Pre-construction Investigation with RTK: Quickly Mapping Existing Conditions

Table of Contents

• Introduction

• What is RTK?

• Importance of Pre-construction Investigation

• Rapid Existing-conditions Mapping Using RTK

• Benefits of Using RTK

• Key Points for Introducing RTK Surveying

• Conclusion

• Simple Surveying with LRTK

• FAQ

Introduction

Before starting earthworks, it is essential to accurately understand the site’s terrain and existing structures through a “pre-construction investigation.” Traditionally, as-built surveys for this purpose required significant time and effort and often many personnel. It is particularly difficult to produce detailed topographic maps in a short period for large development sites or highly undulating terrain. However, in recent years the spread of a high-precision positioning technology called RTK has dramatically improved the efficiency of pre-construction as-built mapping. This article explains how to use RTK technology to quickly map existing conditions and introduces its benefits and points to consider when adopting it.

What is RTK?

RTK (Real-Time Kinematic) is a technology that dramatically improves the positioning accuracy of GNSS (Global Navigation Satellite Systems) such as GPS. While standalone GPS positioning can have errors of several meters, RTK uses correction information from a base station (reference point) to correct positioning errors in real time. As a result, positions can be determined with an accuracy of a few centimeters (a few in). In RTK surveying, a rover and a base station communicate to correct satellite signal error factors (such as atmospheric effects and clock errors), achieving high accuracy. In recent years, *network RTK* (such as VRS) has become widespread, allowing users to take advantage of RTK positioning without installing a dedicated base station by receiving reference-station data over the internet. RTK technology is now used not only in civil surveying but also in many other fields requiring high-precision positioning, such as autonomous tractors in agriculture and autonomous drone flight.

Importance of Pre-construction Investigation

During the planning stage of earthworks, accurately understanding the site’s topography and obstacles is critical. The topographic data obtained from pre-construction investigations form the essential basis for creating design drawings, calculating earthwork volumes, and planning construction. For example, if the existing ground surface elevations are misunderstood, the estimated volumes of fill and cut can be significantly off, which could affect later construction costs and schedules. Also, knowing the locations of buried objects and existing structures can prevent construction troubles such as pipe damage or unexpected obstacles. Pre-construction as-built data also serve as a reference for comparing how the terrain changed after construction, useful for post-completion verification and dispute prevention. In this way, as-built surveying is the foundation for ensuring construction quality and safety. However, spending too much time on investigations delays the start of work and can affect the overall schedule. RTK-based rapid surveying is attracting attention as a means to efficiently obtain detailed as-built data within limited timeframes.

Rapid Existing-conditions Mapping Using RTK

By using RTK, as-built mapping can be carried out far more quickly than before. With a high-precision GNSS receiver (rover), a surveyor can walk the site and rapidly observe elevation points and feature points across a wide area, even by a single person. For example, on a development site of several hectares, a worker carrying an RTK receiver can walk and periodically record points to comprehensively capture the ground surface topography in a short time. The obtained coordinate data can be displayed in real time on a tablet or controller, enabling immediate checks for omissions and data-quality verification.

Furthermore, combining drone (UAV) surveying with RTK greatly increases the efficiency of 3D site mapping. Using RTK-equipped drones to capture aerial images and generating detailed terrain models and orthophotos via photogrammetry is increasingly adopted on many sites. Because RTK enhances the drone’s positional information, the large number of ground control points traditionally required can be greatly reduced, and post-processing of imagery is faster. A short flight can yield point-cloud datasets of millions of points and high-resolution site images, proving powerful for broad topographic understanding and earthwork volume calculations. While conventional ground surveying often yields sparse measurement points, generated point-cloud data reflect fine surface irregularities, contributing to improved design accuracy.

In addition, advances in mobile mapping technologies now allow scanning surroundings with vehicle-mounted or portable sensors while moving. There are many options, such as mobile laser scanners combined with RTK or handheld measurements using smartphones with compact GNSS receivers. The use of these ICT surveying technologies, including drones and mobile surveys, aligns with the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* initiative (measures to improve construction site productivity), and adoption is spreading across the industry. By choosing the optimal RTK-based measurement method based on site scale and conditions, pre-construction investigations can be completed quickly and with high accuracy.

Benefits of Using RTK

RTK-based as-built surveys offer many benefits, including:

• Increased speed: Large areas can be surveyed in a short time, significantly reducing working hours. Surveys that used to take several days can, in some cases, be completed in a few hours to a day using RTK and drones.

