Comparing the Accuracy of GNSS, GPS, and RTK: How to Choose for Sites Requiring Centimeter-Level Positioning

Table of Contents

• Basic accuracy and limitations of GPS

• What is GNSS: Integration of multiple satellite systems

• The advent of RTK GNSS: Achieving cm-level accuracy (half-inch accuracy)

• Impact of accuracy differences in practical fieldwork

• Selection criteria for each technology

• Cost-performance perspective

• Future technology trends

• Summary

In surveying sites, drone surveying, construction, agriculture, and other field operations, accurate positional information is a critical factor that can determine the success or failure of a project. Especially at sites that require centimeter-level accuracy (cm level accuracy, half-inch accuracy), choosing which positioning technology to use becomes a major challenge. With multiple options such as GPS, GNSS, and RTK GNSS, understanding the differences in accuracy and how to apply each directly connects to on-site efficiency and cost optimization.

In this article, we explain in detail the differences between these positioning technologies and propose appropriate options tailored to field requirements. We will systematically provide the knowledge that practitioners need when evaluating them.

Basic Accuracy and Limitations of GPS

GPS (Global Positioning System) is a satellite positioning system developed and operated by the United States. It is built into common smartphones and car navigation systems and is widely used around the world. However, there are clear limitations to GPS accuracy.

The accuracy of standard GPS is typically around 3–5 m (9.8–16.4 ft). In urban areas with tall buildings, errors of 10 m (32.8 ft) or more can occur. This level of accuracy may be acceptable for large-scale civil engineering or the management of vast farmland, but it is insufficient for construction-site tasks where errors on the order of tens of centimeters (several in) are unacceptable, or for agricultural fertilization management that requires precise positioning.

There are several reasons why GPS has accuracy issues. First, signals from satellites experience delays as they pass through the atmosphere. These are called ionospheric delay and tropospheric delay, and these effects vary greatly by region and time of day. Second, in urban areas and regions with dense tree cover, buildings and vegetation can block or reflect signals, causing accuracy to degrade significantly. Third, with GPS alone the number of available satellites can be limited, and accuracy tends to decrease, particularly in high-latitude regions north of 70° N.

Because of these kinds of limitations, standalone GPS positioning cannot meet the requirements of tasks where positional accuracy is critical. However, understanding the basic performance of GPS serves as a starting point when selecting more advanced systems.

GNSS: Integration of Multiple Satellite Systems

GNSS (Global Navigation Satellite System) is a technology that uses multiple satellite positioning systems in an integrated manner in addition to GPS. Specifically, it simultaneously receives and processes satellite constellations independently operated by several countries and regions, such as the systems operated by Russia, Europe, and China.

By adopting GNSS, it becomes possible to receive more satellite signals than with a single satellite system. Combining multiple satellite systems increases the number of satellites that can be received simultaneously compared to conventional GPS, typically allowing reception of more than 20 satellite signals. The greater the number of satellites, the higher the accuracy and the shorter the periods during which signals cannot be received.

GNSS accuracy is typically about 1-3 m (3.3-9.8 ft). This is an improvement over GPS, but it still does not reach the centimeter-level accuracy (cm level accuracy (half-inch accuracy)) required for construction sites and drone surveying. However, GNSS offers high stability of signal reception and can provide relatively stable positioning even in urban areas, so it provides sufficient accuracy for general navigation apps and smartphone location services.

When using GNSS, the accuracy of satellite orbital information and timing information is important. This information is provided by the operating agencies of each satellite system. As satellite systems continue to improve in accuracy, GNSS accuracy is expected to gradually improve in the future.

The advent of RTK GNSS: Achieving cm level accuracy (half-inch accuracy)

RTK (Real-Time Kinematic) GNSS is a technology that dramatically improves GNSS accuracy.

This technology enables real-time centimeter-level accuracy (half-inch accuracy), and in some cases millimeter-level accuracy (0.04 in).

