Differences Between GPS Surveying and RTK Surveying:
Key Points for Improving Accuracy on Civil Engineering Sites

What is GPS Surveying? Basic Mechanism and Applications

GPS surveying is a method of measuring one's position using signals from GPS satellites (GNSS satellites). In standard GPS functionality (standalone positioning), a single receiver receives signals from at least four satellites overhead, measures the distance to each satellite, and calculates the position coordinates.

This standalone positioning is widely used in GPS devices such as car navigation systems and smartphone map apps, and it has various applications such as in aircraft and ship navigation, car navigation, and displaying the current location on smartphones.

However, the accuracy of GPS surveying using standalone positioning is limited to a few meters. This is because it is affected by errors such as satellite orbit inaccuracies and signal delays caused by atmospheric layers (troposphere and ionosphere), resulting in errors of approximately 5 to 10 meters. Therefore, standalone positioning is not suitable for fields like civil engineering surveying, which require higher accuracy.

For example, tasks such as map creation and boundary measurement, where precision is crucial, cannot be accurately performed with standalone GPS positioning due to the large errors. High-precision positioning methods are required for such tasks.

In summary, GPS surveying (standalone positioning) is simple and convenient but has limited accuracy, typically within a few meters, making it unsuitable for applications that require high precision, such as those in civil engineering sites. This is where high-precision positioning technologies, such as RTK surveying, come into play.

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What is RTK Surveying? The Mechanism of High-Precision Positioning

RTK surveying stands for "Real Time Kinematic," a high-precision positioning method also referred to as real-time dynamic positioning in Japanese. RTK is classified as relative positioning, and it uses two GNSS receivers: a base station (with a known position) and a rover (which moves while performing measurements). Both receivers simultaneously receive signals from at least four satellites, and by exchanging data between the base station and the rover, measurement errors are corrected, and high-precision positioning is achieved. Since errors caused by satellite orbit inaccuracies and atmospheric influences are canceled out in relative positioning, RTK greatly enhances positioning accuracy.

The main feature of RTK surveying is its ability to maintain positioning accuracy within a few centimeters of error. Although some small errors remain, they are much smaller compared to the several meters of error associated with standalone positioning, and the error is kept within a few centimeters.

RTK surveying, which provides high-precision data in real-time, is increasingly used in fields that require more accurate positioning, such as autonomous agricultural machinery, machine guidance for construction equipment, and autonomous drone flights.

For example, in the construction industry, RTK is now used for controlling the positions of heavy machinery and for as-built management. In drone autonomous navigation, RTK prevents deviations in flight paths, enhancing safety.

In summary, RTK surveying is a method that achieves centimeter-level high-precision in real-time through positioning between two points: the base station and the rover. This method meets the high-precision positioning needs that traditional GPS surveying could not address, and its use is expanding in various fields, including civil engineering surveying.

Although RTK surveying involves initial setup costs and effort, it significantly improves the productivity of surveying work and the reliability of results, providing real-time centimeter-level accuracy.

Recently, by using the electronic reference points provided by the Geospatial Information Authority of Japan or private correction information services (e.g., VRS or satellite augmentation), it is becoming possible to perform RTK surveying without setting up a base station on-site, lowering both costs and operational hurdles.

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The Importance of RTK in Civil Engineering Surveying (Improved Accuracy, Work Efficiency, and Construction Management)

High-precision RTK surveying is becoming increasingly important in the civil engineering and construction sectors. One reason for this is the high positioning accuracy required in infrastructure construction and maintenance. In civil engineering, there are many situations where even a few centimeters of deviation are not acceptable, such as in road alignments, the placement of bridges, and tunnel excavation directions. By using RTK, high-precision surveying can be conducted even at sites far from reference points, ensuring accurate construction and as-built management that aligns with the design plans. For example, in the measurement of railway track distortion or monitoring road subsidence, RTK-based fixed-point observations can capture time-dependent changes with an accuracy of a few centimeters. In this way, RTK's high precision is essential in infrastructure maintenance.

Additionally, RTK surveying significantly improves the efficiency of civil engineering surveying tasks. Traditionally, topographic surveys and as-built checks required setting up total stations and working with multiple people, or connecting height differences through leveling surveys. With RTK-GNSS surveying, you can simply carry the surveying equipment and move around the site to rapidly obtain 3D coordinates for multiple points, completing large-scale surveys in a short amount of time.

Particularly in construction sites with open sky visibility, such as in large-scale development areas or for long structures, RTK increases mobility, leading to personnel reductions and project timeline shortening. The Ministry of Land, Infrastructure, Transport and Tourism also recommends the use of RTK-GNSS as part of ICT construction (i-Construction) to enhance on-site productivity.

For example, combining LiDAR (Light Detection and Ranging) functionality on smartphones or tablets with RTK surveying GNSS receivers has been reported to enable high-precision 3D surveying without the need for expensive laser scanners. In fact, the Ministry of Land, Infrastructure, Transport and Tourism has also recommended this method in their guidelines.

