Best Practices for Using Control Points in RTK Surveying

Table of Contents

• Introduction

• What is RTK Surveying

• Control Points Required for RTK Surveying

• What Are Ground Control Points (GCP)

• How GCP Placement Changes with RTK Adoption

• Best Practices for Using Control Points in RTK Surveying

• Benefits and Caveats of RTK Surveying

• Simple Surveying with LRTK

• FAQ

Introduction

In recent years, RTK surveying (real-time kinematic positioning) has been attracting attention in surveying and construction fields. RTK is a technology that achieves dramatically higher accuracy (on the order of a few centimeters (a few in)) compared to standalone GPS positioning by correcting satellite positioning errors in real time. Its adoption has greatly streamlined surveying tasks that previously required significant time and effort. RTK is also being promoted under the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* initiative.

However, achieving high-precision positioning requires points with known coordinates to serve as control. In drone photogrammetry and civil engineering surveys, many ground control points (GCPs) have traditionally been installed to guarantee the accuracy of survey results. Using RTK surveying can significantly reduce the effort required to install such control points. This article explains the concept of control points in RTK surveying and the role of GCPs, and explores key points and best practices for conducting efficient, high-precision surveys.

What is RTK Surveying

RTK surveying is a method that uses two GNSS receivers (a base station and a rover) to correct positioning errors in real time and achieve centimeter-level (inch-level) high-precision positioning. RTK stands for Real Time Kinematic and is a type of satellite positioning technology. With standard GPS positioning (standalone), errors of several meters (several ft) can occur due to ionospheric delay and satellite orbit errors, but RTK observes and estimates these error factors at the base station and sends corrections to the rover, allowing errors to be reduced to within a few centimeters (a few in).

In RTK, one receiver is installed at a point with known coordinates as the base station, and the other is carried around as the rover. Both receivers receive signals from the same satellites; the base station calculates error information from its own positioning data and transmits it to the rover via radio or communication lines. The rover applies the received correction information to its own position in real time to obtain accurate coordinates. For example, even in Japan’s geodetic system, using RTK allows immediate computation of coordinates in the World Geodetic System at centimeter-level (inch-level) accuracy on site, enabling precision surveys that were previously difficult at the field level.

A communication link for exchanging correction data between the base and rover is essential for RTK surveying. Common methods include sending signals directly from the base station using UHF radios, or connecting to a correction information service over the Internet using network RTK (Ntrip). In the latter case, virtual reference station (VRS) methods that use the Geospatial Information Authority of Japan’s permanent station network data allow high-precision positioning without setting up your own base station. However, because real-time methods lose accuracy or cease positioning when communications are interrupted, a stable communication environment is important. Also, RTK errors tend to increase with distance from the base station (longer baseline), so in practice surveying within a range of a few kilometers is recommended.

Control Points Required for RTK Surveying

To achieve high accuracy with RTK surveying, control points for placing the base station are indispensable. A control point here refers to a location whose coordinates are known accurately in advance. When using RTK at a construction site, you place the base station antenna on a nearby known point such as a public triangulation point or benchmark, or on a point whose coordinates have been established in advance by total station survey. Using the known coordinates of that control point as the foundation for RTK computations gives absolute reference to the coordinates obtained by the rover.

If there are no suitable known points near the site, there are alternative approaches. One is to use a network RTK service that leverages the Geospatial Information Authority’s permanent station data to obtain corrections from a surrounding fixed reference station network (VRS). Another is to temporarily set up a base station at an arbitrary point, perform RTK surveying with provisional coordinates, and later re-determine that base station position precisely via static surveying or control surveying and apply an offset correction to all measured data. Thus, methods for obtaining control point coordinates include using existing known points, using real-time services, or applying post-calculation corrections. In any case, in RTK the accuracy of the coordinate assigned to the base station directly affects the overall positioning accuracy. It is important to operate with the understanding that the technology is fundamentally "dependent on control points."