The basic principle of RTK GNSS is to establish a reference point (reference station) and pre-measure its precise coordinates. By comparing signal information between the receiver at the reference point and the rover receiver, error sources such as ionospheric delay and tropospheric delay are canceled out. This differential processing dramatically improves accuracy.

The accuracy of RTK GNSS is generally around 2–5 centimeters (0.8–2.0 in) horizontally and 3–8 centimeters (1.2–3.1 in) vertically. This represents an improvement of tens to about a hundred times compared with conventional GNSS. It is suitable for tasks that require high accuracy, such as staking out on construction sites, precision fertilization in agriculture, and accurate coordinate acquisition in drone surveying.

How RTK GNSS achieves high accuracy also depends on the communication method between the reference station and the rover. In traditional systems, the reference station and the rover exchanged correction information directly via radio communication, but nowadays systems that distribute correction information using cellular networks or Internet connections are also widely used. In such network RTK, more reliable correction information is generated by integrating and processing information from multiple reference stations.

When implementing RTK GNSS, it is necessary to consider multiple factors such as the surveying accuracy of reference points, the stability of the communication environment, and the system’s operating costs. However, at sites where high precision is essential, it is well worth incurring these costs and efforts to implement it.

Impact of Accuracy Differences in Practical Work Environments

Let's consider specifically how these technologies will affect real-world sites.

Assuming foundation work at a construction site, if the position of a building’s foundation pile is off by 50 cm (19.7 in), the entire structure built on it may become distorted. GPS that allows errors of 3–5 m (9.8–16.4 ft) is not suitable for such applications. The error must be kept to tens of centimeters or less, and RTK GNSS is essential.

In the field of precision agriculture, accurately managing the application locations of fertilizers and pesticides enables more efficient use of inputs and reduces environmental impact. When performing multiple operations on the same field, if positional accuracy is not at the centimeter level (half-inch accuracy), application locations can overlap with previous ones or, conversely, be missed. RTK GNSS is also effective in this area.

In drone surveying, precise 3D terrain maps and building models can be generated only when aerial images are tied to accurate ground coordinates. Plain GPS often fails to provide sufficient positional accuracy, and the quality of survey deliverables frequently deteriorates. On sites where high-accuracy survey results are required, the adoption of RTK-capable drone systems is becoming standard.

On the other hand, there are many applications for which meter-level positioning accuracy is sufficient, such as assessing crop growth across vast farmland or monitoring the channels of large rivers. In these cases, standard GNSS is adequate and investment in RTK GNSS is unnecessary.

Selection Criteria for Each Technology

Choosing a positioning technology begins with accurately understanding the on-site requirements. It is important to consider the following perspectives.

First, the required level of accuracy. Is accuracy on the order of several meters (m (ft)), on the order of tens of centimeters or less (cm (in)), or at the millimeter level (mm (in))? This requirement is the most important decision criterion. If the accuracy requirement is not clear, it will lead to one of two failures: either excessive investment, or conversely implementing an insufficient system that prevents work from proceeding.

Next is the work environment. Whether work is performed among tall buildings in urban areas, in open locations with good visibility, or in heavily treed areas, the performance of each system can vary significantly with the environment. In urban areas, receiving satellite signals can become difficult, so more advanced technology is often required.

The duration and frequency of the work are also important. Is it a limited surveying task performed once a day, or a task that requires continuous daily positioning? That distinction changes how you evaluate operational costs. If you need high-accuracy positioning continuously every day, systems such as network-based RTK can be more cost-effective.

You also need to verify the availability of the communication environment. In particular, with RTK GNSS, communication between the reference station and the mobile receiver is essential. In mountainous or remote areas, communication infrastructure may not be available, and in such cases you need to install your own reference station or consider an alternative positioning method.

Human resources and the operational framework are also factors to consider. Even if an advanced positioning system is deployed, without personnel capable of operating and maintaining it, it will be a wasted asset. Before deployment, it is important to establish the operational framework and formulate a personnel training plan.

