How far can buried-pipe AR be done with only a smartphone? Three accuracy tiers and how to use them

The "Buried Pipe AR" technology, which can visualize the positions of pipes and cables buried underground through a smartphone camera, is now attracting attention across a wide range of infrastructure fields, from construction and civil engineering to water and sewer, gas, electricity, and telecommunications. Because workers can intuitively grasp underground piping routes and equipment locations on site simply by pointing a smartphone, it is expected to reduce the time spent interpreting drawings and lead to improved work efficiency and safety.

However, many people have doubts about how accurately alignment can be achieved using only a smartphone. Especially if the positions of buried objects are misidentified, it can lead to serious risks such as human error during excavation or damage to other buried utilities. So, how much practical accuracy can smartphone-only AR for buried pipes provide? And if higher accuracy is required, what methods are available?

In this article, we organize the accuracy of buried-pipe AR using smartphones into three levels — "smartphone's built-in GPS only," "using an external GNSS receiver," and "using RTK-GNSS" — and explain the expected accuracy benchmarks and suitable use cases for each. We cover the key points for selecting the right technology based on your intended use, and hope this serves as a hint for maximizing the use of buried-pipe AR in the field.

What Is AR for Buried Pipes?

"Buried Pipe AR" is a technology that virtually overlays the positions of underground pipes and cables onto the camera feed of a smartphone or tablet. Also called AR (augmented reality), it visualizes the locations of buried utilities by superimposing digital data onto real-world footage, making it appear as if you can see through the ground. Specifically, pre-prepared map data—such as water and sewer pipes, gas pipes, and power and communication cables (GIS data and design drawings)—are loaded into a smartphone app and displayed to align with the real world based on GPS location and the phone’s orientation and tilt sensor information. When you view the ground through the phone’s screen, the routes and locations of underground buried pipes and installations are shown as colored lines and markers over the actual scenery, creating an experience as if you were seeing the underground structures on site.

This technology eliminates the need to imagine the location of buried pipes from drawings in your head or to infer them from surface landmarks. Field personnel can intuitively grasp underground piping layouts, which can reduce construction errors and accidental excavation risks and shorten work time. Also, because many field personnel already carry smartphones, they can check the location of buried pipes on the spot whenever they want without purchasing or carrying dedicated equipment. For example, the ease of being able to respond with just a smartphone is a major advantage when instantly confirming pipe locations at emergency repair sites or getting a rough sense of the relative positions of buried objects during preliminary surveys. Furthermore, smartphones enable integration with network connectivity and camera functions, allowing flexible operations such as reporting conditions from the field with photos or sharing the AR view on site to seek advice from remote specialists.

Thus, smartphone-based AR for buried pipes is a groundbreaking means of improving on-site productivity and safety, but display accuracy is essential for it to deliver its full effect. Even though smartphones themselves are convenient, AR displays with positional errors can lead to misunderstandings. From the next chapter, we will take a detailed look at the factors that determine smartphone AR accuracy and the accuracy benchmarks and use cases for each technical configuration.

Accuracy and Use Cases When Using Only a Smartphone's Built-in GPS

First, consider the case of using only the built-in GPS (GNSS) that comes standard on smartphones. The typical positioning error of a consumer smartphone GPS is generally on the order of a 5–10 m (16.4–32.8 ft) radius. Even in open areas with good satellite reception, deviations of several meters are unavoidable, and in environments with poor signal conditions such as urban canyons or under elevated structures the reported position can jump even more. Many people have likely experienced their location marker in a smartphone map app appearing off the road, or their recorded walking track being significantly displaced from the actual route. These errors are caused by factors such as errors contained in the satellite signals and radio reflections from surrounding buildings (multipath), and they are unavoidable when trying to derive a user’s precise position using only the smartphone’s built-in GNSS sensor.

In recent years, the positioning performance of smartphones has gradually improved. Models supporting multi-GNSS, which can use multiple satellite positioning systems simultaneously, and those equipped with high-performance positioning chips that can receive signals on multiple frequency bands such as L1 and L5 have also appeared. Also, within Japan, some models can receive satellite augmentation signals transmitted from the Quasi-Zenith Satellite System and use them to augment positioning accuracy. With such latest smartphones, under good conditions, there have been reports of errors being limited to about 1-2 m (3.3-6.6 ft) when stationary. However, that still falls far short of centimeter-level accuracy (cm-level accuracy). In particular, for management and construction of buried pipes, even a deviation of tens of centimeters (tens of in) can be critical. For example, an error of 5 m (16.4 ft) could lead to confusing a pipe under an adjacent road, and even an error of 1 m (3.3 ft) could result in the actual pipe not being at the excavation site (it being buried 1 m (3.3 ft) away). Therefore, AR for buried pipes based solely on a smartphone’s built-in GPS, which yields meter-level positioning accuracy, should be regarded only as guideline-level reference information.

