RTK Accuracy Explained: GPS/GNSS vs RTK for Construction Surveying

Table of Contents

• Difference between GPS and GNSS

• What is RTK?

• Accuracy comparison: GPS/GNSS vs RTK

• Positioning accuracy required on construction sites

• Barriers to RTK adoption

• Simplified surveying with LRTK

• Frequently Asked Questions

Have you heard the terms GPS, GNSS, and RTK? In recent years the construction industry has been advancing ICT utilization, and these satellite positioning technologies have attracted attention. They are important technical elements within the Ministry of Land, Infrastructure, Transport and Tourism’s promoted initiative known as i-Construction. The accuracy of location information varies greatly depending on the technology used, and understanding these differences is important for selecting equipment and planning construction work on site. However, many people still wonder, “What is the difference between GPS and GNSS?” or “How much does accuracy improve when using RTK?” In this article, under the theme “RTK vs GPS vs GNSS”, we explain the differences between each technology and the differences in accuracy. We will cover how much positioning accuracy is required for construction projects and touch on the new simplified surveying method LRTK, presenting the latest on high-precision positioning.

Now, let’s take a closer look at the characteristics of each positioning technology and their accuracy differences.

Difference between GPS and GNSS

First, let’s clarify the difference between GPS and GNSS. GPS (Global Positioning System) is the name of the satellite positioning system operated by the United States. GNSS (Global Navigation Satellite System) is the collective term for multiple satellite positioning systems, including GPS. For example, Russia’s GLONASS, Europe’s Galileo, and China’s BeiDou are all part of GNSS.

In common usage we often say “GPS,” but strictly speaking GPS is just one type of GNSS. GNSS-capable receivers can receive signals from multiple satellite constellations, so they can track more satellites than when using GPS alone. This increases the likelihood of obtaining a position even with some obstructions and improves positioning stability. However, using GNSS does not automatically yield dramatically higher accuracy. As noted later, whether GPS or GNSS, standalone positioning without corrections typically remains on the order of a few meters. Incidentally, using a wide-area augmentation system (SBAS) that uses geostationary satellites can sometimes improve GPS/GNSS standalone positioning errors to under 1 m (3.3 ft). Still, to achieve centimeter-level accuracy you need more advanced correction methods like RTK described below. While GNSS offers benefits in stability and success rate due to increased satellite counts, fundamental accuracy improvements require other measures.

What is RTK?

So what is RTK? RTK (Real Time Kinematic) is a method that applies real-time error corrections to GNSS (such as GPS) positioning to compute highly accurate positions. Standard GPS/GNSS positioning incurs errors on the order of meters due to atmospheric effects, satellite clock errors, and so on. RTK performs relative comparisons with the precise position of a nearby reference station (base station) and corrects error factors. Simply put, a base station with a “known position” and a mobile receiver (rover) receive the same satellite signals; the error information measured at the base station is sent to the rover in real time so the rover can improve its own position to centimeter-level accuracy.

RTK uses carrier-phase measurements, enabling much finer measurement resolution than conventional code-based positioning. As a result, high-precision positioning within a few centimeters horizontally and vertically can be achieved. However, RTK requires radio communication (or network communication) between the base and rover, and an environment that can deliver correction information within the positioning area. RTK also performs best in open skies without tall buildings or obstacles. That said, when conditions are met, RTK can provide real-time accuracy on the order of a few centimeters, making it a powerful tool for surveying and construction sites.

Note that RTK is broadly a form of differential GPS (DGPS), but the major difference is that traditional DGPS (mainly code-based corrections) provided accuracies of meters to sub-meters, whereas RTK achieves orders-of-magnitude improvements by analyzing carrier-phase data.

Accuracy comparison: GPS/GNSS vs RTK

Now let’s compare the accuracy difference between standalone GPS/GNSS positioning and RTK positioning. Standalone positioning means determining position with a single receiver without base station corrections — the method typically used by our smartphones and car navigation systems.

• GPS (GNSS) standalone positioning: Typical errors are about 5–10 m (16.4–32.8 ft). In ideal conditions errors may be a few meters, while in urban or mountainous environments with disturbed satellite signals errors can exceed 10 m (32.8 ft). For example, you may have experienced map apps showing your position offset from the road or your location jumping around every few seconds — this is caused by GPS error. Vertical (elevation) errors are often larger than horizontal errors and can also exceed 10 m (32.8 ft). Therefore, measuring height accurately with standalone positioning is practically difficult.

