Can Mobile RTK Provide Sufficient Accuracy for Construction Surveying?

Table of Contents

• Introduction

• What is RTK Surveying?

• Why Centimeter-Level Accuracy Is Necessary in Construction Surveying

• Traditional RTK Surveying and Its Challenges

• Background Behind the Feasibility of Mobile RTK Surveying

• Advantages of Mobile RTK Surveying

• What Mobile RTK Surveying Can Do

• Accuracy and Performance of Mobile RTK Surveying

• Simple Surveying with LRTK

• FAQ

Introduction

With the recent spread of smartphones, we can now easily make use of location information. Smartphone GPS is useful in many daily and work situations, such as checking your current position in a map app or recording the coordinates of a photo’s location. However, the typical smartphone GPS position accuracy has an error on the order of several meters, which is insufficient for precise surveying on construction sites. For tasks such as setting out building positions (batter boards), measuring site boundaries, and managing as-built conditions in civil works, deviations of a few centimeters can exceed acceptable tolerances and lead to serious problems. This is why RTK surveying using GNSS (Global Navigation Satellite System) — a centimeter-level (half-inch accuracy) high-precision positioning technology — has attracted attention. In this article, we answer the question “Can mobile RTK provide sufficient accuracy for construction surveying?” by explaining how RTK surveying on mobile devices works and what performance it can achieve. We also introduce solutions that enable high-precision positioning using only a smartphone, without relying on traditional surveying instruments.

What is RTK Surveying?

RTK surveying, short for Real Time Kinematic, is a surveying method that uses GNSS to correct positioning errors in real time and obtain high-precision positions. Ordinary GPS or GNSS positioning with a single receiver accumulates small delays and clock errors in satellite signals, resulting in errors on the order of meters, which does not meet the accuracy required for construction surveying. RTK surveying uses two or more receivers (a reference station and a rover) simultaneously, calculating the relative error between the fixed reference station’s known position and the rover’s measurements in real time to correct the rover’s positioning results. By precisely correcting the relative position with respect to the reference point, RTK can achieve extremely high positioning accuracy within a few centimeters. In other words, RTK is a technology that dramatically improves accuracy by canceling error factors contained in satellite signals via a reference station.

Why Centimeter-Level Accuracy Is Necessary in Construction Surveying

Why is centimeter-level accuracy necessary for surveying on construction sites? Because a difference of a few centimeters can mean a large difference on site. For example, in setting out building positions (batter boards), if reference lines or stakes are not placed at the exact positions shown on the design drawings, the entire building could be tilted or encroach on neighboring land. In civil engineering, if the alignment of a road or bridge is off by a few centimeters, joints between structures may malfunction, or construction may stray beyond property boundaries. As-built management (verifying that completed structures or formed terrain match the design) also requires the ability to detect small height and positional deviations. For instance, a difference of a few centimeters in embankment height can affect drainage slopes, making precise measurement key to quality control. From these site requirements, RTK surveying, which can measure positions with an accuracy of ± a few centimeters, plays a very important role in construction and civil engineering. While centimeter-level accuracy is also useful in fields like agriculture and disaster response, it is particularly essential in construction surveying to proceed with work confidently.

Traditional RTK Surveying and Its Challenges

RTK surveying itself is not new; it has long been used as a means of high-precision positioning. However, traditional RTK surveying had the issue that it required expensive, large dedicated equipment. Typically, a high-precision GNSS receiver mounted on a tripod would be set up as a reference station, and another receiver would be used as a rover to visit survey points. Just acquiring two receivers, tripods, and communication equipment could cost several million yen, making adoption difficult for individuals or small sites. The bulky equipment also made transport and setup time-consuming, and technicians with positioning expertise were needed to configure and operate the systems. In some cases, radio licenses or base-station permissions were required to transmit correction information, making the method far from easily accessible to everyone. Because of these high hurdles, RTK surveying was mostly used by a limited number of surveyors or on large-scale projects.