AR display on a smartphone alone is useful only in situations where you want to grasp the rough positional relationships or for purposes that do not require high accuracy. For example, it can be used in the initial stages of a field survey to get a general idea of buried pipe routes, or to roughly overlay existing drawings with the actual site appearance to check for any obvious discrepancies. Also, when used for in-house training or safety education demonstrations, smartphone AR is unlikely to cause problems even if the positioning is somewhat off. The important point is to understand that buried-pipe AR displayed using only a smartphone GPS has limitations in accuracy. It is wise not to use it for decisions about major construction or for accurately marking excavation locations, but rather to treat it as reference information that, with a safety margin, indicates "the buried object is roughly around here." You can experience buried-pipe AR with only a smartphone, but its accuracy is limited, and caution is required when applying it in practical work.

Accuracy and Use Cases When Using an External GNSS Receiver

Next is the case of combining a smartphone with an external GNSS receiver. An external GNSS receiver, as the name implies, is a high-precision positioning device that connects to a smartphone or tablet and provides more accurate position information to the phone instead of the built-in GPS. You use it by attaching a device with a small antenna and a positioning-dedicated chip to the smartphone itself or by connecting wirelessly via Bluetooth. Traditionally, GNSS equipment capable of centimeter-level (half-inch level) positioning required expensive, tripod-mounted surveying instruments, but in recent years products have appeared that pair palm-sized receivers with smartphones to enable high-precision positioning easily. External GNSS receivers not only efficiently receive signals from satellites thanks to their high-sensitivity, high-performance antennas, but can also capture signals on multiple frequency bands to reduce sources of error. Furthermore, by working with a smartphone they offer extensibility such as obtaining position correction information over the Internet (they also support RTK, which is discussed later). In other words, an external GNSS receiver is an attachment that transforms a smartphone into a full-fledged positioning device and is a key piece of equipment for easily approaching survey-grade accuracy in the field.

So, how much does accuracy actually improve by using an external GNSS receiver? Simply replacing a built-in GPS with an external receiver can significantly improve positioning accuracy and stability due to better reception sensitivity and improved positioning algorithms. In favorable conditions such as under an open sky, it is not uncommon for errors to be roughly less than 1 m (3.3 ft), often on the order of tens of centimeters (e.g., 30 cm (11.8 in)). This is because external receivers can use signals from multiple satellites simultaneously that a built-in GPS could not reliably capture, and can take advantage of signals corrected for satellite orbit and clock errors. Also, some receivers can further improve accuracy by using augmentation systems provided by countries or regions. For example, a receiver that supports the sub-meter service of Japan’s Michibiki satellites can improve accuracy in real time to a level below an error of 1 m (3.3 ft). However, even with such external GNSS receivers, securing satellite signals can be difficult in downtown areas with many high-rise buildings or in underground spaces, and accuracy can deteriorate significantly. External devices can raise accuracy from meter-class to decimeter-class (about 10 cm (3.9 in)), but depending on the situation errors of several meters may remain, and perfect accuracy is not always achievable.

Even so, compared with using a smartphone alone, the accuracy improvement achieved by introducing an external GNSS receiver is significant. This is because the AR overlay misalignment of buried pipes becomes visibly smaller, greatly reducing the risk of misidentification in the field. A concrete use case is the on-site verification of the approximate position of buried pipelines. For example, when a municipal infrastructure officer checks on-site the routing of water and sewer pipes beneath a road, the error using only a smartphone was so large that it was unclear even which part of the road they ran under. However, using an external GNSS receiver can reduce the error to on the order of several tens of centimeters (on the order of several tens of in), allowing correct grasp of coarse positional relationships such as “running toward the center of the roadway” or “closer to the sidewalk.” Also, during the planning stage of excavation work when considering clearances to surrounding buried utilities, if meter-level uncertainty (uncertainty on the order of meters, i.e., several ft) is resolved, one can more confidently determine whether construction can proceed according to the drawings. Furthermore, it is useful for simple surveying-type purposes, such as newly discovering buried utilities not shown on drawings in the field and feeding their positions back into GIS data. An error of about 1 m (3.3 ft) would make recording to existing maps ambiguous, but if positions can be recorded with accuracy of 50 cm or less (19.7 in or less), they are suitable for updating asset information.

By leveraging an external GNSS receiver in this way, the practicality of smartphone AR is greatly improved. However, the true value of an external receiver lies in the centimeter-level positioning provided by RTK, as discussed below. In the next section, we will look at how to achieve accuracy on a smartphone comparable to surveying equipment using RTK-GNSS, and at examples of its applications.