• High-accuracy data: Positioning accuracy at the centimeter level improves the reliability of obtained topographic data, allowing confident use for design and earthwork calculations.

• Labor savings and reduced personnel: More tasks can be performed by a single person, streamlining work that previously required multiple staff. This helps mitigate labor shortages and reduce labor costs.

• Improved safety: RTK enables remote positioning in hazardous slopes or unstable footing, helping ensure worker safety. Drone use can reduce the risk of surveying at heights or steep slopes.

• Real-time results: Because measurement results can be checked on-site immediately, missing or incorrect data can be identified and remeasured on the spot. This enables rapid data provision to designers and enhances responsiveness.

• Use of 3D data: By combining photogrammetry or laser scanning, dense point clouds and detailed 3D models can be created. These support advanced analyses such as comparisons with final models, earthwork calculations, and simulations.

• Immediate use of digital data: RTK-acquired survey data are electronic from the start, making it easy to share via the cloud or import into drawing software. Up-to-date data can be shared between field and office immediately, speeding up post-survey design and review work.

Key Points for Introducing RTK Surveying

When introducing RTK technology on site, several points should be noted:

• Ensure good satellite reception conditions: RTK positioning requires a clear view of the sky. In urban areas surrounded by tall buildings or in forests, satellite signals can be blocked, degrading accuracy or causing unstable positioning (float solutions). Keep the antenna’s sky view unobstructed during positioning and secure as wide a field of view as possible.

• Reference points and coordinate systems: For accurate surveying, it is important to start positioning based on appropriate known points (reference points with known coordinates). When producing results in public coordinate systems (World Geodetic System, Japan Geodetic System), align reference-point coordinates using electronic reference stations where available. Even when working in a local coordinate system, calibrate the RTK to local known points or reference stakes and confirm there is no offset in measurements.

• Communications and correction data: For network RTK, the rover needs an internet connection. Confirm in advance that the site has network coverage and prepare communication methods such as mobile routers or tethering. As a contingency for unstable communications, consider switching to post-processing kinematic (PPK) or planning to set up your own base station.

• Managing positioning accuracy: RTK displays accuracy indicators in real time (whether the solution is FIX or FLOAT, estimated errors, etc.). Aim to maintain FIX solutions, and when observing critical points, take several measurements to confirm stable values. Vertical accuracy is generally more susceptible to error than horizontal accuracy, so when strict height control is required, it is ideal to double-check with leveling surveys or other verification methods.

• Combining with conventional methods: RTK is not omnipotent, so combining methods as appropriate is important. For example, for boundary setting that requires millimeter-level accuracy or for establishing reference points for important structures, final confirmation with a total station or optical level may be preferable. Use RTK to maximize efficiency where appropriate and supplement with conventional methods for high-precision requirements to balance efficiency and accuracy.

• Equipment management and weather conditions: Ensure GNSS receivers and mobile devices are fully charged for long work sessions and carry spare power supplies. Even for equipment with dust and water protection, do not force operation in heavy rain or strong winds—deciding to survey only in safe conditions is important.

Conclusion

With the advent of RTK, pre-construction as-built investigations can now be conducted with unprecedented speed and accuracy. Being able to obtain precise topographic data in a short time directly accelerates planning and reduces rework. Even on surveying sites where labor shortages are a concern, RTK enables a small team to cover a wide area, reducing workload. Feeding obtained 3D data back into design and construction enables more reliable construction management and shape control.

As technology advances, surveying is shifting from work that relied on “the experience and intuition of skilled personnel” to work that anyone can perform using digital tools. RTK is a key technology in this change, but practical on-site use depends on user-friendly equipment and systems. Recently, solutions that make RTK easy to adopt without expensive dedicated equipment have emerged. One such solution is LRTK.

Simple Surveying with LRTK

LRTK is a new solution that enables anyone to perform RTK surveying easily by combining a high-precision GNSS receiver with a smartphone app and cloud services. Traditionally, RTK surveying required specialized equipment and complicated setup, but with LRTK a small GNSS antenna is attached to a smart device and intuitive app operation is used for surveying. Even without complex operation or deep knowledge, simply walking with the device on site automatically acquires centimeter-level position data (half-inch accuracy), and the collected data can be managed and shared in the cloud.