What Are Ground Control Points (GCP)

GCPs (Ground Control Points) are ground-installed targets with known coordinates used as references in aerial photogrammetry and UAV (drone) surveys. In Japanese they are generally called "ground reference points"; cross marks or panels are placed on the ground and their precise coordinates are determined by prior surveying. When performing 3D surveys (such as SfM analysis) using drone imagery, if these GCPs appear in the photos, the model generated in an arbitrary image coordinate system can be aligned to real-world coordinates. In other words, GCPs function as true positional anchors that give the photogrammetric model its real-world coordinates.

In traditional photogrammetry, multiple GCPs are typically placed around and within the site to increase accuracy. For a large survey area, for example, GCPs would be placed at the four corners and near the center to correct distortions in the produced point cloud data and terrain model. With a sufficient number of properly distributed GCPs, the entire model can be matched to geographic coordinates with high accuracy. However, installing and surveying GCPs requires manpower and time, and in places that are difficult to access such as forests or steep slopes, installation itself can be challenging.

How GCP Placement Changes with RTK Adoption

The advent of RTK technology is changing how GCPs are handled in drone aerial photography and photogrammetry. Using RTK-equipped drones or cameras, the geotag of each photo is highly accurate from the outset. Therefore, in theory, it may be possible to align photogrammetric models to map coordinates accurately without installing many GCPs that were traditionally required.

In practice, validations using RTK drones have reported cases where 3D surveys achieved errors on the order of a few centimeters (a few in) even without any GCPs. In one experiment, aerial data acquired in RTK mode using a fixed base station produced a terrain model with accuracy comparable to the case with GCPs (errors of about 2–3 cm (0.8–1.2 in)). Of course, in real-world conditions there is a risk of signal loss or RTK reception failures, so it is safer to install a few verification checkpoints (a small number of GCPs) as a precaution. Nevertheless, RTK adoption can greatly reduce the number of ground control points needed, eliminating the necessity to scatter 10 or 20 GCPs as before, which is a major benefit.

This reduction in GCPs is also an important theme in the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* initiative. The bottleneck of “installing many ground control points” that hindered surveying productivity is being alleviated by GNSS technologies such as RTK and PPK. As a result, reductions in surveyor workload, shorter work times, and improved safety (less entry into hazardous areas) can be expected.

Best Practices for Using Control Points in RTK Surveying

To perform RTK surveying more reliably and efficiently, there are best practices to keep in mind regarding the handling of control points. The main points are listed below.

• Use known points: Where possible, install the base station on nearby known control points such as triangulation stations or benchmarks and use those accurate coordinates as the reference for surveying. Using known points gives definite absolute accuracy to the positioning results.

• Use network RTK: If there are no suitable known points nearby, consider obtaining correction information from a network RTK service (VRS method) that uses the Geospatial Information Authority’s permanent station network. Using correction data from a virtual reference point enables high-precision positioning without installing a physical control point on site.

• Post-correction of the base station: If you placed a provisional base station at an arbitrary point for surveying, calculate the exact coordinates of that point afterward through static positioning or combined surveying with known points, and apply a global offset correction to all measured results. Determining the control point coordinates afterward allows final deliverables to be aligned to the designated coordinate system.

• Monitor communications and reception status: Continuously monitor the communication status between base and rover during surveying, and record positioning data logs in case the link is lost. Also check satellite reception; in environments with insufficient satellites, postpone measurements or switch to post-processing (PPK) as a risk mitigation measure.

• Ensure clear sky view and mitigate multipath: Choose locations with as open a sky view as possible for both base and rover to avoid satellite signal blockage and multipath interference from trees and buildings. For points requiring high precision, pay close attention to surrounding conditions during observations.

• Verify accuracy with checkpoints: For important surveys, install and measure a small number of independent checkpoints (verification control points) to compare against RTK positioning results and confirm errors. Such verification enhances quality assurance of RTK surveys.