Background Behind the Feasibility of Mobile RTK Surveying

Recently, this situation has been changing significantly. Technological advances have produced small, affordable RTK-capable GNSS receivers that can be used with smartphones and tablets. Smartphones themselves have become high-performance enough to obtain raw positioning data from internal GNSS chips and process it in real time. At the same time, the spread of network infrastructure has popularized a mechanism called Ntrip for receiving correction information over the Internet. Ntrip distributes correction data from government or private continuously operating reference station networks and positioning services, enabling real-time corrections without having to set up a dedicated base station. In Japan, the Quasi-Zenith Satellite System “Michibiki” also offers the CLAS centimeter-class augmentation service, which allows compatible receivers to obtain high-precision correction information even without an Internet connection. These technological foundations have made it possible to realize RTK surveying with just a smartphone plus a small GNSS receiver. Devices that once provided centimeter accuracy as fixed, bulky instruments are now handheld and pocketable, bringing a major change to surveying workflows on site.

Advantages of Mobile RTK Surveying

Mobile RTK surveying using smartphones offers many advantages compared with traditional methods. The main benefits are summarized below.

• Mobility and ease of use: All you need is a smartphone and a palm-sized small receiver, eliminating the need to transport bulky equipment. The ability to carry a pocketable device and use it whenever needed increases site operational mobility. It’s easy for one person to walk to multiple points and measure them.

• Low cost: Initial costs are greatly reduced compared to dedicated GNSS surveying instruments. There is no need to share expensive equipment within a team, making a one-per-person deployment realistic and improving on-site productivity.

• Ease of operation: Intuitive smartphone app interfaces allow surveying without specialized knowledge. With a tap on the screen you can record points and perform various calculations, lowering the barriers to surveying work.

• Real-time sharing: Since smartphones have connectivity, positioning data can be uploaded to the cloud from the field and shared with office staff immediately. This smooths communication between the field and office and enables quick decision-making and feedback based on measured data.

• Multi-function integration: By combining smartphone cameras and various sensors, one device can handle photo measurement, AR display, 3D scanning, and more. Tasks that previously required separate devices and processes can be handled all-in-one with smartphone RTK surveying.

In these ways, mobile RTK surveying lowers the barriers of equipment, cost, and operability, making field surveying more accessible and efficient.

What Mobile RTK Surveying Can Do

Combining a smartphone with an RTK-capable receiver enables a wide range of on-site capabilities. Below are the main functions and use cases made possible by mobile RTK surveying.

• High-precision point surveying: By holding the receiver over the point to be measured and tapping a button on the smartphone screen, you can record the latitude, longitude, and height of the point with centimeter-level (half-inch accuracy) precision. You can add timestamps, point names, and notes for each measured point, making it easy to manage and review data later.

• Simplified setting out: Marking positions on site according to coordinates specified in design drawings is easy with smartphone RTK. By checking direction and distance to the target point in a smartphone app while moving, one person can accurately perform setting out that previously required transits or total stations and multiple people. It can also guide stake-driving positions and find reference points hidden by vegetation or snow.

• 3D point cloud scanning: By using a smartphone equipped with a LiDAR sensor, you can 3D-scan surrounding structures and terrain to obtain high-density point cloud data. Combining the position corrections from RTK with the scanned data allows you to assign accurate coordinates to the point cloud and place it to scale on a map. This enables quick as-built evaluations and creation of BIM/CIM models based on on-site scans.

• Photogrammetry and records: High-precision location tags can be automatically attached to photos taken with the smartphone camera and saved to the cloud. For example, photos of concrete cracks or disaster sites will be plotted accurately on a map, so when you return to the office you can immediately identify the locations of images, streamlining survey records and reports.

• Instant distance, area, and volume calculations: Measured coordinate data can be analyzed on site to compute distances between points, areas of enclosed regions, and cut-and-fill volumes automatically. For example, you can roughly estimate the soil quantity to be transported during earthworks and decide the number of dump trucks to arrange right from the field.

• AR visualization: By loading design drawings or 3D models into the smartphone, AR technology can overlay them on real-world images. Thanks to the high-precision positioning of mobile RTK, you can project design models accurately on site without noticeable drift, enabling stakeholders to share the expected finished appearance and intuitively verify as-built conditions.

