What is an RTK cross-sectional diagram? On-site digital transformation realized through surveying with centimeter-level accuracy (cm level accuracy, half-inch accuracy)

Table of Contents

• What is RTK?

• What is a cross-sectional drawing?

• A new method for creating cross-sectional drawings using RTK

• On-site DX brought by centimeter-accuracy surveying (cm level accuracy (half-inch accuracy))

• Simple surveying using LRTK

• FAQ

What is RTK?

RTK stands for "Real-Time Kinematic" and is a technique that corrects positioning errors from satellites such as GPS in real time, allowing positions to be measured with centimeter-level accuracy (half-inch accuracy). Typical smartphone built-in GPS has errors of several meters (several ft), and its vertical accuracy is also insufficient. However, RTK uses correction information from a reference station to cancel out error sources in the satellite signals, enabling positioning accuracy to be improved to errors of only a few centimeters in both horizontal and vertical directions (a few in). In recent years, RTK technology has been miniaturized and reduced in cost, and by being made usable with smartphones, high-precision positioning has become easily accessible even without specialized equipment.

To perform RTK positioning, you need to receive correction data from a base station (a point with known coordinates). In Japan, you can use network RTK that utilizes the Geospatial Information Authority of Japan’s network of continuously operating reference stations, or the Quasi-Zenith Satellite "Michibiki" provides the Centimeter-Level Augmentation Service (CLAS), which offers cm level accuracy (half-inch accuracy). By attaching an external RTK-GNSS receiver to a smartphone and obtaining correction information via a mechanism called Ntrip over a mobile data connection, or, for compatible devices, by receiving CLAS signals directly from the satellites, you can achieve high-precision positioning in real time regardless of location. By taking advantage of RTK-capable GNSS devices and services in this way, centimeter-precision surveying (cm level accuracy (half-inch accuracy)) that formerly required surveying instruments costing on the order of several million yen can be accomplished with palm-sized equipment and just a smartphone.

What is a cross-sectional drawing?

A cross-sectional drawing is a diagram that shows the internal shape when terrain or a structure is cut vertically along a given section. For example, for roads, embankments, tunnels, and the like, the longitudinal and transverse cross-sectional shapes are measured at prescribed locations after completion to verify that the finished work matches the design. In civil engineering and construction, cross-sections (transverse sections) and longitudinal sections are indispensable for quality control. Cross-sectional drawings are important deliverables used for as-built (shape after construction completion) management, checking differences from the design, and even earthwork volume calculations. For instance, in road works the road surface elevation and pavement thickness are compiled as cross-sections to verify whether construction was carried out according to the design. Cross-sections are also utilized in a variety of situations, such as slope (inclined surface) stability evaluations and checking cross-sectional shapes in river works. In other words, a cross-sectional drawing is a highly reliable record of a site cross-section and provides data that is broadly useful—from construction management and quality assurance to consideration of design changes.

However, conventional creation of cross-sectional drawings required a great deal of time and effort. In the common method, skilled surveyors used surveying instruments such as levels and total stations to measure elevation and distance at each point along the cross-section line and connected those points to produce the drawing. For each cross section, two or more staff members (a measurer and a staff-rod holder, for example) were required, and surveying long sections meant repeatedly moving tripods and equipment. Naturally, taking multiple cross sections involved repeating this work, and it was not uncommon for the process from field surveying to drawing production to take several days to several weeks. Furthermore, at measurement locations such as steep slopes or beside busy roads, the work itself involved safety risks like falls or contact with vehicles. Manual cross-section surveying also limited the number of points that could be acquired, creating the risk of overlooking fine undulations or some defects between measured points. In addition, converting field-recorded data into CAD drawings at the office was cumbersome and carried the risk of recording and transcription errors inherent in manual work. the burden on manpower and time, safety concerns, and limits of data accuracy and completeness were among the various challenges in creating cross-sectional drawings by conventional methods.

