Accuracy Comparison of RTK-enabled GNSS and GPS: Positioning Accuracy Required at Civil Engineering and Surveying Sites

Table of Contents

• Evolution of traditional GPS and high-precision positioning technologies

• Principles of RTK positioning and dramatic improvement in accuracy

• Characteristics of conventional GPS positioning and use cases

• Examples of civil engineering sites requiring RTK positioning accuracy

• Relationship between correction information services and positioning accuracy

• Improved stability through integration of multiple satellite systems

• Verification and management of positioning accuracy on site

• Integration of smartphones and RTK positioning technology

• Considerations when introducing RTK positioning

• Relationship between high-precision positioning and site efficiency

• Emergence of iPhone-mounted high-precision GNSS positioning devices

• Future developments of high-precision positioning technologies

On construction and surveying sites, obtaining accurate positioning information greatly affects the quality of work. There is a significant difference in accuracy between conventional GPS positioning and the latest RTK (Real-Time Kinematic)-enabled GNSS positioning. Understanding these technical differences is important knowledge for field personnel. In this article, we compare the accuracy of RTK-enabled GNSS and GPS and explain in detail the characteristics of each and the points for choosing between them.

The Evolution of Conventional GPS and High-Precision Positioning Technologies

In the past, GPS positioning was the primary means of obtaining location information at surveying sites. Conventional GPS positioning is a system that calculates position based on signals transmitted from satellites. By receiving signals from multiple satellites, a three-dimensional position can be determined. However, this conventional GPS positioning had major drawbacks. The accuracy obtained often remained on the order of several meters to several tens of meters (a few to several dozen ft), and it was difficult to achieve the centimeter-level accuracy (cm level accuracy, half-inch accuracy) required at civil engineering sites.

In recent years, satellite positioning technology has advanced rapidly. In addition to conventional GPS satellites, environments have been established where multiple satellite positioning systems are available. By simultaneously receiving satellite signals from different systems, the reliability and accuracy of positioning have greatly improved. The technology of integrating and utilizing multiple satellite positioning systems in this way is generally called GNSS (satellite positioning systems in general). GNSS does not rely solely on GPS satellites; by leveraging signals from multiple satellite systems, it enables more robust and accurate positioning.

The Principles of RTK Positioning and the Dramatic Improvement in Accuracy

RTK-enabled GNSS positioning has brought revolutionary improvements over conventional positioning technologies. In the RTK method, the difference between satellite signals received at a reference station with known coordinates and those received by the mobile receiver is calculated. By using this differential information, satellite orbit errors and signal delays caused by the atmosphere can be substantially corrected.

The accuracy achieved by RTK positioning is on the order of a few centimeters (a few in) horizontally and a few centimeters (a few in) vertically. It can largely meet the accuracy requirements needed for pile driving, setting out (layout marking), and as-built management at civil engineering construction sites. This improvement in accuracy has brought greater efficiency and higher quality to surveying operations. The level of accuracy that previously required combining multiple surveying techniques can now be achieved with a single system.

Characteristics and Use Cases of Conventional GPS Positioning

The accuracy of conventional GPS and GNSS standalone positioning (basic positioning that does not use correction information) is generally about 2-10 m (6.6-32.8 ft) horizontally. Even in construction work, if the goal is rough position awareness, there are situations where this level of accuracy is sufficient. For example, in general surveys of large sites or in coarse progress management, the accuracy obtainable from standalone positioning can be utilized.

The advantage of standalone positioning is that no external infrastructure, such as reference stations, is required. With a receiver, you can perform positioning independently anywhere. This is suitable for activities in environments where it is difficult to receive correction signals from reference stations, such as remote mountainous areas and islands. However, for practical construction management and centimeter-level surveying, this level of accuracy is often insufficient, so transition to RTK positioning is progressing.

Examples of civil engineering sites where RTK positioning accuracy is required

On civil engineering construction sites, centimeter-level accuracy (cm level accuracy (half-inch accuracy)) is essential in many situations. For example, pile driving in road construction and setting out in bridge construction must be carried out strictly according to design tolerances. Using RTK positioning for these tasks can greatly reduce the time spent surveying on site.

As-built management in land development works is another field where the use of RTK positioning is rapidly advancing. By visualizing the difference from the design elevation in real time on site, it becomes easier to verify construction accuracy. In earthwork quantity calculations as well, obtaining three-dimensional data of the existing ground with centimeter-level accuracy (half-inch accuracy) enables more accurate quantity estimation.

The Relationship Between Correction Information Services and Positioning Accuracy

The accuracy of RTK positioning depends heavily on the quality of the correction information. Correction information is calculated at the reference station and delivered to the mobile receiver via a communication link. For high-precision positioning to be realized, it is essential that this correction information arrives in a timely manner and is accurate.

Currently, multiple correction information distribution services are in operation. These services compute and distribute correction information in real time from networks of reference stations deployed nationwide or across wide areas. By utilizing network-based correction information services, stable RTK positioning has become possible at any site. Previously, it was necessary to install reference stations on one's own, but by using external services, that effort and cost can be reduced.

Distribution of correction information typically utilizes cellular networks and LTE communications. However, in mountainous or no-coverage areas, communications may become unstable. To support positioning in such environments, RTK positioning techniques that do not rely on communications have also been developed.

