5 perspectives to confirm subsidence risk at solar power plants using drone surveying

In solar power plants, it is necessary to continuously monitor terrain conditions over a wide area, such as graded land, slopes, drainage channels, around support structures, and on-site roads. In particular, settlement can lead to tilting of power generation equipment, poor drainage, slope deformation, cable-related problems, and increased operation and maintenance costs, so it is important to detect signs early. Drone surveying can be effectively used in practice to check settlement risk because it can record changes in terrain while providing a broad overview of the site. This article organizes five perspectives that field personnel should pay attention to when checking settlement risk at solar power plants using drone surveying.

Table of Contents

• Reasons to Check for Settlement Risk at Solar Power Plants

• Perspective 1 Compare the changes in ground surface elevation over time

• Viewpoint 2 Check the tilt and unevenness of the mounting racks and panel rows

• Viewpoint 3: Check drainage paths and locations where puddles form

• Viewpoint 4: Check for deformation of slope faces and embankment boundaries

• Viewpoint 5: Organize survey results so they can be retained as inspection records

• Precautions when using drone surveying for subsidence monitoring

• Summary: Identify subsidence risks early through wide-area monitoring and continuous comparisons

Why you should check subsidence risk at solar power plants

Solar power plants are installed in locations with a variety of ground conditions, including mountainous areas, developed sites, reclaimed or landfill sites, land converted from agricultural use, and sloping terrain.

Even places that appear flat can experience settlement and unevenness over time at the boundary between fill and cut, in low spots where drainage concentrates, or in areas susceptible to groundwater or rainwater. Settlement does not necessarily progress suddenly; in the early stages it often appears as slight offsets or poor drainage, so it can be missed during routine inspections.

The reason settlement becomes a problem at a solar power plant is not simply that the ground sinks. If the foundations around the mounting structures settle, it can affect the tilt of the panel surfaces and the alignment of the rows. If drainage channels or side ditches settle, rainwater may not flow in its intended direction and standing water tends to remain on site. If level differences appear on internal roads or maintenance walkways, movement of inspection vehicles and workers can be impeded. Furthermore, if settlement is related to deformation of slopes or embankments, it must be treated as a maintenance and management risk for the entire site, not just the power generation equipment.

Traditional patrol inspections often involve personnel walking the site and visually checking for anomalies. While visual inspections make it easy to find nearby abnormalities, they are limited in understanding elevation differences across an entire plant or wide-area subsidence trends. In particular, solar power plants have large sites and rows of panels arranged in similar patterns, so localized changes can be masked by the surroundings. This is where drone surveying, which captures aerial imagery and organizes it as terrain data and orthophotos, is useful.

By using drone surveying, you can record large areas under consistent conditions and more easily check for changes by comparing with previous data. It also makes it easier to share among inspectors, managers, contractors, and maintenance personnel subtle micro-topographic changes that are hard to notice on site, low spots where rainwater tends to collect, irregular alignment of rack rows, and slope deformations. However, conducting drone surveys does not automatically allow you to determine subsidence risk. It is important to decide what to look at, what area to compare, and what accuracy to record, and to combine that with on-site verification when making judgments.

Perspective 1: Compare changes in ground surface elevation over time

The most basic step in checking subsidence risk is to compare changes in ground surface elevation over time. At solar power plants, keeping survey data at multiple times—such as at completion, after the start of operation, after heavy rainfall, and during regular inspections—makes it easier to track how the ground surface is changing. A single survey can show the terrain condition at that moment, but it can be difficult to judge whether that shape is the original one or has changed during operation. Therefore, for the purpose of assessing subsidence risk, it is important to continuously record the same area using the same standards.

With drone surveying, you can create terrain models and orthomosaic images from captured photos and compare them with past survey results. For example, if you find areas that are lower than before—such as parts of on-site roads, around rack foundations, along drainage channels, or near the toe of slopes—this can be an indication to suspect subsidence. Of course, vegetation growth, shadows, shooting conditions, and surface conditions can change how things appear, so you should not draw conclusions based solely on changes in the data; judgments should be made in conjunction with on-site verification.

When observing height changes, it is important to clarify the reference used for comparison. If the handling of reference points and survey marks changes each time, the entire dataset will be offset, making it difficult to tell whether it is actual subsidence or a difference in surveying conditions. When checking subsidence risk, relative changes—how much lower a location is compared to other locations within the power plant—are as important as absolute elevation. Observing patterns such as a specific row being lower than its surroundings, continuous lowering along drainage channels, or changes along the edges of embankments makes it easier to narrow down locations that should be prioritized for on-site inspection.