Such user-friendly systems have greatly lowered the barrier to adopting RTK surveying. Even less-experienced technicians can accurately perform basic as-built surveying and stakeout tasks using LRTK. In practice, tasks that traditionally required two people for staking have been performed safely and accurately by a single person using LRTK’s AR navigation features. The LRTK series supports *i-Construction* and is well suited for small and medium-sized contractors and local governments that lack a dedicated surveying department. Simple surveying with LRTK, which maximizes the benefits of RTK, is likely to further change common practices on construction sites.

FAQ

Q: What is the difference between RTK and ordinary GPS? A: Standalone GPS positioning typically has errors on the order of several meters, whereas RTK uses correction information from a reference station to cancel those errors and achieve centimeter-level accuracy. In short, RTK is a method to make GPS more accurate. RTK positioning requires dedicated receivers and reception of correction data via communications, but it dramatically improves positioning accuracy.

Q: Why is RTK suitable for pre-construction investigations? A: Pre-construction investigations require accurately capturing wide-area terrain in a short time. RTK enables a single person to efficiently measure many points, and when combined with drones, can acquire detailed 3D models of the entire site in a short time. It collects data faster than conventional methods while offering high accuracy, making it reliable for earthwork calculations and design. For ensuring accurate site understanding within tight schedules, RTK is a highly effective tool.

Q: What equipment is needed to start RTK surveying? A: Basically, you need a GNSS receiver (rover) that supports centimeter-level positioning (half-inch accuracy). To receive correction information, either provide another receiver for a base station or connect to a correction service via a network. Nowadays, it is common to use public or private correction services (such as electronic reference-station data) over a smartphone connection, eliminating the need to operate your own base station. In such cases, an RTK-capable GNSS device and a communications-capable terminal (or built-in SIM) enable real-time high-precision positioning on site. Using integrated products like LRTK, where the receiver and app are combined, minimizes equipment setup and configuration effort. If you operate your own base station, place it as close to the site as possible, because accuracy declines with excessive distance between base and rover.

Q: Is RTK necessary for drone surveying? A: To create more accurate topographic maps efficiently, RTK is recommended for drone surveying. Photogrammetry is possible with non-RTK drones, but it often requires placing ground control points and post-processing corrections. An RTK-capable drone records the aircraft’s position with high accuracy during flight, reducing the number of ground control points needed and enabling faster survey results. On large development sites, RTK drone surveying can quickly produce accurate orthophotos and point clouds, greatly benefiting subsequent design and construction management. Even without RTK, post-processing kinematic (PPK) can improve accuracy after the flight, but RTK provides the advantage of immediate on-site results.

Q: How accurate is RTK surveying? A: Under good conditions in open environments, RTK-GNSS surveying accuracy is typically about 2–3 cm (0.8–1.2 in) horizontally and about 3–5 cm (1.2–2.0 in) vertically. This level of accuracy is generally sufficient for ordinary civil surveying and earthworks management. However, in environments with tall surrounding obstacles or poor signal conditions, temporary errors of several tens of centimeters can occur. Therefore, for important measurements, pay attention to environmental conditions and, as needed, perform conventional measurements or double checks. When millimeter-level accuracy is required, it is common to use more time-consuming static observations (static method) or total-station surveys rather than RTK.

Q: Is RTK a technology beginners can master? A: Recent RTK equipment and software have become quite user-friendly, and basic operations are not difficult. Systems like LRTK, which guide users via smartphone apps, allow those with limited expertise to proceed without confusion. However, several practical tips help maximize accuracy: avoid placing obstacles above the antenna, avoid moving the antenna during positioning, and confirm that a sufficient number of satellites are being tracked. Experience will reduce accuracy variability. Starting practice in open areas and verifying results while learning will enable beginners to handle RTK surveying in a short time. Utilizing equipment manufacturers’ support can also help resolve questions during training.

Q: What is simple surveying with LRTK? A: LRTK is an RTK surveying solution that combines a compact high-precision GNSS receiver with a smartphone app and cloud services. It enables anyone to perform centimeter-level surveying (half-inch accuracy) easily without specialized heavy equipment or complex configuration. For example, attaching an LRTK receiver to a smartphone and walking the site automatically collects high-precision position data and saves and shares it in the cloud. Tasks that once relied on veteran surveyors’ experience—such as staking and as-built verification—can be accurately performed by beginners using LRTK’s intuitive AR navigation. In short, simple surveying with LRTK is “a new surveying method that delivers RTK-level accuracy with simpler equipment and operation.” Because it can dramatically improve on-site efficiency while maintaining high accuracy, widespread adoption of this approach could significantly change industry practices.