Thus, mobile RTK surveying is not limited to point measurement; it can handle everything from measurement and records to construction support in an all-in-one workflow. The ability to use measured data on site for immediate visualization and subsequent tasks strongly supports digital transformation (DX) at construction sites.

Accuracy and Performance of Mobile RTK Surveying

Now to the main question — “Can mobile RTK provide sufficient accuracy for construction surveying?” — we will answer by looking at concrete accuracy figures. In short, with appropriate conditions, positioning using a smartphone plus an RTK receiver more than meets the accuracy required for construction surveying. Actual verification results indicate that the typical positioning error for a smartphone combined with a small RTK receiver is about ±1–2 cm (±0.4–0.8 in) horizontally and about ±3 cm (±1.2 in) vertically. This means that in many site scenarios positions can be determined within a few centimeters, which is acceptable for routine construction surveying tasks. Furthermore, by averaging measurements taken while stationary for several tens of seconds, accuracies below 1 cm (<0.4 in) can be achieved. In one experiment, averaging continuous measurement data for about 30 seconds (roughly 60 samples) yielded a horizontal error of 8 mm (0.31 in), an astonishing level of accuracy.

The accuracy of smartphone RTK surveying can rival first-class GNSS surveys that use national reference points. In one comparison between a high-end traditional surveying instrument and a smartphone RTK device measuring the same point, the coordinate difference was reported to be within 5 mm (≤0.20 in). In other words, the positioning accuracy of mobile RTK has reached a level comparable to specialized, expensive instruments. Of course, achieving the best accuracy requires favorable conditions such as an open sky for stable satellite reception; like traditional surveying, mobile RTK is also affected by environmental conditions. Nevertheless, modern receivers that support multiple satellite systems and dual frequencies are now common, and cases of successful positioning in challenging environments such as mountainous areas or urban canyons between tall buildings have increased. Considering this environmental robustness, the performance of mobile RTK is fully capable of providing stable centimeter accuracy that meets the demands of typical construction surveying.

Simple Surveying with LRTK

As we have seen, smartphone-based mobile RTK surveying and its advantages are making high-precision surveying accessible to everyone. One notable practical solution for implementing mobile RTK on site is LRTK. LRTK is an innovative product/service that transforms a smartphone into a centimeter-level surveying instrument and is designed so that even non-experts can perform simple surveying on site. A dedicated small GNSS receiver (weighing about 125 g and slightly over about 1 cm (about 0.4 in) thick) attaches to the smartphone, and by launching the dedicated app you can begin high-precision positioning immediately without complex setup.

For example, if there is a point you want to measure on site, simply hold the LRTK receiver attached to the smartphone over that point and tap the screen button. The latitude, longitude, and elevation of the point are recorded instantly, and the data is saved on the smartphone along with point names, notes, and timestamps. Recorded point data can be synchronized to the cloud with a single tap, allowing office PCs to check and share results in real time. This enables a single field worker to perform surveys and immediately report and share results with the office, streamlining operations that used to take surveying professionals half a day to complete. Using LRTK, on-site tasks can be finished in a short time and the data shared with stakeholders on the spot, dramatically improving work efficiency.

LRTK has a low adoption barrier and is priced much more affordably than traditional surveying instruments. It is designed for a one-per-person era and can be used intuitively by field workers and inspection staff without specialized qualifications. LRTK supports iPhone as well as some Android devices, adapting flexibly to devices already in use on different sites. In addition to single-point positioning, LRTK provides an all-in-one suite of features that take full advantage of mobile RTK surveying, such as point cloud scanning, AR-based as-built checks, and guidance to stake-driving coordinates. For example, you can assign global positioning coordinates in real time to point cloud data captured by an iPhone LiDAR scanner, reflecting an accurate 3D model on a map immediately after scanning. The AR function can overlay the pre-construction design model accurately in physical space, allowing intuitive assessment of discrepancies with the as-built state.

Although LRTK is described as “simple surveying,” its accuracy and functionality rival those of professional surveying instruments. Surveying tasks that once required specialists are becoming more approachable, and an era in which anyone can use their smartphone to access centimeter-level accuracy has arrived. If you are skeptical about whether such high-precision surveying can really be done with just a smartphone, we recommend trying out simple surveying with LRTK. Experience the new era of surveying enabled by mobile RTK on site.

