First Steps in RTK Surveying: Essential Knowledge for Civil Engineers

RTK surveying is a technology that enables high-precision satellite positioning in real time. RTK stands for Real Time Kinematic, meaning “dynamic interferometric positioning” in Japanese.

Compared to traditional standalone GPS surveying, it enables position measurement with dramatically higher accuracy, leading to its expanding use in civil engineering surveying sites.

This article explains everything from the basic concepts and mechanisms of RTK surveying, its differences from GPS surveying, and application methods, to preparation for beginners and tips for improving accuracy. Furthermore, it introduces an easy implementation method using LRTK and information on requesting free materials. Civil engineers and surveying personnel interested in RTK surveying should definitely refer to this.

1. What is RTK Surveying? Basic Concepts and Differences from GPS Surveying

RTK surveying is a high-precision satellite positioning method that utilizes position correction data from a base station (reference station). While conventional GPS surveying (standalone positioning) using a single receiver yields positional data with errors of several meters, RTK surveying can achieve accuracy within a few centimeters.

To achieve this high precision, RTK employs two GNSS receivers. One receiver is fixed as the base station (installed at a known point), while the other operates as the mobile station (survey target). Precise positioning is achieved by determining the relative position between these two stations.

In RTK surveying, simultaneous GNSS observations are made at the point of interest (rover) and the known point (base station). The rover's position is then corrected in real-time using correction data transmitted from the base station. This cancels out various error factors inherent in satellite signals, enabling positioning with significantly higher accuracy than standalone positioning. For example, even within Japan's geodetic system, RTK enables the determination of XYZ coordinates with centimeter-level accuracy, allowing precise surveying tasks—previously difficult—to be performed immediately on-site.

2. How RTK Surveying Works (Base Station, Rover, Correction Data)

A base station is a fixed station with precisely known coordinates. On construction sites, it is set up by placing an antenna at a known point or a reference point near the site. The rover, on the other hand, is a receiver that moves with an antenna to each point where positioning is desired. Both stations simultaneously receive signals from multiple GNSS satellites like GPS or GLONASS. The base station transmits the observed data in real time to the rover. The rover's receiver instantly analyzes its own observation data and the correction data sent from the base station to calculate highly accurate position coordinates. This is the fundamental mechanism of RTK (Real-Time Kinematic) surveying.

Wireless communication is used to transmit the correction data, typically employing methods such as:

• Dedicated Radio Communication Directly connects the base station and rover station via radio using bands like UHF or low-power radio (920MHz band). This method operates even without an internet connection on-site and offers stable communication for delivering correction data over short distances. However, it is limited by line-of-sight range and susceptible to obstacles, potentially requiring repeaters for large sites.

• Network RTK (Ntrip Protocol) The rover unit connects to a correction data distribution service via mobile networks or the internet. Instead of a base station, it receives real-time correction data from sources like the Geospatial Information Authority of Japan's electronic reference point network. Also called the Virtual Reference Station (VRS) method, it utilizes the nationwide network of established base stations, enabling RTK positioning without requiring the installation of a dedicated base station. While reducing initial field investment, this method requires a contract with a service provider and a stable communication environment.

In RTK surveying, accuracy tends to decrease with increasing distance (baseline length) between the base station and rover due to communication delays and atmospheric errors.

Generally, high accuracy can be maintained for short baselines of up to several kilometers. However, excessive distance causes error accumulation, making it difficult to obtain a fixed solution. To address these challenges in long-distance surveying, networked RTK-GNSS utilizing the Geospatial Information Authority of Japan's electronic reference point data has been put into practical use. This enables positioning with accuracy comparable to short baselines even over long baselines. In short, by using networked RTK that provides wide-area correction data without requiring a dedicated base station, centimeter-level positioning is achievable even at sites far from the base station.

3. Applications of RTK in Civil Engineering Surveying

RTK surveying, which provides high-precision, real-time position coordinates, is utilized in various scenarios within civil engineering and construction. Specifically, RTK's advantages can be leveraged for the following applications:

• Base Surveying and As-Built Surveying For tasks like road and development site cross-section surveying and topographic surveying, which traditionally required significant time using total stations or leveling, RTK-GNSS allows surveyors to measure numerous points quickly while moving. This proves highly effective for creating topographic maps and understanding existing conditions for design purposes.