• RTK positioning: When positioning with corrections from a base station, errors shrink dramatically. If RTK achieves a “FIX” solution, horizontal positions are within about ±1–2 cm (±0.4–0.8 in) and vertical (height) within about ±3 cm (±1.2 in). This is orders of magnitude more accurate than typical GPS. To put it bluntly, an error of 5 m (16.4 ft) versus 5 cm (2.0 in) is the difference between the size of a car and the tip of a finger. RTK also enables accurate height measurements, allowing precise measurement of terrain elevation and structure heights.

As described above, RTK greatly improves GPS/GNSS accuracy. However, achieving RTK accuracy requires the solution to transition from a “float” to a “fix,” which can take on the order of tens of seconds; in poor environments it can take longer or may fail to fix. Once fixed, RTK maintains stable centimeter-level accuracy in real time.

Positioning accuracy required on construction sites

So what level of positioning accuracy is required on construction sites? In short, typical GPS accuracy (a few meters) is often insufficient. Construction sites require placing structures per design drawings and strictly managing elevation differences, so meter-level offsets are unacceptable.

For example, if a building foundation is placed 5 m (16.4 ft) off the intended position it would be a major problem, and a 1 m (3.3 ft) error in earthworks elevation could disrupt drainage slopes. Civil and architectural surveying typically requires centimeter-level accuracy (half-inch accuracy). Traditionally, optical surveying instruments such as transits and total stations were used by experienced surveyors to carefully set out points and check elevations. However, these methods are time-consuming, labor-intensive, and require skilled personnel.

In recent years, GNSS-based surveying, especially RTK high-precision positioning, has gained attention. With RTK-GNSS, in open outdoor conditions you can quickly establish survey benchmarks and measure as-built shapes. Errors of a few centimeters (a few inches) allow accurate assessment of ground elevation and earthworks extent, improving quality control and as-built verification efficiency. Additionally, GNSS receivers are increasingly mounted on construction machinery such as bulldozers and excavators to enable machine guidance/machine control using RTK high-precision information. This allows operators to cut and fill accurately without relying solely on experience or intuition. In promoting such ICT construction, high-precision GNSS (RTK) is indispensable.

In this way, differences of a few centimeters directly affect quality and safety on construction projects. High-precision surveying prevents rework due to positional errors and contributes to shorter schedules and cost savings. Since standalone GPS with meter-level accuracy cannot meet these needs, high-precision positioning technologies like RTK have become essential.

Barriers to RTK adoption

While the usefulness of RTK is clear, there are several barriers to practical on-site use. First, RTK positioning requires preparation of dedicated equipment and an appropriate environment. Traditionally, high-precision GNSS receivers necessitate either setting up your own base station or subscribing to a commercial or public correction service (for example, the Geospatial Information Authority of Japan’s reference station network or VRS services). If you set up a base station, you need to pre-survey the exact coordinates of that station.

Second, there are cost and expertise issues. High-precision GNSS receivers and base station equipment can be expensive, and configuring and operating them requires knowledge of positioning. Moreover, the construction industry faces a shortage and aging of survey personnel, increasing the need for positioning solutions that do not rely heavily on specialists. Small contractors or rural sites often do not have dedicated surveyors on staff, so mastering RTK can be challenging. RTK positioning is also affected by radio conditions and communication environments, so in some locations centimer-level stability may be difficult to achieve.

For these reasons, RTK has sometimes been perceived as “something for advanced surveyors” or “difficult to introduce except for large projects.” However, recently solutions that lower these barriers and enable anyone to easily use RTK surveying have emerged.

Simplified surveying with LRTK

One such new solution is simplified surveying with LRTK. LRTK is a system that achieves RTK centimeter-level positioning simply by combining a dedicated small GNSS receiver device with a smartphone. Even without complex equipment operation or specialist knowledge, users can follow a smartphone app to set up the device and automatically acquire correction information via the Internet, enabling positioning within ± a few centimeters (± a few inches).