• Confirm fixed solution (Fix): Always confirm that the RTK solution is a fixed solution (Fix) while observing. A float solution (Float) results in lower accuracy, so generally record survey points only after a Fix solution is obtained. Continuously monitor solution status on the receiver display or software; if the solution becomes Float, wait until sufficient accuracy is achieved or improve positioning conditions.

Benefits and Caveats of RTK Surveying

Using RTK technology in the field provides many benefits, but there are also points to be cautious about in operation. Below are the main benefits and considerations.

Benefits of RTK Surveying:

• Immediate high-precision positioning: Centimeter-level coordinates can be obtained on site, allowing on-the-spot data checks and comparisons with design values. This reduces re-measurement effort and enables quick decision-making.

• Dramatically improved work efficiency: By carrying a GNSS antenna, many points can be measured quickly. Large-area terrain surveys that previously required a total station and multiple line-of-sight setups can be completed with fewer personnel and much faster using RTK.

• Simplified GCP installation: As noted earlier, RTK can greatly reduce the work of placing and removing many GCPs during drone surveys, directly shortening field time and reducing labor costs.

• Strong performance in wide or difficult areas: RTK drones can survey from the air and GNSS receivers can position from a distance in swamps, steep slopes, and other places that are hard to access on foot. This reduces work in hazardous locations and improves safety.

• Applications such as machine guidance: The real-time, high-precision position information is being increasingly applied to machine guidance for construction equipment and autonomous support for heavy machinery. Immediate, accurate positioning feedback contributes to automation and efficiency in construction processes.

Caveats of RTK Surveying:

• Dependence on communication environment: Real-time positioning requires a continuous communication link from the base to the rover. In mountainous areas or downtown areas with high-rise buildings, radio signals can be blocked and RTK positioning can be interrupted. If communications are unstable, consider temporarily switching to static surveying or post-processing (PPK) as a mitigation.

• Influence of satellite reception conditions: In locations without an open sky view (under trees or in building shadows), satellite signals are difficult to receive and RTK accuracy may decline. When conducting GNSS surveys, be mindful of obstructions and observe in as open an environment as possible.

• Accuracy management of control point coordinates: If the coordinate used for the base station has errors, those errors are directly reflected in the overall positioning results. Determine control point coordinates as rigorously as possible and, if necessary, cross-check with known points or perform post-survey verification. Applying offset corrections by statically surveying the base station after the survey is also effective.

• Initial investment cost: Introducing RTK requires some initial investment in GNSS receivers, communication modems, and possibly subscription fees for correction services. However, low-cost GNSS devices and correction services have emerged in recent years, lowering the barrier to adoption.

• Skills required for operation: Using RTK effectively requires a certain level of expertise. For example, it is important to understand the difference between fixed (Fix) and float (Float) solutions and to monitor that a Fix solution is being obtained. Also be careful not to misconfigure coordinate systems, such as mixing up the World Geodetic System with a local coordinate system.

Simple Surveying with LRTK

We offer a portable GNSS receiver called "LRTK" as a solution to easily use high-precision RTK surveying on site. LRTK is a small device that works with a smartphone or tablet and is an all-purpose surveying tool that field surveyors can operate intuitively. RTK positioning that previously required specialized equipment and extensive experience can be started easily by anyone using LRTK.

One feature of LRTK is its ability to flexibly switch positioning methods according to the communication environment. Normally, it connects to a network RTK (Ntrip) service via the smartphone’s Internet connection to obtain corrections on site and achieve centimeter-level (inch-level) positioning. In areas where mobile communications are unavailable, such as mountainous regions, LRTK can also utilize the Quasi-Zenith Satellite System “Michibiki” centimeter-class augmentation service (CLAS). With a dedicated antenna, LRTK provides the flexibility to perform real-time high-precision positioning even where the Internet is not available.