The cost-performance perspective

When selecting positioning technologies, economic considerations are, of course, important. Generally, the higher the accuracy, the greater the system costs.

When using standard GPS or GNSS, the initial investment for receivers is relatively small and operational costs are almost negligible. However, because accuracy is limited, it cannot be applied to tasks where accuracy is essential.

When introducing RTK GNSS, multiple investments are required, such as installing reference points, procuring receivers, and setting up the communication environment. In addition, ongoing costs arise, including system maintenance and management, calibration, and communication fees. However, because high accuracy can be achieved, on-site work efficiency improves significantly, and re-surveys and rework are reduced. In many cases, these efficiency gains justify the system investment.

When making investment decisions, it is important to evaluate the total cost of ownership from a medium-term perspective over multiple years, rather than on a single fiscal year.

For systems used daily, it is not uncommon for the value of the initial investment to be recouped after about one year of operation.

Future Technology Trends

Positioning technologies are evolving rapidly.

Looking ahead, there are several key points to note.

Improvements in satellite system accuracy will continue. Each satellite operator is continually investing in improved orbital accuracy and signal quality, and this is expected to lead to improvements in basic GNSS accuracy.

There is also a trend toward improved positioning accuracy for mobile devices such as smartphones. Traditionally, smartphone GNSS reception functions were basic, but the latest smartphones are being equipped with receivers that support multiple satellite systems. This is making it increasingly possible to achieve GNSS-level accuracy on smartphones.

What is even more noteworthy is the advancement of technology to equip everyday devices such as smartphones with RTK-capable GNSS modules. While RTK positioning had traditionally been possible only with dedicated surveying equipment, an environment is now being put in place that will allow positioning with cm level accuracy (half-inch accuracy) even on portable devices such as smartphones.

In this trend, iPhone-mounted GNSS high-precision positioning devices have emerged. By using such devices, high-precision positioning functionality can be added to existing iPhones, creating a system that combines the convenience of a smartphone with high-precision positioning. For practitioners working at construction sites, in agriculture, and in surveying operations, the benefit of obtaining high-precision location information with the smartphone they carry every day is immeasurable.

With an iPhone-mounted device, you avoid the weight and handling complexity of traditional dedicated surveying equipment, and you can check positioning results through intuitive operation. In addition, since the existing application environment can be used as is, system deployment effort is greatly reduced. Cloud integration is also easy, enabling seamless accumulation, analysis, and sharing of positioning data.

In work sites where high-precision positioning is required, it is well worth considering the use of such modern solutions. Now that centimeter-level accuracy (cm level accuracy (half-inch accuracy)) from RTK GNSS can be achieved on portable devices rather than the expensive and complex systems of the past, new possibilities for improving operational efficiency and productivity are opening up across fields such as surveying, construction, and agriculture.

Summary

GPS, GNSS, and RTK GNSS are positioning technologies, each with different accuracy characteristics and use cases. GPS is sufficient for tasks that can be handled with approximately 3-5 m (9.8-16.4 ft) accuracy, and GNSS is suitable when approximately 1-3 m (3.3-9.8 ft) accuracy is required. However, for tasks where centimeter-level accuracy (cm level accuracy (half-inch accuracy)) is indispensable—such as construction site foundation work, precision fertilizer management in agriculture, and drone surveying—the introduction of RTK GNSS is essential.

It is important to organize the specific on-site requirements and, after comprehensively considering the required accuracy, communication environment, and work frequency, select the appropriate technology. In recent years, the commercialization of smartphone-mounted RTK-capable GNSS devices has made it possible to deploy centimeter-level positioning solutions (cm level accuracy, half-inch accuracy) at more sites. Compared with conventional dedicated surveying instruments, such solutions—superior in operability, portability, and integration—contribute greatly to improving on-site operational efficiency and productivity.

By selecting the optimal positioning technology to meet the site’s specific needs, operational staff can simultaneously strengthen business competitiveness and improve on-site work efficiency.