Also, the appearance of changes in ground surface elevation can vary depending on the season and rainfall conditions. When vegetation is tall it is difficult to accurately capture the ground surface, and immediately after rainfall the terrain can look different because of mud and puddles. If the purpose is to check for subsidence, it is desirable, as far as possible, to keep the timing of imaging and ground-surface conditions consistent and to retain data that are easy to compare. In power plants with a lot of vegetation in particular, surveying after vegetation has been cleared or during periods with good visibility makes it easier to confirm the condition of the ground surface.

The important thing is not to chase fine numerical values to detect subsidence, but to read the trend of change. If you judge based only on small local differences, you may mistake surveying errors or the influence of surface conditions for subsidence. On the other hand, if there are trends such as continuous decreases in the same direction, low areas expanding year by year, or ground surface lowering in places that coincide with poor drainage, those sites should be prioritized for on-site inspection. Drone surveying is effective as an entry point for grasping such wide-area trends.

Viewpoint 2 Check the tilt and unevenness of the mounting structure and panel rows

Settlement risk at solar power plants can appear not only on the ground surface but also in the way mounting racks and panel rows look. If settlement progresses around the foundations of the racks, the alignment of panel rows may become disturbed and some support posts may show an unnatural tilt. When inspected on foot from the ground, the overall picture can be difficult to grasp because the viewer’s eye level and the arrangement of the panels obstruct the view, but recording from above with drone surveying makes it easier to confirm the straightness of entire rows and their relationship with the surroundings.

Unevenness in panel rows is not necessarily caused solely by settlement. Various factors are involved, such as installation errors during construction, design steps to match the terrain, differences in racking types, and adjustment status during maintenance. Therefore, if signs appear—such as a row looking wavy in aerial images, the spacing between rows appearing unnatural only at specific locations, or steps and bare ground visible around support posts—it is important to cross-check with design drawings, construction records, and past inspection records. Rather than concluding settlement from the outset, it is necessary to organize potential abnormalities as candidate causes.

The advantage of drone surveying is that it allows panel rows to be understood as surfaces. In solar power plants, similar rows of panels are spread over large areas, so it can be difficult to distinguish differences between individual rows from the ground. From above, the alignment of rows, foundation positions, their relationship to maintenance access routes, and distances to drainage channels can be seen together, making it easier to check how subsidence is affecting equipment layout. In particular, rows that cross the boundary between embankment and cut sections, rows near the lower parts of slopes, and rows where drainage tends to accumulate are more likely to show ground changes that manifest in the equipment.

When inspecting mounting structures and panel rows, consider using three-dimensional data in addition to images. If you can check panel surface heights and per-row trends, you can correlate changes in the ground surface with changes in the equipment. However, panel surfaces are prone to reflections and shadows, and data stability can vary depending on imaging conditions. For settlement checks, focusing on the area around the mounting foundations, the alignment of entire rows, and the relationship with the surrounding ground—rather than on the fine shape of the panels themselves—makes the approach more practical for field use.

Settlement around mounting structures can also affect electrical equipment and cable management. Ground subsidence can lead to cable exposure, uneven levels around conduits, rainwater ponding, and deterioration of workspaces. Drone surveys alone cannot confirm every detail, but using wide-area imagery to identify areas of concern and then conducting detailed on-the-ground inspections helps reduce the likelihood of missed issues. Dividing responsibilities between aerial checks and on-the-ground verification is important for managing subsidence risk.

Viewpoint 3: Inspect drainage routes and locations where puddles form

When assessing the settlement risk of a solar power plant, checking the drainage routes is indispensable. If settlement progresses, rainwater may no longer flow in its intended direction and is more likely to accumulate in low-lying areas. Conversely, where water remains for long periods, the ground can become loosened, which can cause settlement and unevenness. In other words, settlement and poor drainage are closely related, and examining only one of them is not sufficient.

With drone surveying, you can take an overhead view of the drainage routes across an entire site. By observing the formed slopes, side ditches, catch basins, drainage channels, small benches on embankments, and the cross slope of internal roads from above, it becomes easier to understand where water is likely to accumulate. Using a terrain model, you can also identify locations that are lower than their surroundings and lines that are likely to become flow paths. However, actual water flow is also affected by surface grass, crushed stone, deposited soil, clogged side ditches, local level differences, and similar factors, so it is important not to rely solely on the gradients shown in the data but to compare them with on-site conditions after rainfall.

Particularly important to check are locations where puddles repeatedly form. If water pooled in a place only once, it may have been caused by temporary rainfall or a clogged drainage channel. However, if water remains in the same spot every time, the extent of puddling has increased compared to before, or water is collecting under the panel rows or around the racking foundations, you should suspect settlement or a change in the drainage gradient. Taking regular drone photographs allows you to compare the locations and extent of puddles with past images, making it easier to share changes that are difficult to convey by verbal explanation alone with stakeholders.