Additionally, LRTK has cloud-linked data management functions. Points positioned with LRTK and photos taken on site are automatically plotted on a cloud map and can be shared in real time within a team. For example, during infrastructure inspections, photos of damage taken with a smartphone connected to LRTK are immediately tagged with accurate coordinates and shared with office staff. This reduces time spent on report creation and speeds up coordination among stakeholders.

By using LRTK, surveying productivity and accuracy on site can be dramatically improved. The compact receiver can be operated with one hand and complex device settings are unnecessary, greatly increasing field mobility. As a solution that contributes to the Ministry of Land, Infrastructure, Transport and Tourism’s push for DX (digital transformation) of construction sites, LRTK has already been adopted at many construction sites. LRTK, as a solution for "making high-precision positioning easier and more accessible," will significantly change future surveying practices.

FAQ

Q1. Why are control points (known coordinates) necessary for RTK positioning? A. In RTK, the base station serves as the foundation for error correction. The base station’s position is treated as "correct," and the rover’s observations are corrected relative to it; if the base station’s coordinates are not accurate, high-precision positioning cannot be achieved. In other words, to cancel errors you must know the starting control point’s coordinates accurately.

Q2. What if there are no known control points at the site? A. There are several options. One is to use a network RTK service that leverages the Geospatial Information Authority’s permanent station network to obtain corrections from a virtual reference station (VRS). This allows high-precision positioning without preparing physical control points on site. Another is to place a provisional base station at an arbitrary point, perform surveying, and later determine that point’s accurate coordinates via static positioning or control surveying and apply a global correction to the measurements. This takes time and effort, but if the control point coordinates are determined afterward, the final deliverables can be aligned to the designated coordinate system.

Q3. Will RTK surveying eliminate the need for GCPs entirely? A. It depends. If RTK-equipped devices consistently maintain high-precision positioning, then in theory surveying can be accurate without GCPs. Cases where only a few checkpoints are sufficient are becoming more common. However, in reality, signal loss or multipath interference can introduce errors. For important survey deliverables, it is advisable for quality assurance to install a few GCPs for verification. While RTK performance improvements have greatly reduced the number of required GCPs, it is prudent to keep safeguards rather than insisting "zero is absolutely fine."

Q4. What is the difference between RTK and PPK? A. RTK (Real Time Kinematic) applies corrections from a base station in real time during surveying to determine high-precision positions on the spot, but it requires a communication environment in the field. PPK (Post-Process Kinematic) combines rover and base station data after data collection and applies corrections in post-processing. PPK does not require communications at the site and can compensate for satellite outages, but results are obtained after processing rather than immediately. Each method trades off real-time capability versus stability, and they are chosen according to site conditions.

Q5. Is there an easy way to start RTK positioning? A. One accessible option is to use a portable GNSS receiver like "LRTK." This device connects to a smartphone and enables centimeter-level positioning in real time without complicated setup. It supports corrections via communication lines and satellite augmentation signals (such as Michibiki’s CLAS), allowing surveys to begin immediately on site. LRTK is recommended for those new to RTK because it is easy to use and enables quick adoption of high-precision positioning.

Q6. What level of accuracy can RTK actually achieve? A. In good environments with proper operation, RTK can achieve approximately 1–2 cm (0.4–0.8 in) horizontal accuracy and about 2–5 cm (0.8–2.0 in) vertical accuracy. Typical RTK-GNSS receiver specifications state values such as horizontal ±(1 cm (±0.4 in) + 1 ppm) and vertical ±(2 cm (±0.8 in) + 1 ppm), and errors are very small when the distance to the base station is short. However, as distance from the base increases, error factors accumulate and maintaining a fixed solution becomes more difficult over long distances of several kilometers. Network RTK (VRS) can greatly mitigate this issue, but in situations with few satellites or poor radio environments, accuracy may degrade and errors of tens of centimeters may occur. In short, RTK accuracy depends on environment and operation, but under favorable conditions it can meet surveying accuracy requirements (errors within a few cm).