• Stakeout and Pile Driving (Survey Layout) RTK also improves accuracy and efficiency in transferring design coordinates to the field (stakeout and pile driving). Traditionally, line-of-sight was required for optical distance measurement at each point. RTK, however, uses GNSS positioning even without line-of-sight, enabling rapid survey layout even in sites with many obstacles.

• As-Built Surveying and Quality Control RTK is also used for measuring the as-built shape of construction work over large areas, such as measuring the form of embankments and cuts for dams and roads, or verifying pavement thickness. It enables rapid, large-scale as-built management and allows for earthwork volume calculations and evaluation of finish accuracy using the acquired 3D data.

• Infrastructure Inspection and Maintenance Inspecting infrastructure like highways, railways, and bridges requires accurately recording the locations of cracks and deformations. RTK-enabled GNSS equipment allows linking inspection photos with their coordinates and comparing measurements over time to track aging changes. Infrastructure inspections, traditionally marked roughly on paper maps, can now be digitally managed with precise location data using RTK.

• ICT Construction & Machine Guidance In recent construction sites, ICT construction—where GNSS receivers are mounted on construction machinery like bulldozers and graders to perform automated grading and excavation—is becoming increasingly common. RTK-GNSS is also used for machine guidance in these construction vehicles. By continuously matching the design surface with the machine's blade position in real time for automatic control, it achieves high-precision construction while reducing manual surveying and stake placement. For example, the Ministry of Land, Infrastructure, Transport and Tourism's i-Construction initiative also recommends construction using RTK-GNSS-equipped machinery.

• UAV Surveying / Aerial Photogrammetry Cases combining RTK with photogrammetry using drones (UAVs) are also increasing. Using RTK-capable drones (or drones with an external GNSS receiver attached to the camera) allows high-precision positional data to be embedded in images captured during flight. This significantly reduces the number of ground control points required. This results in improved work efficiency and accuracy even for large-scale surveys. In civil engineering, UAV photogrammetry is utilized for tasks like earthwork volume calculations and slope displacement measurements. Combining it with RTK enables the creation of more practical and highly accurate 3D models.

Thus, RTK surveying serves a wide range of purposes, from basic topographic surveying to advanced ICT construction, maintenance, and spatial data measurement. Its effectiveness is particularly significant in scenarios demanding “speed” and “accuracy,” and it will likely remain an indispensable technology for civil engineers going forward.

4. Preparing for Beginners to Start RTK Surveying

For beginners looking to start RTK surveying, here are key points to prepare beforehand. Understanding the necessary equipment and procedures will ensure a smooth introduction.

Preparation Items Before Starting RTK Surveying:

• Prepare Surveying Equipment Obtain an RTK-capable GNSS receiver. The basic setup requires two units: one for the base station and one for the rover. If you won't be setting up your own base station, a single rover unit plus a correction service subscription is also acceptable. Ensure you have a complete set ready for field surveying, including accessories like antennas, tripods, poles, and batteries. Recently, compact GNSS receivers that integrate with smartphones have become available; LRTK, discussed later, is one such example.

• Communication Environment Setup Prepare the means to transmit correction data from the base station to the rover. If using a radio, prepare transmit/receive modules and apply for any required licenses for specific frequency bands. For network RTK, equip the rover with an internet-capable smartphone or mobile router and pre-subscribe to an N-trip distribution service (e.g., the VRS service providing electronic reference point data from the Geospatial Information Authority of Japan).

• Securing Known Points and Setting Up the Base Station When setting up your own base station, securing known points with precise coordinates near the site is crucial. Install the antenna at a public control point (such as a Class 4 triangulation point or electronic control point) or a previously surveyed known point to establish the base station. Even if no known points exist, temporary control points can be set up for RTK surveying; however, in such cases, coordinate corrections must be performed later by combining the data with known points. Select an open location with good visibility for the base station and stabilize it using a tripod or similar equipment.

