medium- to long-term data accumulation. Operationally, even without seasoned specialists, automatic flight apps now allow a degree of automated shooting, making photogrammetry increasingly accessible to small businesses and government staff.

• RTK surveying on-site responsiveness: RTK surveying can instantly respond to detailed measurement needs demanded on-site. For example, if an additional survey point not shown on drawings is needed during construction, an RTK receiver can quickly measure and obtain coordinates on the spot and immediately compare them with design values—this quickness is unmatched by photogrammetry. For pinpoint checks like staking or height confirmation, having a single RTK unit on-site greatly improves efficiency. In terms of environmental adaptability, RTK depends on GNSS reception and is weak in some locations, but in open outdoor sites it can be used repeatedly regardless of time of day. Operator skill requirements have improved due to manufacturer support and simplified operation modes. Nonetheless, RTK is not suited to wide-area integrated understanding like photogrammetry, so the common on-site practice is to use photogrammetry for broad situation awareness and RTK for local, immediate measurements. Combining both enables both overview recording and precise point measurements.

So far we have reviewed the characteristics of photogrammetry and RTK surveying in terms of accuracy, efficiency, adoption hurdles, and operability. Each has strengths and weaknesses, and it is wise to use each method appropriately according to site needs. Now, what exactly is the increasingly notable smartphone LRTK, and how can it bridge the above gaps?

Emergence of smartphone LRTK: RTK surveying realized on a smartphone

In recent years, a new approach called smartphone LRTK has emerged. LRTK refers to a smartphone-style RTK surveying system, named from the concept of a “Pocket RTK” that fits in your pocket. It combines an ultra-compact high-precision GNSS receiver device with a smartphone to perform RTK positioning in a palm-sized, one-handed operation. Specifically, a thin receiver terminal weighing around 100–200g is attached to the back of a smartphone (or mounted on a monopod/pole) and connected to the phone via Bluetooth or Wi‑Fi. The small terminal includes a high-precision antenna and receiver, battery, and communication module, and positioning and data recording are controlled from a dedicated app.

The mechanism by which smartphone LRTK achieves high-precision positioning is basically the same as conventional RTK. Correction information can be received via the internet, and advanced models can directly receive the Michibiki satellite-delivered free centimeter-class augmentation service (CLAS). In the latter case, correction data can be acquired from satellites even in mountainous areas without mobile coverage, making it a hybrid type that maintains RTK-level accuracy regardless of communication environment. With smartphone LRTK, there is no need to set up a base station on-site; you can start the app on your phone and obtain a fixed solution (Fix) in just a few tens of seconds. Because operation follows on-screen app guidance, people without specialized knowledge can handle it easily—a major feature.

The arrival of smartphone LRTK has made RTK surveying, which used to be expensive and specialized, much more accessible. The hardware configuration is simply “smartphone + small GNSS receiver,” and initial costs are dramatically lower than purchasing dedicated equipment. Leveraging existing smartphone functions eliminates the need for a dedicated controller, and new features can be added via app updates. In terms of operating cost, in areas with mobile coverage you can use conventional network RTK services, and in mountainous areas you can switch to CLAS satellite augmentation without additional fees—optimizing communication and service costs according to circumstances. Furthermore, smartphone LRTK systems often include automatic synchronization of acquired positioning and point cloud data to cloud services, enabling large-scale point cloud processing and sharing over the internet and reducing the burden of data management and analysis software. Overall, smartphone LRTK can be described as a next-generation surveying solution that “retains RTK-grade accuracy and reliability while combining the ease of use and digital integration of photogrammetry.”

How smartphone LRTK closes the gap between photogrammetry and RTK surveying

How does introducing smartphone LRTK change the gaps between photogrammetry and RTK surveying? The benefits smartphone LRTK brings for each perspective are summarized below.

• Closing the accuracy gap: Smartphone LRTK uses the RTK method and can independently achieve centimeter-level positioning (cm level accuracy (half-inch accuracy)). This can easily compensate for the absolute coordinate accuracy shortcomings of photogrammetry. For example, in drone photogrammetry, control point surveying used to require effort, but with smartphone LRTK you can measure several reference points on-site immediately and correct photogrammetric models to high accuracy. Some smartphone LRTK systems also have point cloud scanning functions, and by combining the smartphone camera or AR technology you can obtain point cloud data with absolute coordinates in real time. This fusion of photogrammetry’s visual 3D data and RTK’s high-precision coordinates enables seamless model accuracy verification and additional measurement.

• Closing the efficiency gap: Photogrammetry is efficient for wide-area data acquisition, while RTK excels at real-time point measurements. Smartphone LRTK incorporates the best of both. If every person carries a smartphone LRTK, they can start high-precision surveying immediately when needed, eliminating time loss caused by “not being able to measure when you need to.” For example, site supervisors and technicians can measure with smartphone LRTK themselves and share data with the office via the cloud, speeding up decision-making. With point cloud scanning, you can perform some 3D modeling on-site in near real time, drastically reducing lead time from data acquisition to utilization. As a result, overall site work efficiency improves, enabling flexible surveying and inspection even under tight schedules.

