Easily achieve high-precision surveying with a smartphone-connected GNSS receiver!

Recently, in the field of surveying, a new style that links smartphones and GNSS receivers—enabling easy, centimeter-level (cm-level accuracy; half-inch accuracy) high-precision surveying—has been attracting attention. Until now, high-precision positioning required large-scale equipment and multi-person operations, but advances in technology have made possible pocket-size GNSS surveying instruments that a single person can carry. Amid labor shortages and workstyle reforms, such smartphone-linked GNSS receivers are expected to serve as solutions that improve on-site productivity. This article explains in detail the basics of GNSS receivers, ways to use them in combination with smartphones, their benefits and concrete use cases, and integration with cloud services. Finally, we touch on a product called LRTK Phone that is easy to introduce, and offer hints for high-precision “one-person surveying” that anyone can easily start.

Fundamentals of GNSS Receivers (Roles, How They Work, and Causes of Error)

First, as basic knowledge, let's briefly cover what a GNSS receiver is, its role and how it works, and the causes of positioning errors.

GNSS (Global Navigation Satellite System) is a general term for satellite positioning systems that measure positions on Earth using multiple artificial satellites. It includes the U.S. GPS, Russia's GLONASS, Europe's Galileo, China's BeiDou, and Japan's Quasi-Zenith Satellite System "Michibiki." A GNSS receiver is a device that receives radio signals transmitted from these satellites and calculates its position (latitude, longitude, altitude) and time. Car navigation systems and smartphone map apps also use GPS receivers to determine position, but a surveying GNSS receiver can determine positions with even higher accuracy.

GNSS positioning mechanism: A GNSS receiver picks up radio signals transmitted from multiple satellites overhead, calculates the distance to each satellite from the signal propagation times, and derives a three-dimensional position solution. In general, when distance information from four or more satellites is available, the three-dimensional coordinates of the receiver can be calculated. However, because GNSS signals are weak radio waves that travel at the speed of light, small factors can introduce errors in positioning accuracy. With a typical smartphone’s built-in GPS, errors on the order of several meters to a dozen or so meters can occur (several m to about 10–19 m (several ft to about 32.8–62.3 ft)), and these arise from the following error factors.

• Atmospheric delay error: Radio waves from satellites slow down and refract as they pass through the ionosphere and troposphere, causing arrival times to be delayed and producing discrepancies in distance calculations. In particular, delays caused by electron density in the ionosphere and water vapor in the troposphere degrade positioning accuracy.

• Multipath error: If radio waves do not arrive directly from the satellite but reach the receiver after reflecting off building facades, the ground, the water surface, or similar surfaces, the indirect path takes more time than the direct wave. This is called multipath (multiple paths), and because reflections cause the measured distance to be longer than the actual distance, they introduce errors into positioning results. Multipath errors tend to be larger in environments with many obstructions such as urban areas or forests.

• Satellite and receiver clock/orbit errors: Slight offsets in the atomic clocks onboard satellites and errors in satellite orbital information also have an impact. Receiver-side internal noise and limitations in receiver clock accuracy are also small but contributing sources of error.

• Effect of satellite geometry: Accuracy also varies depending on the arrangement of satellites overhead. Positioning accuracy improves when satellites are widely dispersed rather than clustered toward the zenith. The indicator of this satellite geometry is called the DOP value; when the value is large (satellites are clustered), accuracy worsens.

Due to these factors, standalone GNSS positioning typically cannot avoid errors on the order of several meters (several ft). Therefore, in surveying, techniques that correct these errors to achieve centimeter-class high accuracy (cm level accuracy (half-inch accuracy)) are used. Representative methods are the RTK (Real Time Kinematic) method and differential GNSS (DGPS). RTK is a method that corrects positioning errors in real time between a base station (reference station) and a rover (mobile station); by transmitting the base station's known position and the differential information of satellite signals to the rover, the rover can subtract the errors from its positioning values. This enables positioning with errors reduced to a few centimeters (a few in). In Japan, network RTK, which uses the Geospatial Information Authority of Japan's electronic reference point network and commercial GNSS reference station networks, has become widespread, and by receiving correction information over the internet on smartphones, etc., via a system called Ntrip, high-precision positioning can be performed without installing one's own base station on site. Also, by utilizing the centimeter-level augmentation service (CLAS) delivered from Japan's quasi-zenith satellite "Michibiki", correction signals can be received via satellite even in environments where mobile communications are difficult, such as mountainous areas, allowing high accuracy to be maintained. In other words, recent GNSS receivers, by incorporating such external correction information, can achieve sufficient surveying accuracy (errors within a few centimeters (within a few in)) anywhere and anytime.