Accuracy and Use Cases When Using RTK-GNSS

Finally, there is the case of using RTK-GNSS (high-precision GNSS positioning by the Real-Time Kinematic method) on smartphones. RTK-GNSS is a technology that receives correction data in real time from a fixed reference station and corrects the positioning errors of the moving receiver to the centimeter level (cm level accuracy; half-inch accuracy). Whereas conventional GPS calculates position from the satellite signal time of arrival, RTK-GNSS analyzes the phase (the wave cycle) of the satellite signals to achieve far more precise positioning. As a result, under ideal conditions it can achieve accuracies comparable to surveying instruments, with errors of about 1–3 cm (0.4–1.2 in) in both horizontal and vertical directions.

To achieve RTK-GNSS positioning on a smartphone, you need an external GNSS receiver that supports RTK computations and a network connection to obtain reference-station correction data. By routing through a reference-station network (network RTK), you can receive correction data in real time without having to provide your own reference station. In Japan, the Geospatial Information Authority’s network of electronic reference points and various commercial RTK networks have been established, creating an environment where RTK positioning can be used relatively easily if compatible equipment is available. Because the receiver’s RTK engine processes satellite signals and correction data in real time, it can continuously update position information at the centimeter-order (in) even while you carry the smartphone. Also, even in mountainous areas outside mobile network coverage where network RTK cannot be used, if the receiver supports the quasi-zenith satellite system’s centimeter-level augmentation service (in), it can receive augmentation signals transmitted directly from the satellite and maintain similar accuracy.

Smartphone AR using RTK-GNSS dramatically improves the display accuracy of underground buried pipes. An error of only a few centimeters (a few in) means that the virtual pipe line on the screen roughly aligns directly above the real pipe. Put extremely simply, the reliability reaches a level where if you excavate at a point drawn on the AR screen you will almost certainly expose the buried pipe (of course, in actual work multiple verification methods should be used for safety, but that is the level of high precision implied). Situations where this accuracy is useful include precisely positioning buried pipes and construction management. For example, when installing a new water pipe and safely working at a crossing with an existing gas pipe, it is necessary to manage the separation between them in centimeters (in inches). By excavating while accurately visualizing the position of the existing gas pipe with RTK-enabled smartphone AR, it becomes possible to expose the pipe with the minimum necessary excavation compared to conventional methods that assume errors of several tens of centimeters (several tens of inches, roughly 1–2 ft) and dig more widely. Also, when recording the as-built locations (the actual buried positions) of buried items after construction, if you obtain survey points directly from the smartphone with RTK-GNSS accuracy and register them in the GIS, you can achieve as-built drawing–level recording accuracy without using surveying instruments again. In this way, smartphone buried-pipe AR combined with RTK-GNSS has the potential to transform underground infrastructure management work that has depended on specialist surveyors and expensive equipment. Because on-site personnel can easily measure positions themselves with centimeter-level accuracy (half-inch accuracy) and proceed while confirming on the AR screen, it not only improves operational efficiency but also greatly contributes to reducing the risk of human error and misrecognition.

Summary

We explained the accuracy of buried-pipe AR with smartphones by dividing it into three stages: smartphone built-in GPS only, using an external GNSS receiver, and utilizing RTK-GNSS. While a standalone smartphone is easy to use, its accuracy is limited to several meters and is mainly suitable for rough checks. On the other hand, using an external GNSS receiver dramatically improves accuracy, providing practical-level reliability for many field tasks. And by combining RTK-GNSS, centimeter-level accuracy comparable to surveying instruments can be achieved, enabling high-precision work such as excavation management and updating asset information.

It is important to choose technology according to the intended use. For emergency on-site checks or educational purposes, a smartphone alone can be sufficient in many cases. For routine on-site checks or basic surveying, introducing an external GNSS receiver can achieve practical-level buried pipe AR. And for full-scale construction management and precise position recording, RTK-GNSS is indispensable. By understanding these three levels of accuracy and selecting the configuration best suited to your company’s operations, you can maximize the benefits of buried pipe AR.

A device attracting attention for bringing such high-precision smartphone AR closer to everyday users is the ultra-compact RTK-GNSS receiver LRTK, which can be attached to a smartphone with one touch. By attaching the LRTK to a smartphone and using a dedicated app, anyone can easily achieve centimeter-level high-precision positioning (cm-level accuracy, half-inch accuracy) and AR display. Precision positioning that until now required specialized technicians and expensive equipment has become more accessible, and the wave of DX (digital transformation) is also surging into the field of buried pipe management. How far buried-pipe AR can go with just a smartphone — the answer is steadily expanding depending on ingenuity and the use of supporting tools. Incorporate high-precision GNSS devices wisely, and please try applying next-generation buried-pipe AR at your own sites.