• Preparing Survey Software and Controllers Prepare software or devices to control the GNSS receiver and verify positioning results. Options include manufacturer-specific controller terminals or applications for tablets/smartphones. For network RTK, configure correction service settings (IP address, port, login info, etc.) within the Ntrip connection app or software. Also verify the coordinate system settings to ensure survey results are in the Japanese Geodetic Datum.

• Functionality Testing and Practice Before heading to the actual site, perform RTK surveying functionality checks and practice using the entire prepared equipment set. For example, you can verify the system is working correctly by setting up a known point as the base station within your company's premises, surveying several surrounding points, and checking the differences with the known point. Also, understand beforehand the time required for the rover to achieve a “FIX solution” (integer multipath resolved) and the satellite acquisition status. If you feel uncertain about the survey procedure, it's reassuring to have an experienced person present or to refer to manufacturer-provided training videos and manuals.

After completing these preparations, you will be ready to begin RTK surveying. Handling the base station and communication settings are particularly common stumbling blocks for beginners, so take your time and prepare thoroughly. Additionally, it's smoother to practice initially in favorable conditions, such as during daylight hours with good satellite reception, and gradually transition to actual surveying.

5. Key Points for Improving Survey Accuracy

To consistently achieve high accuracy in RTK surveying, several techniques and precautions are essential. Below are the main points for improving accuracy:

• Ensuring Accuracy of Base Station Coordinates Use coordinate values for the base station that are as accurate as possible. Ideally, use coordinates from known points. If using an unknown point as a temporary base, you must later precisely measure that base point and apply an offset correction to the entire survey result. A 1-meter error in the base station coordinates will uniformly shift the coordinates obtained by the rover by 1 meter. If you require “absolute accuracy,” the accuracy of the base station is crucial.

• Antenna Setup and Angle/Height Measurement Securely mount the antennas for both the base station and rover, maintaining them as level as possible. Accurately measuring and inputting the antenna height (the height from the antenna reference plane to the measurement point) is also critical. Input errors in antenna height directly translate to vertical errors. Additionally, erect the rover pole as vertically as possible. Even for models with tilt correction, perform calibration before use.

• Optimizing Satellite Reception Environment Pay attention to the surrounding environment at the survey site. Positioning in an open area with a wide view of the sky is ideal. Shaded areas under buildings or trees may result in insufficient satellite acquisition or multipath interference (radio wave reflections), degrading positioning accuracy. If surveying in obstructed areas is unavoidable, extend measurement time to stabilize data or choose times with fewer obstacles (when satellite constellation is favorable).

• Utilizing Multiple Frequencies and Satellite Systems If possible, use a receiver supporting multi-GNSS and multiple frequencies. Utilizing multiple satellite systems like GLONASS, Galileo, or QZSS (Michibiki) alongside GPS improves satellite configuration, enhancing accuracy and positioning stability. Receivers supporting multiple frequencies like L1/L2 also improve ionospheric delay correction accuracy, enabling faster acquisition of a FIX solution. Some modern RTK services receive signals from QZSS's Centimeter-Level Augmentation Service (CLAS) to obtain correction data. Using compatible equipment enables wide-area corrections without requiring communication links.

• Regular Checks and Verification Regularly check accuracy during surveying operations. For example, manage accuracy by observing known points at the start and end of each day's work and verifying the coordinate differences. If significant errors occur, this data serves as a basis for deciding whether to correct or re-measure that day's survey points. Additionally, if the rover loses its FIX solution and reverts to a FLOAT solution (an uncertain state) during positioning, immediately reset it to regain a FIX solution before resuming measurements. Continuously monitoring solution status and RTK residual values, and investigating any anomalies, is essential for maintaining high accuracy.

• Smoothing and Averaging Positioning Results When necessary, improving accuracy can be achieved by smoothing the obtained positioning results or taking time averages instead of using them directly. While RTK's real-time capability is a key feature, further precision gains are possible by, for example, holding a single point for several seconds to tens of seconds and averaging the coordinates obtained during that period. Elevation values, in particular, tend to fluctuate significantly, so averaging multiple observations is advisable when feasible.

