Comparison of RTK Positioning and Standalone GPS Positioning Accuracy: The Correct Choice for Sites Requiring cm-level Accuracy (half-inch accuracy)

For field practitioners struggling with on-site positioning accuracy, choosing between RTK positioning and standalone GPS positioning is an important decision that directly affects the quality of work. In this article, by presenting concrete numerical values for the accuracy differences between the two methods, we explain why RTK positioning is chosen at sites where centimeter-level accuracy (cm level accuracy (half-inch accuracy)) is required, and the reasons and selection criteria.

Table of Contents

• Accuracy of Standalone GPS Positioning and Its Limitations

• How RTK Positioning Achieves Centimeter-Level Accuracy

• Accuracy Comparison Table by Positioning Method

• Specific Scenarios Where Centimeter-Level Accuracy Is Required

• Operational Considerations That Affect Accuracy

• Choosing Based on the Balance Between Cost and Accuracy

• Compact RTK Positioning Achieved with LRTK

Accuracy and Limitations of Standalone GPS Positioning

Standalone GPS positioning (standalone GPS) is a method in which radio signals transmitted from satellites are received by a single receiver, and the signal arrival times are used to calculate the position. Because this method has a simple structure and low equipment cost, it is widely used for everyday applications such as smartphones and car navigation systems.

However, positioning accuracy is about 3-5 m (9.8-16.4 ft) horizontally even under ideal conditions. This is because ionospheric delay and tropospheric delay that occur when satellite signals pass through the atmosphere, satellite orbital errors, receiver clock errors, and so on are not corrected. In urban areas with densely packed buildings, near tunnels, or in mountainous areas with dense tree cover, satellite signals can be blocked or reflected (multipath), which can cause errors of up to 10-20 m (32.8-65.6 ft).

For tasks at civil engineering and construction sites, such as boundary determination, batter-board installation, and the placement of ground control points for drone surveying, this level of error is completely unacceptable in practice.

How RTK Positioning Achieves Centimeter-Level Accuracy

RTK (Real-Time Kinematic) positioning is a technique that uses two receivers, a base station and a rover, to substantially cancel out error sources in satellite positioning.

The reference station is fixedly installed at known coordinates and receives satellite signals to continuously calculate the difference between the "value computed as its own position" and the "known true coordinates." This difference constitutes correction information that includes error components such as ionospheric, tropospheric, and orbital errors, and is transmitted in real time to the mobile station via radio or cellular networks. Because the mobile station performs positioning while receiving this correction information, the same error components are canceled out and a highly accurate position is calculated.

Furthermore, RTK positioning performs calculations using the carrier wave phase (carrier phase). By counting the phase cycles of the radio waves transmitted by the satellites, it achieves far higher resolution compared to conventional code-based positioning. Once it reaches the state called the "FIX solution", stable measurements of horizontal accuracy of 1–2 centimeters (0.4–0.8 in) and vertical accuracy of 2–3 centimeters (0.8–1.2 in) can be obtained.

Accuracy Comparison Table by Positioning Method

When comparing the three methods—GPS standalone positioning, SBAS augmentation (such as Michibiki’s L1S signal), and RTK positioning—they have the following characteristics, respectively.

Standalone GPS positioning has a horizontal error of 3–5 m (9.8–16.4 ft) — in unfavorable environments 10 m (32.8 ft) or more — and because reception of correction information is unnecessary, the equipment is inexpensive and operation is simple. However, the stability of accuracy is low, and it varies greatly depending on the on-site environment.

SBAS augmentation (satellite-based augmentation system) has a horizontal error of about 1–2 m (3.3–6.6 ft), and by using correction information transmitted from geostationary satellites, it improves accuracy compared with standalone positioning. Applications to smartphones and agricultural equipment are progressing, but it does not meet the accuracy required for surveying and construction management.

RTK positioning has horizontal errors of 1-2 cm (0.4-0.8 in) and vertical errors of 2-3 cm (0.8-1.2 in), and uses a dedicated reference station or a network correction service. It meets the accuracy required for surveying, construction, precision agriculture, and drone operations.

Specific situations that require centimeter-level accuracy (cm level accuracy (half-inch accuracy))

In staking out (stake setting) work that accurately reproduces the coordinates from design drawings on site, construction quality is directly affected if errors exceed several centimeters (a few in). For constructing exactly to the design values—such as road centerlines, curb locations, and structure foundation positions—RTK positioning is effectively a prerequisite.

The accuracy of orthophotos and point cloud data produced by drone surveying depends on the coordinate accuracy of ground control targets (ground control points) placed on the ground. By using RTK positioning to measure the control points, the absolute accuracy of the entire drone dataset can be kept within a few centimeters (a few in).

In agriculture, RTK positioning used for steering control of auto-steer tractors and seeders ensures that tractors follow the previous pass precisely. In seeding operations with 20–25 cm (7.9–9.8 in) spacing, managing errors of 2–3 cm (0.8–1.2 in) is a critical factor directly linked to yield.

Operational considerations affecting accuracy

The nominal accuracy of RTK positioning (1-2 cm (0.4-0.8 in)) is a figure for favorable conditions only. In field operations, several factors affect accuracy.

Verification of a FIX solution is essential. If the RTK receiver is not outputting a FIX solution, accuracy cannot be guaranteed. To avoid mistakenly using measurements from a FLOAT solution or SPPS state, make it a habit to always check the device’s status display.

As the baseline length (the distance between the reference station and the rover) increases, the error components common to the reference station and the rover that cannot be fully cancelled increase, and accuracy decreases. Network RTK mitigates this issue, but it depends on the operational status of the distribution service.

The antenna mounting surface is also important, as the orientation of the receiver and the surrounding radio environment affect multipath. Before measurement, check the number of satellites in view and the PDOP value (the degradation rate of accuracy due to satellite geometry), and avoid taking measurements under poor conditions to help ensure stable accuracy.

Choose Based on the Balance Between Cost and Accuracy

Conventional RTK positioning equipment was expensive, costing several hundred thousand yen to over one million yen per set, so distributing them to all workers or deploying them simultaneously across multiple sites imposed a significant cost burden.

In recent years, compact RTK receiver modules that attach to smartphones such as the iPhone have appeared, making it possible to achieve RTK accuracy at a significantly lower cost compared with conventional equipment. Because the need to purchase additional dedicated hardware is minimized and operation is completed via a smartphone app, they are valued for being easy for on-site staff who are not surveying professionals to use.

When selecting equipment, it is important to comprehensively evaluate the required accuracy, frequency of use, on-site communication environment, ease of operation, and implementation cost.

Compact RTK positioning achieved with LRTK

LRTK (an iPhone-mounted GNSS high-precision positioning device) is a device that enables RTK positioning simply by attaching it to an iPhone. It supports network corrections and can obtain a FIX solution over a wide area within the country.

The large antennas and dedicated controllers that were required for conventional RTK equipment are no longer necessary, allowing on-site personnel to use the iPhones they carry every day as-is. The measured data can be instantly checked and managed in the app, and data integration with CAD and business systems is smooth.

Accurately understanding the accuracy difference between standalone GPS positioning and RTK positioning, and selecting a positioning solution that meets the site's requirements, is the quickest way to eliminate rework and improve overall construction quality.