A New Method for Creating Cross-Sectional Drawings Using RTK

These methods for creating cross-sectional drawings have been changing significantly due to recent digital technologies. By combining high-precision positioning with RTK and 3D scanning technologies, data acquisition for cross-sections has been dramatically streamlined. Specifically, the site is scanned using a smartphone-mounted RTK-GNSS receiver and the smartphone’s built-in LiDAR (light detection and ranging sensor) or camera, recording wide-area terrain and structures as point cloud data. Point cloud data are three-dimensional data that represent surrounding shapes as a collection of countless points, with each point containing X, Y, and Z coordinate values (east-west, north-south, height). Because point clouds obtained with RTK-capable devices are assigned accurate coordinates in the World Geodetic System (absolute coordinates) on site, later processes such as comparison with design drawings and drafting become easy.

Once you have scanned the entire site as a point cloud, the way cross-section creation is done fundamentally changes. Traditionally the procedure was "set survey lines at X-meter intervals and measure on site each time," but if you have acquired point cloud data, you can freely extract cross-sections of the required locations afterwards. For example, if you specify a line connecting any two points on the acquired point cloud model, you can immediately display the cross-section at that position. This avoids situations like having to re-survey because you forgot to measure somewhere on site, and even if discrepancies with the design drawings are found after completion, you can inspect additional cross-sections in detail within the data. In short, it triggers a shift in thinking from "measure in order to create cross-sections" to "first record the site in 3D, then create as many cross-sections as you need". Compared with traditional cross-section surveying, which merely connected points, the point cloud scanning method digitally records every detail of the site, dramatically increasing both the amount of information and the accuracy reflected in the cross-sections.

Furthermore, this new method also simplifies the data processing and drawing workflows. The acquired point cloud data can be uploaded from a dedicated app to a cloud service for analysis and visualization. By displaying the point cloud model in the cloud and specifying an arbitrary cross-section line on the browser screen, a cross-section at that location can be generated automatically. The generated cross-section can not only be previewed on the screen but can also be instantly downloaded in formats usable by CAD software, such as DXF. This eliminates the need to manually draw cross-section lines from scratch based on coordinates obtained on site, greatly speeding up the drawing creation process. Of course, since the point cloud data already contains position information in an absolute coordinate system, the output cross-sections are also accurately positioned in the public coordinate system and can be used directly for overlaying with design drawings and quantity calculations. With point cloud data and cloud utilization, an era has begun in which the process from acquisition to creation and sharing of cross-sections is completed in a single, one-stop workflow.

On-site DX Brought by Surveying with cm level accuracy (half-inch accuracy)

By introducing digital technologies to the field, surveying and measurement operations can be dramatically streamlined, enabling data to be utilized with unprecedented speed and quality. Surveying with RTK at cm level accuracy (half-inch accuracy) and the use of point cloud data are precisely the driving force behind on-site DX (digital transformation). Below, let’s look at the specific effects brought about by the new RTK-based methods.

*A smartphone with an RTK receiver attached and fixed to a monopod (pole) being used to perform surveying. High-precision positioning becomes possible safely and efficiently even by a single person.* Labor savings and improved safety: High-precision cross-sectional data acquisition becoming possible by 1 person leads to significant labor improvements. Cross-sectional surveying that previously required 2–3 people can be completed by a single operator with RTK-enabled smartphone surveying. This not only reduces the effort of arranging personnel and labor costs, but because sites can be operated with fewer people, it also helps alleviate chronic labor shortages. In addition, because there is less need to enter hazardous slopes or areas near roadways for surveying, reducing safety risks is another major benefit. Since terrain can be scanned from a distance, workers can acquire data without remaining at hazardous locations for long periods. Through on-site DX, surveying and measurement can be conducted "with fewer people, safely".