New Applications Through Integration of Smartphones and GNSS Receivers

GNSS receivers enable high-precision positioning, but traditionally they required large stationary units and dedicated control terminals, and operation was specialized. However, recently, by pairing smartphones with GNSS receivers, a new surveying style that anyone can easily use has emerged. Let’s look at the novel uses made possible by combining smartphones and GNSS receivers.

Using a smartphone as the controller: In conventional high-precision GNSS surveying, in addition to the receiver unit itself, a dedicated handheld computer or controller was required. Nowadays, the smartphone or tablet you already have serves as the receiver’s controller and display terminal. By connecting the GNSS receiver and the smartphone via Bluetooth or a cable and using a dedicated app, satellite reception status, current coordinates, and accuracy information can be checked intuitively on the smartphone screen. Because the surveying instrument can be controlled with the smartphone’s familiar touch controls, there is no need to learn the complex operations unique to specialized equipment.

Real-time Communication and Data Utilization: Because smartphones are also communication terminals, there are significant advantages when they are linked with GNSS receivers. For example, it is possible to obtain the correction information of the aforementioned network RTK in real time via 4G/5G networks. Traditionally, corrections could only be received within a range of a few kilometers from the base station using radio modems or wireless communications, but via a smartphone you can receive correction data anywhere nationwide over the Internet. Another new feature is that positioning result data can be sent to the cloud on the spot or displayed in conjunction with mapping services, allowing you to immediately use the obtained position information as digital data. With smartphone integration, you can not only verify measured coordinates on site, but also instantly check them on office PCs or import them into a Geographic Information System (GIS) with a single tap.

Fusion with AR and sensors: Smartphones have various sensors built in, such as cameras, accelerometers, gyros, electronic compasses, and, in the latest models, LiDAR (Lidar) sensors. By combining high-precision positioning information from GNSS with these smartphone sensors, a new sensory surveying becomes possible. For example, with an AR (augmented reality) feature that overlays the positions of surveyed points and target points from design drawings onto the live site footage captured by a smartphone camera, you can draw virtual survey stakes and lines in the real world. Also, by using a smartphone’s LiDAR function, you can scan nearby structures and terrain to convert them into point cloud data, and if you fuse GNSS positioning information with this, you can assign accurate coordinates to all points in the point cloud. In this way, GNSS × smartphone integration brings a visual and intuitive approach to surveying work and provides an experience that conventional equipment could not deliver.

In addition, by leveraging the app extensibility unique to smartphones, you can automatically add location tags to photos taken in the field, leave notes via voice input, and thus expand the scope of data collection. In short, it can be said that by combining a smartphone with a GNSS receiver, GNSS—which used to simply measure positions—has evolved into a comprehensive tool that 'measures, shows, communicates, and records'.

Advantages of Smartphone-Connected GNSS Receivers (Lightweight, Single-Person Operation, Low Cost, High Accuracy, etc.)

So, what specific advantages do GNSS receivers that pair with smartphones offer? Below, we organize their main benefits, including comparisons with traditional methods.

• Lightweight, compact, and easy to carry: Smartphone-connected GNSS receivers are mostly palm-sized compact devices. Despite housing the antenna, receiver, and battery, they are lightweight designs of a few hundred grams or less, and some can be attached to the back of a smartphone. Unlike conventional stationary GPS receivers and large antennas, their portability—being able to carry them in a pocket while moving around a site—is attractive. Transporting equipment into narrow sites or deep mountains also becomes easy, dramatically improving mobility.