When inspecting drainage routes, pay attention not only to low-lying areas but also to places where sediment flows in and where deposition is likely. If soil washed from slopes accumulates in side ditches or drainage channels, water can overflow and spread, leading to erosion of the ground surface or localized subsidence. Areas such as beside on-site roads, bends in drainage channels, collection points, and the toes of slopes are places where changes are easy to spot even in drone imagery. Signs such as changes in soil color, washed-away crushed stone, or vegetation that differs from the surroundings are also clues indicating poor drainage or concentration of water.

In solar power plants, because equipment is spread over a wide area, poor drainage may affect only part of the generation equipment. Even if it does not appear to be a major problem for the entire site, persistent standing water in specific areas can interfere with inspection work and lead to deterioration around equipment. When using drone surveying, it is practical to consider changes in ground elevation and water flow together on the same map rather than separately. Locations where low spots and standing water overlap should be prioritized as candidates for on-site inspection.

Viewpoint 4: Check for deformations at slope faces and embankment boundaries

Settlement risks at solar power plants appear not only on the flat areas where panels are installed but also on surrounding slopes and embankment boundaries. On developed sites, ground changes can easily occur at the boundary between cut and fill, at slope shoulders, slope toes, small benches, and around drainage facilities. Even when settlement seems confined to the flat area, it may actually be related to slope deformation or poor drainage. Therefore, in drone surveys it is important to check not only directly beneath the generation equipment but also the site perimeter and slopes.

Slope deformations exhibit various signs such as surface sliding, cracks, bulging, erosion, sediment accumulation, and disturbances to vegetation. These may be only partly visible from the ground, but when photographed by a drone from above or at an oblique angle they are easier to grasp as continuous changes. In particular, conditions such as linear changes visible along the slope shoulder, sediment accumulating at the lower part of the slope, or ongoing erosion along drainage channels require attention as possible indicators of subsidence or ground deformation.

Embankment boundaries are also important points to check. At the boundary between fill and the existing ground, the degree of compaction and the way water flows tend to change, and over time steps and unevenness can develop. In solar power plants, sites are often developed to secure level areas, and even if no problems are visible immediately after completion, changes may surface depending on rainfall, drainage conditions, and ground conditions. Checking elevation differences and surface patterns with drone surveys and looking for changes along embankment boundaries helps with early detection of settlement risk.

When checking slopes and embankment boundaries, cross-referencing past topography and development plans is also effective. Understanding which areas are fill and which are cut, and how drainage facilities are planned, makes it easier to interpret survey results. If field personnel look only at images without understanding the background of the terrain, they may confuse natural topographic variations with deformations. Conversely, if the development history and ground conditions are known, it becomes easier to judge the significance of changes detected by drone surveys.

Changes in slope conditions also affect safety management. If they impact nearby support structures, maintenance walkways, fences, or drainage channels, they will influence planning for maintenance inspections and restoration work. When conducting drone surveys to confirm settlement risk, it is important to consider site maintenance and management as well as the operating status of the power generation equipment. If changes in slopes or fill boundaries can be detected at an early stage, it becomes easier to move on to emergency response, detailed investigations, and repair planning.

Perspective 5: Organize surveying results so they can be retained as inspection records

When using drone surveying to assess subsidence risk, it's important not to stop at simply capturing images. Because subsidence needs to be evaluated over time, survey results should be preserved in a form that allows later comparison. By organizing images, terrain data, location information, capture date, coverage area, weather, ground surface conditions, reference points used, and observations made on-site, it will be easier to compare under the same conditions during the next inspection.

To make inspection records easy to use, it is first important to know the location. A simple photograph alone can make it difficult later to identify which panel row and which spot it is. If you organize them as orthophotos or in a form close to a plan view, managers and field personnel can more easily confirm the same location. For areas suspected of subsidence, recording the location, extent, inspection details, whether on-site photos are available, and whether a follow-up check is necessary will also help with inspection handover.

Also, when explaining subsidence risk, visualization is important, not just numbers. Materials that color-code the amount of ground-surface change and low-lying areas, comparisons of past and current images, and maps showing the locations of poor drainage, for example, make it easier to align understanding among stakeholders. In particular, when stakeholders occupy different positions—power plant managers, maintenance companies, contractors, landowners, etc.—materials that convey the situation to people who have not seen the site are necessary. The outputs of drone surveys are valuable because they can be readily used as materials for sharing a broad overview of site conditions.

