Drone Point-Cloud Surveying for Mega-Solar Construction! RTK Support Removes Need for Ground Control Points

Surveying Challenges at Mega-Solar Sites and the Background of Drone Use

Construction of mega-solar (large-scale solar power plants) requires surveying vast areas spanning tens of hectares and terrain with significant undulations. Traditional manual surveying over wide areas can take several days or more, and work on steep slopes or rough ground prior to grading poses labor and safety challenges. Moreover, precise surveying to accurately capture land elevation differences and sunlight conditions is essential for efficient earthwork planning and maximizing power generation. Against this backdrop, surveying using drones (unmanned aerial vehicles) has attracted attention in the construction industry in recent years. Drone surveying, which can acquire wide-area terrain data from the air in a short time, is a countermeasure to the worsening shortage of skilled personnel and, together with initiatives such as the Ministry of Land, Infrastructure, Transport and Tourism’s “i-Construction” promotion, is transforming surveying operations on large sites like mega-solar facilities.

Advantages of Point Cloud Data and the Importance of Survey Accuracy

In drone surveying, analysis of aerial photographs can produce a collection of many three-dimensional coordinate points known as point cloud data. Advantages of point cloud data include:

• Because the entire vast site can be recorded as a high-density 3D point cloud, subtle terrain irregularities and structures can be captured without omission.

• After acquisition, heights, distances, slopes, and other measurements at arbitrary locations can be taken from the data, enabling various analyses without additional field measurements.

• Accurate 3D models and contour maps can be generated from point clouds, which can be overlaid on design drawings for comparison and verification, and used for earthwork planning simulations.

• They can be used for construction management tasks such as calculating cut-and-fill volumes and visualizing construction progress, allowing the site’s status to be understood through objective data.

• For solar panel layout planning, point clouds enable shadow simulations that consider terrain slopes and surrounding obstructions, helping to optimize power generation efficiency.

However, to fully leverage the usefulness of such point cloud data, survey accuracy must be ensured as a prerequisite. For example, if a terrain model has an error of several tens of centimeters, large discrepancies can arise in earthwork volume calculations. Likewise, positional shifts in panel or pile installation can lead to misalignment of components during construction. Civil engineering work typically requires accuracy on the order of several centimeters, and the Ministry of Land, Infrastructure, Transport and Tourism’s workmanship management standards set allowable errors at around ±5 cm (±2.0 in). Therefore, point cloud data acquired by drones must meet equivalent accuracy. Traditionally, achieving high-precision point cloud surveys required the establishment of ground control points, but in recent years new methods using RTK have emerged. The next section first reviews the issues of the traditional ground control point method.

Challenges of Ground Control Points in Traditional Drone Surveying

As noted above, to obtain high-accuracy point cloud data it is necessary to tie the captured images to accurate coordinates. In conventional drone photogrammetry, it was common to place multiple known-coordinate markers on the ground called ground control points (GCPs) and synchronize them with the captured images. The coordinates of the control points are pre-measured by GNSS surveying or total station, and during aerial-photo analysis these are used as references to align and scale the entire model.

However, placing ground control points involves the following issues:

• On vast sites many control points are required, and simply installing them and measuring accurate coordinates can take half a day or more.

• In forests or steep slopes, installing control points can be difficult, and poor footing raises safety risks such as worker slips and falls.

• Survey staff entering a construction site where heavy machinery is operating requires work coordination to ensure safety, and sometimes the overall site progress must be temporarily halted.

• If the terrain changes due to earthworks, existing control points become unusable, requiring new points to be installed for each survey.

• Installation and surveying of control points require personnel with specialized knowledge, increasing labor costs.

Thus, although necessary for ensuring accuracy, preparing ground control points involved significant time and cost. A method attracting attention as a solution to these issues is RTK-capable drones, which can minimize or eliminate the need for ground control points. The next section explains the surveying innovations brought by RTK-equipped drones.

Surveying Innovations with RTK-Capable Drones

RTK stands for real-time kinematic, a technology that enables centimeter-level positioning by correcting GNSS (satellite positioning) errors in real time. RTK-capable drones are equipped with high-precision GNSS receivers on the airframe and receive correction information from base stations or network reference stations on the ground during flight, which enhances the positional data of each captured photo. This makes it possible to generate point cloud models in accurate coordinates without the many ground control points that used to be essential.