• Surveying tasks can be completed by one person: Traditionally, surveying was done by teams of two or more (for example, one person operating the surveying instrument while another placed a target at a distance). However, with smartphone-connected GNSS, you can perform everything yourself from setup to observation and recording — all by one person. High-precision GNSS surveying with real-time corrections does not require line-of-sight, so you can measure the position of the tip of your rod or pole without assistance. As a solution to the on-site lament "I wish I had another person...", single-person surveying has become a reality.

• Low initial cost: By utilizing a smartphone, dedicated controllers and other devices become unnecessary, significantly reducing initial setup costs. While high-precision GNSS receivers may bring to mind prices in the millions of yen, smartphone-connected products are markedly cheaper. Some offerings even provide subscription (monthly usage) plans, allowing you to reduce upfront expenses. Also, because one person can perform the work, labor cost reductions are possible, and overall the fact that they can be operated at low cost is a major advantage.

• Centimeter-level high-precision positioning (cm level accuracy (half-inch accuracy)): Even small receivers that pair with smartphones offer positioning accuracy comparable to conventional stationary GNSS. They support network RTK and Michibiki's CLAS signal, and when correction information is properly applied, positioning with errors within a few centimeters (within a few in) is possible. This is in stark contrast to the accuracy of typical smartphone built-in GPS (errors of about 5–10 m (16.4–32.8 ft)), and provides sufficient precision on-site for setting boundary stakes and construction quality control. Because you can obtain high-precision absolute coordinates on your own, tasks that used to require hiring a surveyor can now be performed in-house.

• Intuitive operation via smartphone app: Many apps for smartphone GNSS receivers are user-friendly and allow intuitive operation. You can tap a point on the map to specify the location to measure or simply press a large positioning button on the screen to complete a recording, so no specialized knowledge is required. The smartphone's familiar UI and touch controls enable even newcomers unfamiliar with surveying instruments to become proficient in a short time, allowing them to contribute on-site sooner. In addition, the smartphone's high-resolution display can render maps and drawings clearly, making it easier to check detailed instructions in the field.

• Versatile, all-in-one use: It is noteworthy that smartphone-connected GNSS receivers are versatile in a single unit. Not only single-point coordinate observation, but also stakeout position assistance as described later, 3D scanning, AR navigation, and more — with this one device you can broadly cover everything from surveying to measurement and design verification. Tasks that would previously have required separate equipment and software can be completed with just a smartphone + GNSS receiver and the appropriate app. Truly as an "all-purpose surveying instrument," the flexibility to be used in scenes from construction and civil engineering sites to disaster surveys and infrastructure inspections is a major advantage.

Surveying examples using LRTK Phone (coordinate acquisition, stakeout, point cloud scanning, AR guidance, etc.)

Now, let's look at some concrete examples of the surveying and measurement tasks you can actually perform using a smartphone-connected GNSS receiver. Here, using the ultra-compact RTK-GNSS receiver "LRTK Phone" developed by a venture originating from Tokyo Institute of Technology as an example, we introduce four use cases.

Acquiring Coordinates and Streamlining the Recording of Survey Points

The most fundamental task is obtaining and recording the coordinates of any chosen point on site. Using a smartphone-mounted GNSS receiver like the LRTK Phone, you can measure and save the latitude, longitude, and elevation of a targeted point with a single tap. For example, simply press the "Positioning" button on the smartphone screen at the location you want to measure, and the high-precision position coordinates at that moment will be recorded. Date/time and positioning accuracy information are automatically saved at the same time as the recording, and point names are automatically assigned sequentially, so there is no need to write them down in a field notebook. This eliminates the hassle of taking paper notes on site and prevents mistakes caused by missed point records or recording errors.

The acquired coordinate data are displayed in a real-time list on the smartphone app and can also be plotted on a map for verification. Furthermore, if you take a photo with the smartphone camera at that point, the image is tagged with location information (geotag) and saved to the cloud. You won’t have to worry later back at the office about “where was that photo taken?”, and because you can manage coordinates with photos, preparing report materials becomes smoother. Positioning datums supported include the World Geodetic System (WGS84) and the Japan Geodetic Datum (JGD2011), and conversions to the plane rectangular coordinate system are also supported, allowing results to be directly reflected in design drawings on site. In this way, a smartphone-linked GNSS receiver integrates point coordinate acquisition and record organization, enabling fast and reliable surveying tasks.