FAQ

Q: What do I need to perform mobile RTK surveying? A: Basically, you need a smartphone, an RTK-capable small GNSS receiver, and a dedicated app for high-precision positioning. Both iPhone and Android compatible devices exist, but it is common to use a receiver that supports multi-GNSS and dual-frequency for higher accuracy. For example, attaching an LRTK-type receiver to your smartphone and launching the dedicated app will let you start centimeter-level (half-inch accuracy) positioning immediately. There is no need to prepare a private base station; correction information is received via the smartphone’s connectivity from public continuously operating reference stations or satellite augmentation services.

Q: What is the difference between conventional GPS positioning and RTK surveying? A: The main differences are in accuracy and mechanism. Conventional smartphone or car navigation GPS positioning has errors on the order of meters because it cannot correct for atmospheric delays or small satellite clock errors. RTK surveying, as described above, uses relative positioning with a reference station to correct those error factors in real time. As a result, RTK achieves accuracy of a few centimeters horizontally and vertically. RTK also drastically improves vertical (height) accuracy, enabling precise height measurements similar to leveling surveys. In short, compared with standalone GPS positioning, RTK surveying is overwhelmingly more accurate and provides position information suitable for strict tasks like construction surveying.

Q: Is the accuracy of smartphone RTK surveying really reliable? A: Yes. Under appropriate conditions, measurements from a smartphone plus an RTK receiver are highly accurate and can be comparable to professional instruments. Demonstrations and experiments have confirmed that their accuracy is not inferior to high-performance traditional surveying equipment. Horizontal errors typically fall within a few centimeters, and in some cases accuracy below 1 cm (<0.4 in) has been reported. To obtain the best accuracy, measure in an open environment with good satellite reception and consider remaining stationary for several tens of seconds to average measurements.

Q: Is an Internet connection required for mobile RTK surveying? A: Generally, a mobile data connection is needed to receive real-time correction information. Mobile RTK commonly obtains correction data over the Internet from continuously operating reference station networks maintained by government agencies or private services. Therefore, it is important that the smartphone has connectivity while surveying. However, in Japan you can also use the CLAS signal from the Quasi-Zenith Satellite System Michibiki to receive corrections offline. With a compatible receiver, you can continue positioning even in areas without cellular service, allowing you to choose the method that best fits the site conditions.

Q: In what situations can mobile RTK surveying be used? A: Mobile RTK surveying can be used broadly — from general civil and construction surveying to disaster site investigations and precision agriculture. It is effective for as-built management in earthworks, setting out on construction sites, measuring volumes in roadwork, and rapidly surveying disaster sites to aid recovery planning. It is also useful for infrastructure inspection, where accurately measuring small displacements helps track long-term changes, and for agriculture, where centimeter-level control is needed for autonomous tractors and field parcel measurements. Because mobile RTK makes tasks that previously required expensive equipment and specialized skills easy to perform, its range of applications is expected to expand further.

Q: Can non-specialist users operate mobile RTK systems? A: Yes. Mobile RTK solutions are designed so that relatively untrained users can operate them without special qualifications. Many mobile RTK systems include user-friendly apps that guide users through the process, and difficult configuration steps are automated, so beginners who don’t even know the term “RTK” can still use them. While basic surveying knowledge can help make better use of the tools, they are intended as everyday tools for field staff. In practice, products like LRTK are being used successfully by site managers and inspection personnel to increase productivity.

Q: How can I use the data obtained from surveying? A: Survey data collected on a smartphone can be flexibly utilized via the cloud. For example, in the LRTK app, measured latitude, longitude, and elevation for each point are automatically plotted on a cloud map and can be checked from an office PC. Data can be exported in CSV or DXF formats for import into CAD or GIS software for drawing and analysis. The app can also automatically convert to Japan’s plane rectangular coordinate systems or the World Geodetic System (WGS84) and apply geoid height corrections, so you obtain coordinates in the coordinate system you need for practical work. Unlike the days of handwritten field notebooks, mobile RTK surveying lets you directly leverage measured data digitally.