Major advantages (innovations) brought by RTK-equipped drones include:

• Significant reduction of ground control points: With RTK drones, surveying is possible with almost no ground control point installation, dramatically reducing survey preparation work.

• Maintenance of high accuracy: Even without ground control points, point cloud data can be obtained with accuracy comparable to conventional methods (horizontal and vertical errors on the order of several centimeters). Only a very small number of checkpoints are required for accuracy verification.

• Reduced working time: Because on-site preparation is almost unnecessary, work can be completed with just a flight plan and data processing. Surveys that used to take several days can be finished with a single flight (about several tens of minutes) and a few hours of analysis in some cases.

• Cost reduction: By reducing workdays and personnel, surveying costs can be drastically compressed. Cases have been reported where drone surveying cost less than one-quarter of conventional methods.

• Improved safety: Since humans no longer need to enter hazardous areas, surveying can be conducted safely from the air, reducing the risk of occupational accidents associated with surveying work.

• High-frequency surveying: The low burden makes it easy to conduct surveys as needed, enabling weekly or monthly regular measurements or ad hoc surveys in line with construction progress.

With the advent of RTK drones, surveying on vast mega-solar sites has been dramatically streamlined. The next section looks at concrete examples of how the high-precision point cloud data and 3D models obtained in this way can be applied to actual earthworks, pile driving, and construction management.

Use Cases in Mega-Solar Earthworks, Pile Driving, and Management

In mega-solar construction projects, high-precision data obtained by RTK-capable drones is used in various situations. Below are the main use cases: earthworks, pile-driving operations, and construction management.

Use in earthworks: During the land preparation phase, drone surveying is used before construction to grasp the current terrain and assist in developing the design plan (earthwork plan). A pre-construction model is created from the acquired point cloud, and by comparing it with the planned ground elevation, cut-and-fill volumes can be calculated accurately. After construction begins, periodic drone surveys allow early detection of deviations from the planned sections and tracking of weekly or monthly volume changes. On vast mega-solar sites it is difficult to manually grasp earthwork progress across the entire area, but using drone point cloud data you can generate a 3D map that shows at a glance which areas have been formed according to design. This prevents unnecessary over-excavation and enables appropriate allocation of construction equipment according to progress, contributing to improved efficiency and quality of earthworks.

Use in pile-driving operations: RTK and drone surveying are powerful tools for pile-driving work that supports solar panel racks. Normally, positioning thousands of piles accurately across a large site requires a surveying team to stake out positions and install indicating stakes or markings. Using RTK-capable equipment greatly streamlines this staking process. If the pile position coordinates from design drawings are loaded into a GNSS-equipped handheld device, the device will display in real time the offset between the user’s current position and the target pile position. Workers simply follow the screen guidance, move to the indicated spot, and mark the position, enabling pile staking to be completed much faster and more accurately than before. Satellite positioning works reliably even in mountainous areas with poor line of sight or for nighttime work, making it efficient on large mega-solar sites. After pile driving, drones can be used again to measure the pile heads in 3D and easily inspect whether all piles are within the design position and elevation. If pile misalignments are found, corrective actions can be taken immediately to prevent cumulative deviations in later stages and ensure smooth panel installation.

Use in construction management: Drone point cloud data also has great power in construction and workmanship management. In workmanship inspections, completed terrain and structures must be compared with designs to confirm they are within specified tolerance ranges, and high-precision 3D models obtained by drone surveying enable comprehensive workmanship checks in a short time. For example, comparing the ground model obtained from drone surveying at completion with the design model and visualizing any height or slope discrepancies can eliminate labor-intensive spot measurements. Analyzing point cloud data with specialized software makes it easy to create cross-sections and longitudinal/transverse profiles for the entire site, shortening the time required to prepare reports and inspection materials.

Furthermore, the detailed recorded data from drone surveying aids information sharing with clients and stakeholders. Site conditions that are hard to convey on 2D drawings can be intuitively understood using 3D models and orthophotos (composite nadir images), helping all parties share the same understanding. Using point cloud data for project progress reporting allows convincing presentation of site status to distant contractors and investors. In addition, drone surveying is useful for material management; for example, scanning stockpiles of soil or crushed stone on site and periodically measuring their volumes enables accurate tracking of material consumption and remaining quantities.