Speed-up (reduced work time): Once point cloud data has been captured, you can generate the required cross-sections as many times as needed afterward, eliminating the need to "go to the site and measure every time you want to check a section." Traditionally, surveying on site, returning to the office to draft and analyze drawings, and obtaining results could take several days, but with a digitized workflow it is possible to complete the process from data capture to drafting and sharing on the same day. For example, you can scan the site in the morning, create cross-sections in the cloud and share them with the design team by midday, and begin necessary corrective work in the afternoon—enabling a rapid response. This can shorten construction schedules and improve responsiveness, and also accelerate the site's decision-making cycle (PDCA).

Quality Improvement and Data Utilization: The point cloud data obtained through digital surveying contains far more information than partial, manually conducted surveys and becomes a comprehensive record of the site (網羅的な記録). As a result, variations in as-built conditions and localized anomalies are less likely to be overlooked, improving the accuracy and reliability of cross-sectional drawings. Also, because measurement data are automatically digitized, human errors such as missed records or transcription mistakes (ヒューマンエラー) are reduced. Since up-to-date site data are continuously accumulated in the cloud, they become a valuable asset that can be used for comparisons with the past and for future maintenance management. Furthermore, cloud-based data sharing enables real-time information sharing (リアルタイムな情報共有) between the field and the office. For example, it is easy to instantly share cross-sections and point cloud models online with stakeholders to review and discuss the current conditions together with clients and designers. The need to email drawings or gather on site is reduced, increasing the speed of communication and decision-making. In this way, centimeter-level surveying (cm level accuracy (half-inch accuracy)) and data utilization are enabling on-site DX that achieves both efficiency and quality improvement (効率化と品質向上を両立する現場DX).

The above effects also align with *i-Construction* (Ai-Construction), promoted by the Ministry of Land, Infrastructure, Transport and Tourism. i-Construction is an initiative to improve productivity across the entire construction process through ICT and data utilization, and 3D surveying using RTK and cloud utilization can be said to be representative measures. The introduction of RTK cross-sectional drawings is a good example of DX (digital transformation) that renews the conventional "givens" of on-site work and further raises the quality and efficiency of construction management.

Simple Surveying with LRTK

Finally, as a solution that lets you easily put into practice the RTK-based cross-section creation discussed so far, we introduce a system called "LRTK". LRTK (pronounced "Eru Aru Tī Kē") is a positioning and point-cloud measurement system conceived to transform a smartphone into a high-precision, all-purpose surveying instrument. It consists of a small RTK-GNSS receiver device that attaches to a smartphone and a dedicated app with an intuitive interface, enabling anyone to easily start centimeter-level surveying (cm level accuracy (half-inch accuracy)).

*A pocket-sized LRTK receiver (a black device) attached to a smartphone. With just this small terminal, a smartphone is quickly transformed into a high-precision surveying instrument.*

Handy deployment and operability: The defining feature of LRTK is, above all, the ease of deployment and operation. There is no need to purchase dedicated large equipment; by simply attaching a dedicated device to your existing smartphone, you can start high-precision surveying as soon as the next day. The device itself is lightweight and compact, weighing only a few hundred grams, making it easy to carry and ready for use whenever needed on site. The app's UI is simple and intuitive, so even those without surveying expertise can follow the guidance to perform point measurements, point-cloud scans, and even AR-based visualizations. Because it does not require lengthy training, adoption on site is smooth. Tasks that were previously left to specialized departments or external contractors can, with the introduction of LRTK, be handled so that on-site personnel themselves can immediately obtain high-precision data. If you want to quickly check a cross-section, you can take out your smartphone, scan on the spot, and check the results—such rapid response becomes possible.

Low cost, high functionality: Traditionally, if you wanted to perform centimeter-level 3D measurements (half-inch accuracy), you had to purchase expensive GPS surveying equipment and laser scanners and train operators. Compared to those, LRTK can be introduced at a relatively low cost, and maintenance costs are also kept down, making it suitable for adoption at small- and medium-sized sites and at the departmental level. Nevertheless, the positioning accuracy achieved is top-class, realizing high precision with horizontal and vertical errors within a few centimeters (a few in).