By paying attention to the above points, you can further enhance the accuracy and reliability of RTK surveying. In summary, the keys to ensuring accuracy are: “accurate reference points,” “good reception environment,” “maximizing equipment performance,” and “constant monitoring.” The more high accuracy is required, the more important it is to return to the fundamentals. Ensure reliable operation to maximize the benefits of RTK surveying.

6. Easy Introduction to RTK Surveying Using LRTK

High-precision RTK surveying is appealing, but acquiring all the equipment from scratch can be costly, presenting a significant barrier for small-to-medium-sized civil engineering firms and beginners looking to adopt it. This is where LRTK, a convenient RTK solution utilizing smartphones, has emerged in recent years. LRTK is a digital positioning technology brand provided by Refixia Inc. It is a product series designed to enable anyone to easily achieve centimeter-level positioning by combining a dedicated compact GNSS receiver with a smartphone app.

Among these, the LRTK Phone is particularly recommended for beginners. The LRTK Phone is an add-on GNSS receiver for smartphones, an all-in-one device integrating an antenna, GNSS receiver, battery, and wireless communication functionality. Its operation is extremely simple: just attach the LRTK Phone to your existing smartphone (Android or iPhone), install the dedicated “LRTK App,” and launch it. That's all it takes to start acquiring centimeter-level positioning information via RTK positioning on your smartphone. A major feature is that it requires no complex setup or specialized knowledge like traditional RTK equipment, allowing you to start surveying with intuitive operation.

The LRTK Phone supports Bluetooth and Wi-Fi connections for communication, linking wirelessly with your smartphone. It also supports the 920MHz band low-power radio, enabling direct exchange of correction data between an LRTK acting as a base station and another LRTK acting as a mobile station within the communication range. If you don't have your own base station, you can connect to network RTK services like electronic reference points via your smartphone's mobile network to receive correction data for positioning (supports Ntrip client functionality).

In other words, it supports both methods: deploying your own base station or receiving corrections via the network. This allows flexible implementation based on site conditions and operational requirements.

A specific field application involves automatically tagging photos taken with a smartphone with high-precision coordinates obtained via LRTK and storing the data in the cloud. For example, in crack inspections of concrete structures or recording conditions at disaster sites, simply taking a photo with a smartphone records its location (longitude/latitude) with centimeter-level accuracy, dramatically improving the precision and reliability of reporting materials. It's revolutionary that even infrastructure inspectors without surveying expertise can handle accurate, location-tagged photo data with smartphone-like ease.

Beyond the smartphone-mounted Phone model, the LRTK series offers various other options. For instance, the LRTK Pro2 is a compact, lightweight, stationary RTK surveying unit. It's a rugged, field-ready model featuring dustproof, waterproof, and shock-resistant construction. It integrates everything from the antenna to the battery and wireless module. Even in remote mountainous areas without network coverage, it can receive the CLAS augmentation signal from Japan's Quasi-Zenith Satellite “Michibiki” for positioning. Furthermore, its tilt compensation function ensures accurate positioning even when the pole is tilted.

Additionally, the unique LRTK Helmet is available. This is a helmet with an integrated surveying antenna, enabling automatic surveying simply by the worker wearing the helmet and walking. It incorporates an ultra-thin proprietary antenna and a battery providing up to 12 hours of continuous positioning. Acquired coordinates are transmitted to a smartphone via Bluetooth, enabling completely hands-free surveying.

Even in sites with many obstacles or when both hands are occupied, workers' movements are directly converted into survey data, contributing to improved work efficiency.

By leveraging LRTK in this way, the barriers to adopting RTK surveying are significantly lowered. Even non-specialist surveyors, such as site supervisors or construction managers, can now handle high-precision positioning themselves. This brings benefits like reducing delays caused by waiting for survey results and enabling real-time verification of work progress for immediate incorporation into construction. Price-wise, subscription (monthly usage) plans are available, offering flexibility to reduce initial costs. Truly democratizing RTK surveying, LRTK will be a strong ally for small civil engineering firms and those new to RTK.