Enhancing Stability through Integration of Multiple Satellite Systems

In modern GNSS positioning, it has become common to use multiple satellite navigation systems simultaneously. Receiving satellite signals from multiple systems—such as the U.S. GPS, Russia's GLONASS, Europe's Galileo, and China's BeiDou—greatly improves positioning reliability.

Especially in urban canyons (environments surrounded by high-rise buildings) and sites with many trees, there are situations where receiving satellite signals is difficult. By utilizing satellites from multiple systems, it becomes more likely to secure a sufficient number of satellites even in such environments and to continue positioning. In RTK positioning, accuracy tends to improve as the number of satellites increases, so the integrated use of multiple systems offers significant practical benefits.

Verification and Management of On-site Positioning Accuracy

When using RTK positioning in the field, it is important to appropriately manage the reliability of the positioning results obtained. By checking indicators such as the DOP value (Dilution of Precision) displayed by the receiver and the number of resections, you can grasp what level the current positioning accuracy is at.

In particular, whether the ambiguity resolution (integer value determination) required for RTK positioning has been completed greatly affects accuracy. The state in which ambiguity resolution has been completed is called "fixed", and only in this state is centimeter-level accuracy guaranteed. In the "float" state, where this condition has not been reached, accuracy degrades and it should not be used for fieldwork.

The receiver's screen displays status information like this. It is important for field personnel to always check the positioning status before starting work and confirm that it is in a fixed state before beginning surveying and stakeout operations.

Integration of Smartphones and RTK Positioning Technology

In recent years, the integration of high-precision GNSS positioning capabilities into smartphones has been advancing. RTK positioning functions, which were formerly limited to specialized surveying instruments, are becoming available on more general-purpose devices.

By attaching an RTK-capable external GNSS device to a smartphone, semi-professional positioning accuracy has become achievable. This approach has made it possible to obtain high-precision location information on demand even in settings beyond traditional surveying. Its use is beginning to spread across a variety of fields, including construction site progress management, precision agriculture, and infrastructure inspection.

Surveying and positioning applications that run on smartphones are emerging one after another, making positioning tasks that traditionally required specialized knowledge accessible to a broader range of stakeholders. This is further accelerating improvements in on-site productivity and operational efficiency.

Considerations When Introducing RTK Positioning

When introducing an RTK positioning system to a site, there are multiple considerations. The first thing to check is the communications environment at the target site. Whether a communication line is available to receive correction information greatly affects the feasibility of RTK positioning.

Next, it is important to verify the consistency between the required positioning accuracy and the accuracy achievable with current technology. In situations that require millimeter-level precision (mm (0.04 in)), combining RTK positioning with additional surveying techniques may be necessary. Conversely, if an accuracy on the order of several tens of centimeters (cm (several tens of in)) is sufficient for the task, standalone positioning may be adequate.

Staff training must not be overlooked. Practical knowledge, such as how to operate RTK positioning equipment, how to assess positioning status, and how to respond to anomalies, is indispensable for safe and accurate on-site work.

The Relationship Between High-Precision Positioning and On-Site Efficiency

The advancement of high-precision GNSS positioning technology has brought not only improved surveying accuracy but also increased efficiency across the entire worksite. With improved setup accuracy, tasks that previously required repeated measurements can now be completed in fewer attempts.

By combining 3D scanning, the differences between the existing ground and the design drawings can be visualized, allowing you to see at a glance which areas should be worked on and to what extent. This visualization improves the speed and accuracy of on-site decision-making and dramatically enhances construction management efficiency.

Moreover, by sharing the same high-precision positioning data among multiple work crews, there is the advantage that the consistency of work across crews is automatically ensured. Traditionally, the site agent had to reconcile the progress of each crew, but unified, data-driven management becomes possible.

Introduction of an iPhone-Mounted High-Precision GNSS Positioning Device

Recently, high-precision GNSS positioning devices that can be attached to smartphones have been attracting attention. These devices combine the portability of smartphones with the high accuracy of RTK positioning. Compared with conventional specialized surveying equipment, they are simple to operate and easy to access on site.

By utilizing such a device, you can integrate it with surveying applications that run on a smartphone to carry out practical surveying and setting-out operations more efficiently. Setting accuracy can be achieved at the centimeter level (half-inch accuracy), meeting the requirements of many civil engineering sites.

Furthermore, by combining the acquired positioning data with 3D scanning capabilities, on-site construction quality control can be further enhanced. Because data can be processed in real time on a smartphone, on-site decision-making is faster and the overall efficiency of construction management is increased.

A smartphone-based approach has lowered the barriers to surveying and setting-out operations, allowing more field personnel to carry out work based on accurate positional information. Its convenience is particularly evident when someone needs to complete a survey alone or must work while moving nimbly between multiple sites.

Future Developments in High-Precision Positioning Technologies

GNSS positioning technology continues to evolve. Improvements in satellite positioning system performance, expansion of correction information distribution networks, and the miniaturization and cost reduction of receivers are progressing on multiple fronts.

In particular, by combining AI and cloud technologies to enable automatic positioning determination, anomaly detection, and real-time data processing, it is expected that the use of high-precision positioning will expand to a wider range of field sites. The increasing performance of general-purpose devices such as smartphones will usher in an era in which the required precision can be achieved without specialized surveying equipment.

The importance of positional information accuracy at civil engineering and surveying sites is expected to increase further. Improving setting accuracy has become an indispensable factor for ensuring construction quality, enhancing on-site safety, and streamlining construction management. For field practitioners, understanding the characteristics of high-precision positioning technology and using it appropriately will become increasingly important.