However, when preparing documents, it is also important to avoid making overly definitive statements. Even if drone surveying shows a downward trend, that alone does not allow definitive determination of the cause or the level of risk of ground subsidence. In documents, it is desirable to separate and organize the facts confirmed by the survey, items that require on-site confirmation, and items for which additional investigation is desirable. For example, limit expressions to statements such as "a specific location appears lower than its surroundings," "the extent of standing water has increased compared with past images," or "some irregular alignment is observed in part of the rows of mounting racks," and, if necessary, follow up with specialized investigations.

Continuously keeping inspection records provides material for future decision-making. To determine whether settlement is temporary or gradually progressing, comparison with past data is indispensable. Power plants operate for long periods, and personnel may change. Therefore, organizing results in a way that anyone can understand is important for long-term maintenance management. By incorporating drone surveying not as a one-off investigation but as a method for recording regular inspections, you can enhance your ability to respond to settlement risks.

Precautions when using drone surveying to check subsidence

Drone surveying is useful for understanding subsidence risk, but if used incorrectly it can lead to wrong judgments. The first thing to be aware of is that survey results are affected by capture and processing conditions. Sunlight, shadows, wind, ground vegetation, moisture, flight altitude, image overlap, and the arrangement of reference points all influence how results look and their accuracy. If the conditions for the previous and current surveys differ greatly, the differences may appear due to changes in data creation conditions rather than actual terrain changes.

When checking for settlement, it's important not just to broaden the survey area but also to clearly define priority locations. Broadly photographing the entire solar power plant is useful, but if you anticipate locations with a high risk of settlement in advance, verifying the results becomes more efficient. Areas to focus on include the embankment boundary, areas near slope faces, along drainage channels, locations where puddles have formed in the past, level differences on internal roads, and around racking foundations. By combining an overall aerial view with focused inspections of priority areas, it becomes easier to reduce oversights.

Combining drone surveys with on-site verification is also indispensable. Even if a drone survey identifies potential anomalies, it can be difficult to judge surface conditions, conditions around foundations, clogged drainage channels, structural damage, or vegetation overgrowth without close-up on-site inspection. Drones are well suited to accessing areas that are difficult for people to approach and to covering wide areas, but they do not replace all detailed assessments. A practical workflow is to use wide-area surveys to extract locations of concern and then use ground inspections to confirm the causes and whether any response is necessary.

Also, when assessing subsidence risk, a plan that takes the power plant’s operation and safety into account is necessary. When flying drones, you must check workers on site, surrounding facilities, electrical and transmission equipment, weather conditions, and so on, and establish a safe flight plan. Inside the power plant, attention must also be paid to reflections and wind effects, distances to mounting structures and fences, and securing takeoff and landing areas. It is a prerequisite to have not only surveying accuracy but also a system in place that allows the work to be carried out safely as on-site operations.

Furthermore, how you manage the results is also important. To compare with past data, you need to organize and store information such as file names, capture dates, target areas, coordinate conditions, and inspection details. Even if the data remain, if it is not clear which point in time or which area they represent, they become difficult to use as comparative material. Because settlement checks are well suited to long-term management, it is advisable to decide on a recording method from the initial survey with future comparisons in mind.

Summary: Identify subsidence risk early through broad-area monitoring and continuous comparison

It is important to check settlement risks at a solar power plant from multiple perspectives: changes in ground surface elevation, misalignment of mounting structures and panel rows, poor drainage, deformations at slopes and embankment boundaries, and the accumulation of inspection records. Settlement can be difficult to detect by visual inspection alone, and in its early stages it may appear as puddles, vertical offsets, misaligned rows, or small changes in slopes. By using drone surveying you can capture the entire, extensive plant from above and more easily identify areas of concern by comparing with past data.

However, you should avoid drawing definitive conclusions about the cause or severity of settlement based solely on drone surveys. It is important to consider the influence of imaging conditions and ground surface state, and to judge the situation in conjunction with on-site inspections, design documents, construction records, drainage conditions, and past inspection histories. Survey results are merely an entry point for identifying settlement risk and should be treated as decision-making material to inform further detailed investigations or repair evaluations as needed, making them more useful in practice.

What's particularly important in addressing settlement risk is keeping records that can be compared over time. By retaining the condition at completion and at the start of operation as a baseline and surveying the same area during regular inspections and after heavy rainfall, it becomes easier to grasp trends in change. If areas of concern can be identified while changes are still small, it becomes easier to develop inspection and repair plans, which contributes to the overall maintenance of the power plant.

In managing solar power plants, it is necessary to take a perspective that encompasses not only the generation equipment itself but also the ground, drainage, slopes, access paths, and perimeter areas. Drone surveying is an effective means of capturing that overall picture. By identifying settlement risks early, keeping inspection records, and establishing a system to share them with stakeholders, the maintenance and management of solar power plants can be made easier.