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Furthermore, RTK contributes to improving the accuracy of construction management and quality control. By using RTK-equipped heavy machinery, operators can perform excavation and embankment work while viewing the coordinates, minimizing deviations from the design surface. In as-built management, RTK can be used to measure key points of completed structures and compare them with the design values, allowing for quick as-built inspections. In recent years, the use of RTK-equipped drones to capture and create point clouds of construction sites from the air has become more common, with these point clouds being used for earthwork calculations and inspections. The surveying data obtained with high-precision RTK is highly reliable and can easily be integrated into BIM/CIM (Building Information Modeling/Construction Information Modeling) and GIS data for further use. As such, RTK surveying has become an essential technology for balancing both accuracy and efficiency in civil engineering surveying and construction sites.

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Innovation in Surveying with the Latest RTK Technology "LRTK"

As the importance of RTK surveying continues to grow, the latest RTK technology, "LRTK," has been gaining attention in recent years. LRTK (Lefixea RTK) is a compact RTK-GNSS receiver device developed by Lefixea Inc., a startup originating from Tokyo Institute of Technology, that can be used in conjunction with smartphones and tablets.

This innovative technology makes it possible to achieve centimeter-level positioning, which previously required surveying equipment costing several million yen, with a compact device that attaches to a smartphone. For example, the "LRTK Phone 4C" model is a device weighing only a few hundred grams, with an integrated antenna and battery, that can be attached to an iPhone. By using this device, the positioning accuracy of a typical smartphone GPS, which is usually around 10 meters, can be enhanced to within a few centimeters. In practice, when the smartphone with LRTK attached is mounted on a tripod, it transforms the device into an RTK-GNSS surveying tool, enabling easy and accurate centimeter-level point observations on-site.

 

A person conducting photo-based surveying on-site using a smartphone with the LRTK device attached. The dedicated app displays the latitude, longitude, elevation, and the RTK fixed solution (Fix), allowing the user to record photos with centimeter-level position information on-site.

With the advent of LRTK, it has become possible for a single technician to perform high-precision surveying with just a smartphone in hand. Traditional RTK surveying equipment required a receiver, radio, and battery to be mounted on a tripod and carried around, but with LRTK, all you need to do is attach the device to your smartphone and connect via Bluetooth, eliminating the need for cumbersome cables. This convenience lowers the barrier for surveying tasks on-site, creating an environment where high-precision positioning can be immediately utilized when needed. Additionally, LRTK is equipped with the latest GNSS technology, and with multi-frequency positioning, it is resistant to multipath (signal reflections) and ionospheric errors, making it easier to obtain a stable fixed solution (*).

As a result, even in slightly unfavorable positioning environments, high positioning rates can be maintained, making surveying possible in situations that were previously difficult with traditional equipment. This is a significant advantage in civil engineering sites, such as tunnel entrances, urban areas, or under viaducts, where GNSS positioning tends to be unstable.

Another noteworthy feature of LRTK is that it can be used in areas without mobile network coverage. In traditional network-based RTK surveying, correction data is received from the base station via mobile phone signals, but there was an issue with RTK being unusable in areas with no mobile coverage, such as mountainous regions or tunnels. Lefixea's latest model, the LRTK Phone 4C Off-Grid, is equipped with an option to directly receive centimeter-level correction signals (CLAS) from Japan’s Quasi-Zenith Satellite System (QZSS).

This allows for centimeter-level positioning in areas without mobile network coverage, such as mountainous regions or underground spaces, as long as satellite correction signals can be received, providing significant peace of mind for those managing civil engineering and infrastructure sites.

In practice, the off-grid model of LRTK has proven effective in forest surveying and mountain tunnel construction, areas where mobile coverage is often limited.

LRTK is not only designed for positioning, but also for utilizing the high-precision data it collects. The dedicated app, LRTK App, displays the surveyed points on a map in real-time and logs moving trajectories at a frequency of up to 10 points per second.

Additionally, you can tag photos taken with your smartphone with high-precision positioning coordinates and azimuth information obtained via RTK, and place these photos on a map at their corresponding survey locations.

When integrated with the LRTK Cloud service, point cloud data and photos taken on-site can be instantly uploaded to the cloud and shared with stakeholders. This makes it easy to survey and photograph disaster sites right after an event and share the data with headquarters or relevant agencies via the cloud, facilitating immediate action.

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In this way, the latest RTK technology, LRTK, is bringing significant innovation to civil engineering surveying sites. With the dramatic improvement in the portability, immediacy, and versatility of high-precision positioning, tasks that once relied on specialized surveyors and expensive equipment can now be easily performed by on-site technicians themselves. LRTK has already become a quiet trend among construction companies, surveying firms, and infrastructure managers, paving the way for an era where "one RTK surveying device per person" is a reality.