After a mega-solar facility begins operation, drones also contribute to more efficient equipment inspection. Aerial images can reveal panel contamination or faults, and when combined with point cloud data they can be used to monitor shadow risks from surrounding tree growth, expanding maintenance applications. In this way, from surveying to construction management and maintenance, drone-derived point cloud data can be used at every stage of a mega-solar project, delivering significant benefits in both operational efficiency and data accuracy.

Prospects for Point Clouds, 3D Models, and AR Utilization

As detailed 3D data from drones has become readily available, expectations for augmented reality (AR) technologies that leverage it have grown on site. Combining point clouds and 3D models with AR makes it possible to overlay digital design information onto the real world. For example, if a smartphone or tablet is held over a graded site after earthworks, the screen can display the planned equipment layout or design elevation lines, making it easy to share an intuitive image of the finished result that is hard to grasp from drawings alone. For pile staking, projecting design points onto the ground via AR helps workers locate and drive piles precisely without confusion. Visualizing buried cable conduits and other subsurface installations in AR after completion can also reduce the risk of accidental damage during future maintenance.

Moreover, use of these 3D data extends to support for heavy equipment operation. Machine guidance/machine control technology, which equips excavators and bulldozers with GPS receivers and 3D design data and displays target excavation elevations and slopes on an in-cab monitor, is becoming widespread. This enables operators to perform cut-and-fill accurately by following display guidance without visually checking elevation stakes. Harnessing 3D models for both people and machines dramatically improves construction accuracy and efficiency.

Currently, the Ministry of Land, Infrastructure, Transport and Tourism is also promoting DX on sites, and officially introduces low-cost 3D surveying methods that combine built-in smartphone LiDAR sensors with RTK receivers. 3D surveying that once required expensive laser scanners and advanced surveying equipment is becoming feasible with the simple smartphone + RTK combination. For example, by attaching a small RTK-capable GNSS device to a smartphone to obtain centimeter-level positional information (half-inch accuracy) while scanning the surroundings with the phone’s camera or LiDAR, point clouds can be generated on the spot and displayed in AR. Such technologies are ushering in an era of “smartphone-only surveying” in which site staff can perform surveys and staking immediately as needed. The next section introduces LRTK, a solution that strongly supports smartphone-based surveying.

LRTK RTK Data Integration and the Convenience of Smartphone-Only Surveying

As described above, the use of drones and RTK has greatly improved the efficiency of surveying at mega-solar sites, and LRTK is a technology that further simplifies on-site work. LRTK is our company’s solution that enables high-precision positioning and surveying using a smartphone. By attaching the high-precision GNSS receiver “LRTK Phone” to the back of a smartphone or other mobile device and integrating a dedicated LRTK app with cloud services, anyone can easily use centimeter-level positioning and point cloud measurement and AR functions.

Using the LRTK system, additional field measurements and staking can be performed with a single smartphone in the same coordinate system as orthophotos and point cloud models acquired by RTK-capable drones. For example, if you want to mark an arbitrary point on the 3D model created by drone surveying, you can import the coordinate data into the LRTK app and simply follow the smartphone’s on-screen guidance to arrive at the exact spot. For pile staking and layout marking, LRTK’s coordinate guidance function allows quick location of reference points or pile positions without specialized surveying equipment. Point cloud data and design drawings can be displayed on the smartphone via the cloud and verified in AR against the terrain and structures. These features are presented through an intuitive app UI, making them easy for site staff without surveying expertise to use.

Additionally, LRTK is strong in mountainous sites without internet connectivity. LRTK Phone supports augmentation signals from Japan’s Quasi-Zenith Satellite System “Michibiki,” allowing it to receive correction information directly from satellites and maintain high-precision positioning even outside mobile coverage. With LRTK, work in remote, pathless forest areas or remote mega-solar earthworks can proceed while accurately knowing real-time positions.

In this way, LRTK is highly compatible with data obtained by drones and contributes to further labor saving and sophistication of surveying and construction management at mega-solar sites. By using RTK-capable drones for wide-area surveys and LRTK for detailed measurements, pile staking guidance, and AR checks, tasks that previously required outsourcing or heavy equipment can be handled by on-site personnel. As a result, not only schedule compression and cost reduction but also faster on-site decision-making can be achieved. We expect the trend toward smartphone-only surveying to continue expanding across the civil engineering and construction industry, not limited to mega-solar construction. If you are interested in these new surveying methods, please also see the [LRTK product page](https://www.lefixea.com/phone). The combination of smartphone and RTK technology may become a major force that changes conventional site practices.