Also, the LRTK app integrates with cloud services, and measured data is synced to the cloud with a single tap. This makes it possible to instantly share point clouds and coordinates acquired on site within the company, or analyze them in detail on office PCs. Because cross-sectional views and distance/area measurements can be performed in a browser without dedicated software, the range of data utilization expands. LRTK is precisely a "DX tool anyone can use" that powerfully supports the digitalization of worksites.

FAQ

Q: What is the difference between RTK and regular GPS positioning? A: Regular GPS (standalone positioning) can have position errors of a few meters (a few ft) due to slight errors in satellite signals. RTK positioning, on the other hand, uses correction data from a base station to correct those errors in real time, improving accuracy to an error of a few centimeters (a few in). In other words, RTK is a positioning method that significantly improves GPS accuracy.

Q: Can smartphones and small devices really achieve centimeter-level accuracy (cm level accuracy (half-inch accuracy))? A: Yes, it is possible. By combining a high-precision GNSS receiver attached to a smartphone with RTK correction information, centimeter-class positioning that previously required dedicated equipment can be realized. In fact, systems like LRTK have been confirmed to provide positioning with approximately 2–3 cm (0.8–1.2 in) of accuracy in both horizontal and vertical directions. However, accuracy is affected by the surrounding environment (whether the sky is open or there are no obstructions) and the reception conditions of satellite signals, so ideal accuracy is not always achieved. Even so, it offers far higher precision than a smartphone’s standalone GPS.

Q: Can it be used by someone who is not a professional surveyor? A: Yes. In LRTK, the smartphone app provides comprehensive operation guides and automatic calculation functions, and is designed so that even first-time users can proceed with surveying without confusion. Of course, having basic surveying knowledge is preferable, but since the app automatically handles how to measure points and the generation of cross-sections, it is fully usable by on-site staff. In fact, on-site there are increasing cases where construction management personnel who previously had no involvement with surveying use LRTK to check the as-built condition themselves.

Q: Compared to drone surveying (photogrammetry), what advantages does smartphone RTK surveying have? A: Drone photogrammetry is also a technology that supports field DX and is suitable for acquiring wide-area 3D terrain data. However, there are hurdles such as flight permits, weather conditions, and piloting skills. On the other hand, ground scanning using a smartphone with RTK offers the advantage of being able to measure immediately on site. In narrow areas, indoors, or places that are hard to see from above (such as inside tunnels or under a forest canopy), a person can walk and measure, making it suitable for detailed measurements and inspection work that requires immediacy—areas where drones struggle. Of course, by using drones and smartphone RTK according to the application, you can build a more efficient on-site measurement workflow.

Q: How can the acquired data be utilized? A: The point cloud data and cross-section data acquired with LRTK can be used for a wide range of purposes. You can display cross-sections in the cloud or download them as DXF files for use in creating CAD drawings. The point cloud data itself can also be exported in LAS or PLY formats, allowing terrain analysis to be performed with dedicated point cloud processing software. Also, because data can be shared with stakeholders via the cloud, cross-sections can be used in reporting materials for clients, and terrain comparisons before and after construction can be performed, making it useful in a wide range of on-site scenarios.

Q: As a first step toward on-site DX, what should I start with? A: If you are introducing digital surveying technology for the first time, we recommend starting with simple surveying. Try measuring and creating cross-sections with smartphone RTK over a small area to experience its efficiency and accuracy. Solutions that attach to a smartphone, such as LRTK, keep initial investment low, making them suitable for trial deployments. Once you actually use them, you will be able to obtain high-precision cross-sections in a short time and likely feel you cannot go back to conventional methods. After gaining that confidence, consider advancing your on-site DX efforts in earnest. Smartphones and LRTK can open the door to new field operation workflows.