Use in pile-driving (surveying and layout) operations

At construction sites, staking-out (layout surveying) work that places stakes and markings at the design positions shown on the plans is carried out frequently. Traditionally, this work was done by surveyors in pairs, using a total station or similar equipment to measure angles and distances and establish stake positions. By using a smartphone-linked GNSS receiver, a single person can also perform the staking-out work.

Specifically, you preload the coordinate data for the planned pile-driving positions, obtained from the design drawings, into a smartphone app. At the site, while measuring your current position with GNSS, the app displays in real time the difference from the target pile positions. For example, a guide on the smartphone screen might show "east 5 cm (2.0 in), north 10 cm (3.9 in) to the target point," and if you move as instructed you will arrive at the intended position. Some apps also provide voice guidance or arrow displays, so even on your first visit to a site you can reach the point without getting lost. Using AR technology, virtual arrows and pile markers are overlaid on the camera image to intuitively indicate "this is the design position." This allows you to set out positions accurately by yourself without another person at a distance having to shout "a bit more to the right!" or "there!"

When you can set stakes by yourself, the efficiency of surveying work improves dramatically. You no longer need to coordinate multiple people's schedules, and you can install stakes immediately when you decide to. In addition, stake setting using GNSS absolute positioning automatically links to known points, so consistency is maintained when checking coordinates later. With the introduction of smartphone-connected GNSS receivers, one could say that the era has arrived in which stake setting can be done easily and with high accuracy.

3D Measurement Using Point Cloud Scanning

One of the 3D surveying methods that has attracted attention in recent years is the acquisition of point cloud data using LiDAR and photogrammetry. Some smartphones now come equipped with LiDAR scanners, and by combining these with a GNSS receiver, it is possible to capture site conditions in three dimensions.

When you attach the LRTK Phone to your smartphone and launch the dedicated app’s 3D scan mode, you can capture point clouds while scanning the surroundings with the phone’s camera and LiDAR. Because the phone’s position and orientation and GNSS-derived absolute coordinates are recorded continuously, you can automatically assign global coordinates (latitude, longitude, height) to all captured point clouds. Conventional smartphone-only LiDAR scanning had issues where point clouds would gradually drift and the floor plane would become distorted when walking around. However, when combined with high-precision GNSS, the device position can be accurately determined, enabling stable 3D measurements in which point clouds do not warp during scanning. This makes it possible to easily obtain as-built 3D models with just a smartphone and a compact GNSS receiver, without the need for specialized 3D laser scanners or large equipment.

The acquired point cloud data can be checked on a smartphone screen on site, and analyses such as measuring the distance between two points or the area and volume of a selected region can be performed as needed. For example, calculating the volume of fill or excavated material on site or quantitatively measuring deformation of structures can be completed in a very short time. In addition, point cloud data can be uploaded to the cloud and shared, making it easy to inspect point clouds in detail on an office PC or import them into CAD software to overlay with design data for review. Smartphone-connected GNSS receivers strongly support on-site DX as a tool that enables a single person to carry out advanced 3D measurements.

AR-based Surveying Navigation

A notable feature that leverages smartphones and GNSS is AR (augmented reality) technology-based surveying navigation. This overlays surveying data and design information as virtual objects onto the real-world scene visible on the smartphone screen, intuitively supporting on-site work.

In pile-driving operations, displaying virtual flags or piles at target points in AR makes it immediately clear where the piles should be driven. As an application, AR is also useful for as-built management and inspections. If you load the pre-construction design model (BIM/CIM data) into a smartphone, you can display a 3D model of the planned structure in AR over the site scenery and visually confirm differences between the design and the current conditions. For example, in bridge construction you can overlay the planned bridge girder model on the site to check whether the pier installation positions are misaligned.

Also, it is conceivable to assist safety checks for excavation work by recording the coordinates of underground pipes and cables in advance and visualizing their routes on site with AR. By visualizing the "visualize the invisible" through a smartphone screen, workers can intuitively grasp the situation and prevent mistakes and troubles before they occur.