For example, tasks that previously required hiring a surveying company—such as measuring ground elevation or staking out construction locations—can be performed by the site staff themselves using LRTK. This allows work that formerly required external surveyors to be done in-house in a short time, improving efficiency and reducing costs. Because you don’t need to purchase expensive surveying instruments, the entry barrier is low. In one municipality, such smartphone-linked RTK systems (a combination of a smartphone and a high-precision GNSS receiver) were introduced for disaster recovery sites and enabled rapid surveying of damaged areas. Simplified surveying with LRTK brings high-precision positioning, which previously required experts, closer to everyday use and allows small sites and labor-short sites to utilize accurate location information.

The accuracy achievable with LRTK systems reaches levels comparable to dedicated high-end GNSS surveying instruments: about 1–2 cm (0.4–0.8 in) horizontally and about 3 cm (1.2 in) vertically. At the same time, device costs are lower than traditional surveying instruments and operation is simple. By attaching the device and a smartphone to a dedicated pole (monopod) and pressing a button, height offset calculations and averaged positioning are performed automatically, so people without surveying expertise can operate it.

Thus, simplified surveying with LRTK is an innovative technology that brings the benefits of RTK to a wider audience. In construction projects it can be a trump card to promote on-site DX (digital transformation) while balancing accuracy and efficiency.

Given these advantages, the easy high-precision positioning enabled by LRTK is expected to be increasingly adopted in surveying and construction sites going forward.

Frequently Asked Questions

Q1. How are GPS and GNSS different? A1. GPS is the U.S. satellite positioning system, while GNSS is the collective term for all satellite positioning systems including GPS. Besides GPS, there are systems such as Russia’s GLONASS, Europe’s Galileo, and Japan’s Quasi-Zenith Satellite System (QZSS); collectively these are called GNSS (Global Navigation Satellite System). In common parlance people often say “GPS,” but “GNSS-capable equipment” means devices that can use multiple types of satellite signals.

Q2. How accurate is GPS (GNSS) positioning? A2. Standalone GPS/GNSS positioning typically has an accuracy of about 5–10 m (16.4–32.8 ft). In open areas with good satellite visibility the error may be a few meters, while in urban or mountainous areas it can deviate by 10 m (32.8 ft) or more. Vertical errors are larger than horizontal and can exceed 10 m (32.8 ft). You may sometimes see your smartphone’s map position be off by tens of meters; that is due to GPS error.

Q3. How much does accuracy improve when using RTK? A3. RTK positioning can reduce errors to the order of a few centimeters. Specifically, under conditions with sufficient satellite tracking and available correction information, horizontal accuracy of about ±1–2 cm (±0.4–0.8 in) and vertical accuracy of about ±3 cm (±1.2 in) can be expected. This is far more precise than standard GPS. However, achieving this accuracy requires receiving correction data from a base station and obtaining a “Fix” solution; depending on conditions it may take time to get a Fix or the Fix may be unstable.

Q4. Why is high-precision positioning necessary on construction sites? A4. On construction sites structures must be built to the correct position and elevation, and deviations of a few centimeters can cause serious problems. For example, even a 5–10 cm (2.0–3.9 in) offset in foundation position can lead to construction defects. Typical GPS errors of several meters are too coarse for accurate surveying and as-built management. That is why optical surveying instruments like total stations or high-precision GNSS positioning such as RTK are required. Only with high-precision positioning can safe, high-quality construction management be achieved.

Q5. What is needed to use RTK positioning? A5. To use RTK you generally need a reference station (base) and a rover. RTK cannot be done with a single receiver alone. Specifically, a high-precision GNSS receiver (rover) and correction data from a base station with known coordinates are required. The correction data is delivered to the receiver via radio communication or the Internet (mobile networks, etc.). Instead of setting up your own base station, it is common to use the Geospatial Information Authority’s reference station system or commercial network RTK services. Recently, smartphone-linked easy RTK devices (such as LRTK) have appeared, making it possible to use high-precision positioning without assembling all dedicated equipment.

Q6. What is LRTK? A6. LRTK is a simplified RTK positioning system used in combination with a smartphone. A dedicated small GNSS receiver (LRTK device) connects to a smartphone and acquires correction information via the Internet, enabling centimeter-level positioning without large surveying equipment. The smartphone app automates specialized settings, so people without surveying knowledge can use it. With LRTK, high-precision position information can be obtained quickly by non-experts, making it useful for small-scale work and emergency site surveying.