• Reducing adoption barriers: The biggest impact of smartphone LRTK is lowering barriers to adoption. Existing smartphones or tablets can be used, so only a small receiver is required, significantly reducing initial investment. Without purchasing dedicated equipment costing hundreds of thousands to millions of yen, you can attain centimeter-grade positioning. Intuitive UIs in smartphone apps make operation easy for younger engineers familiar with smartphones, reducing the burden of internal training. Smartphone LRTK helps overcome the challenges of securing specialized personnel and procuring expensive equipment that hampered photogrammetry and RTK adoption, making it easier for small surveying companies and local governments to adopt cutting-edge technology and accelerating industry-wide DX.

• Improving operability and on-site responsiveness: Since smartphone LRTK is based on a familiar smartphone, it is easy to carry and use daily. Traditional RTK equipment was handled as occasional special equipment stored in a warehouse, but smartphone LRTK can be kept in a pocket so that site personnel can measure on the spur of the moment. This reduces outsourcing and rework, enabling immediate response to sudden changes or additional inspection items. With automatic cloud upload of positioning data, on-site measurements can be shared in real time with supervisors or clients in the office, closing the information gap and enabling timely consensus building. Combining photogrammetric 3D models with daily point clouds and coordinates obtained by smartphone LRTK enables continual digital “visualization” of construction progress and as-built control.

As described, smartphone LRTK can fill weaknesses and leverage strengths of both photogrammetry and RTK surveying, greatly advancing how on-site surveying is conducted. It can serve as a bridge that fills gaps in accuracy, efficiency, cost, and operability.

Contribution of smartphone LRTK to on-site DX

Using smartphone LRTK does more than just streamline surveying operations; it strongly supports construction site DX.

First, it promotes data-driven construction management. Combining abundant field data from photogrammetry and point cloud measurement with high-precision position information provided by smartphone LRTK links all information to geographic coordinates. This enables quantification and visualization of as-built and progress as evidence, enhancing quality control and making construction processes more transparent. For example, overlaying point clouds obtained by smartphone LRTK with design 3D data on-site can instantly highlight design deviations (excess excavation, insufficient fill, etc.) by color-coding, enabling early correction and shortening the PDCA cycle to reduce errors and rework.

Next, it facilitates smoother communication. On-site DX requires real-time digital information sharing and collaborative problem-solving among site and office, and between clients and contractors. Measurement data collected by smartphone LRTK can be viewed and used immediately on the cloud, allowing accurate situation assessment from remote locations. This enables appropriate instructions and decisions without travel, reducing travel time and speeding up communication. For local government officials overseeing multiple sites, centralizing measurement data via smartphone LRTK allows progress checks and inspections without site visits, contributing to work-style reform.

Moreover, smartphone LRTK aids human resource development and skill transfer. Advanced surveying has traditionally relied on veteran craftsmen, but intuitive tools like smartphone LRTK allow younger technicians to gain early experience in high-precision surveying. Such simple surveying tools serve as ideal introductory platforms for developing digital personnel needed for on-site DX, bridging the knowledge of experienced technicians with the IT skills of a new generation.

Policies such as the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* strongly encourage the use of ICT earthwork and 3D surveying. Smartphone LRTK is becoming a viable third option alongside photogrammetry and RTK surveying, bringing these policy goals within reach. Providing an environment for digital construction without expensive equipment raises the industry baseline, sharing the benefits of productivity improvements and work-style reforms broadly.

Recommendation for simple surveying using smartphone LRTK

Understanding and combining the advantages of photogrammetry and RTK surveying is increasingly important on future construction sites. By introducing smartphone LRTK as a bridge, simple surveying with the motto “anyone, immediately, anywhere” becomes a reality.

For example, tasks that previously required specialized contractors can now be performed easily by site staff using smartphone LRTK:

• Simple as-built measurements: Scan the shape of fills or excavations with smartphone LRTK and immediately check differences from the design. Calculations of additional fill volumes or decisions to correct excess excavation can be made the same day.

• Quick establishment of reference points: If new reference points are needed near work areas, smartphone LRTK can set them on the spot. Tasks that previously required waiting for orders can proceed without delay.

• Verification of property boundaries and element positions: When rechecking boundary stakes or installation positions of structures, acquire coordinates with smartphone LRTK and immediately compare with design values to prevent disputes and enable quick corrective action.

• Minimal control points for photogrammetry: Before wide-area drone photogrammetry, measuring only a few high-precision points with smartphone LRTK yields high-accuracy models during subsequent photo processing, greatly simplifying control point installation.

Smartphone LRTK’s simple surveying is useful in many on-site situations. No large-scale investment or preparation is required—just attach a small device to the smartphone and you can start. For companies and local governments unsure where to begin with on-site DX, trying smartphone LRTK is a good first step. Making high-precision positioning accessible is likely to reveal new ideas for efficiency and solutions to problems.

This article compared photogrammetry and RTK surveying. The key takeaway is to “use both appropriately and leverage new technologies as connectors.” Smartphone LRTK realizes the best of both worlds, making surveying operations accessible to everyone. To robustly promote on-site DX while balancing accuracy and efficiency, consider introducing smartphone LRTK for simple surveying. Your site is likely to experience impressive efficiency gains and increased confidence thanks to the latest technology.