In this way, AR guidance features use technology to cover the parts of surveying and construction management work that have until now depended on craftsmen's intuition, offering an environment in which anyone can easily and accurately carry out tasks. The adoption of smartphone-linked GNSS receivers is fusing the physical world with digital information, and is set to greatly change how field work is performed.

Utilizing smartphone apps and the cloud (real-time monitoring, records with photos, remote management, etc.)

Smartphone-connected GNSS receivers realize their full potential not only for on-site positioning but also when used in combination with a smartphone app + cloud services. 以下では、スマホアプリとクラウドを活用することによって得られるメリットをいくつか紹介します。

• Share and verify field data in real time: Coordinates and point cloud data obtained with a surveying app can be uploaded to the cloud on the spot. For example, if you send the data to the cloud from your smartphone with one tap immediately after finishing surveying, supervisors and colleagues in the office can instantly view the results in a browser. Work proceeds with a sense of speed such that checks are already completed by the time you return to the office. Even if something was missed in measurement, you can instruct additional measurements on the spot, so you can minimize rework and additional site visits and shorten lead times.

• Reliable information management with photo-attached records: Smartphone GNSS surveying apps have a function to link photos and notes to locations at the same time. If you leave a site photo for each survey point, you can intuitively recall “what kind of place this coordinate was” when reviewing the data later. Because photo images also include shooting direction and angle information, for example you can overlay photos on point cloud data and supplement details that point clouds cannot fully capture (such as cracks or surface conditions) with photographs. Managing data in the cloud creates a database where photos, coordinates, and notes are linked in one place, saving the trouble of creating paper ledgers or photo ledgers. Digitized records are easy to search and are useful for later verification and preparing reports.

• Remote management and collaboration via the cloud: By aggregating positioning data in the cloud, you can grasp site conditions even from geographically distant locations. Managers can check progress in real time from an office PC and immediately communicate instructions such as “please survey here next” to field staff via chat or call. Because multiple people can view the same project data, information sharing and decision-making between the field and the office become speedy. Also, since data is automatically backed up in the cloud, there is no risk of losing data while carrying it back on a USB memory stick. Past survey data can also be organized and accumulated by project, allowing you to leverage data assets for future maintenance and for use at other sites.

By linking smartphone apps with the cloud in this way, real-time collaboration that transcends the boundary between the field and the office becomes possible, and the scope of survey data applications expands dramatically. Rather than simply measuring on-site and stopping there, smartphone-connected GNSS receivers are extremely useful as a comprehensive solution that encompasses post-measurement processing, sharing, and utilization.

Summary: Making high-precision accessible to anyone through simple LRTK surveying

We've examined a new surveying method enabled by the integration of smartphones and GNSS receivers, from fundamentals to practical applications. The smartphone-connected GNSS receiver, which is lightweight and compact, operable by a single person, and low-cost yet delivers centimeter-level high accuracy, holds great potential for future surveying and construction management sites. Already attracting attention in the construction industry and local governments as a trump card for alleviating labor shortages, promoting DX, and improving operational efficiency, various field implementation cases have been reported under keywords such as "one-person surveying" and "smart surveying."

Among them, the LRTK Phone cited as an example in this article is a product that could become a new one-per-person on-site tool: a high-performance RTK-GNSS receiver that anyone can use immediately with just a smartphone. Simply attach it to a smartphone and turn it on, and the site is transformed into a surveying field, enabling an all-in-one workflow from positioning to point cloud measurement, photo documentation, and AR utilization. Because it can be operated without special qualifications or advanced skills, even inexperienced technicians can be deployed to the field as immediate assets.

In the future, surveying is expected to shift from "work carried out by a specialist team lugging heavy equipment" to "something anyone can do with a smartphone and a compact GNSS." If your workplace also has a need for high-precision positioning but is struggling with staffing or budget constraints, please consider using a smartphone-connected GNSS receiver. By adopting the latest solutions, including LRTK, you can remarkably easily and reliably improve the efficiency and accuracy of surveying work. Why not bring this new era of smart surveying to your site as well?