6 Tips for Using Drone Surveys to Inspect Slopes at Solar Power Plants

At solar power plants, checking the condition of the slopes that support the site is as important as inspecting the panels and mounting structures. In particular, at plants located in mountainous areas or on reclaimed/engineered land, small changes in slopes can lead to poor drainage, soil runoff, subsidence, cracks, or collapse risks. Changes that are easy to miss during visual patrols can be more readily recorded—both the overall picture and the details—by combining drone surveying.

Drone surveying not only allows large sites to be inspected from above, but is also characterized by the ability to regularly image the same area, making it easy to compare changes in terrain and surface conditions. However, simply flying does not automatically produce useful results. For slope inspections, operations must be planned and conducted with consideration for the imaging coverage, flight altitude, control points, drainage routes, the influence of vegetation, and how findings are organized in reports.

This article explains six practical tips for using drone surveying when inspecting slopes at solar power plants. It is intended to serve as on-site decision-making material for personnel involved in plant maintenance, inspections, repair planning, and information sharing with partner companies.

Table of Contents

• Organize the objectives of slope inspection by dividing them into terrain changes and maintenance and management.

• Before photographing, confirm the relationship between slopes, drainage, and panel placement.

• Establish criteria for photographing under the same conditions to make changes easier to see.

• Grasp potential anomalies three-dimensionally using point clouds and orthophotos.

• Stabilize verification accuracy by accounting for the effects of vegetation, shadows, and weather.

• Retain as a report and as a record for the next inspection

• Summary

Organize the objectives of slope inspections by dividing them into terrain change and maintenance management

The first important thing when inspecting slopes at a solar power plant is to clarify what you want to verify with drone surveying. Simply taking photos from above may record site conditions, but it is unlikely to produce information that is easy to use for operation and maintenance. By clarifying which changes in the slope you want to observe, how large a change you want to monitor continuously, and who will use the inspection results and for what decisions, the survey plan and the way deliverables are used will change significantly.

On the slopes of solar power plants, there are many things to check: rainwater flow, surface erosion, sediment accumulation, minor collapses, changes in vegetation, clogging around drainage facilities, and how they interface with maintenance access paths. These cannot all be judged from a single flight. In particular, slight undulations of the slope surface and changes in watercourses may only become meaningful when compared with past conditions. Therefore, it is important to view drone surveys not as one-off inspections but as an accumulation of regular records.

For the purpose of confirming terrain changes, the focus is on understanding the shape and gradient of slopes, the movement of sediment, and the extent of collapses. Capture images so that the height and form of the ground surface can be compared, and, if necessary, organize them as point clouds or terrain models. In this case, the reproducibility of imaging conditions and the accuracy of alignment are important. If, when overlaying the previous and current results, it is unclear whether the differences are actual changes or due to differences in imaging conditions, the data becomes difficult to use as a basis for judgment.

On the other hand, for maintenance and management purposes the focus is on determining the priority of on-site responses. For example, checking whether sediment has accumulated around drainage ditches, whether collapsed soil has flowed down to the toe of slopes, whether scouring is progressing near fences, or whether the water flow paths beneath the bottoms of panels have changed. In such cases, conducting high-density surveys across the entire area is not necessarily the right approach. Clear images, location information, and comparison materials that allow potential anomalies to be found quickly and that support decisions on field inspection and repair are important.

If you shoot without clarifying the objective, you are likely to encounter problems afterward such as “I don’t know which part of the slope to look at,” “there are many photos but they’re hard to judge,” and “I can’t explain the differences from the previous inspection.” Conversely, if the objective is clear, it becomes easier to decide the shooting area, flight route, overlap rate, deliverables to check, and the structure of the report. For slope inspections at solar power plants, the first step to using drone surveying effectively is to distinguish whether you are looking for changes in topography, searching for maintenance-related anomalies, or both.

Confirm the relationship between slopes, drainage, and panel layout before photographing

Before conducting drone surveys, you should check the relationship between the slopes and the overall layout of the power plant, rather than viewing the slopes in isolation. In a solar power plant, panel rows, mounting structures, maintenance access paths, drainage ditches, sedimentation facilities, balancing ponds, fences, and slopes all influence one another. If changes appear on a slope, their cause may not lie within the slope itself. They can be related to rainwater inflow from upslope, surface flow beneath the panels, the gradient of access paths, or clogging of drainage facilities, among other factors.

Before photographing, what you should pay particular attention to is the flow — where water enters, where it passes, and where it goes. Locations where water tends to gather from the slope shoulder onto the slope face, spots under panel rows where rainwater tends to concentrate, areas where flow from maintenance access walkways is directed toward the slope face, junctions where drainage channels converge, and places at the slope toe where sediment tends to accumulate are all worth recording carefully during the shoot. Rather than simply photographing the entire site uniformly, identifying in advance the locations likely to undergo change will make the deliverables easier to interpret.

In solar power plants, shadows from the panels and the mounting racks affect how the ground surface appears. From above, the ground between panel rows and beneath the racks can appear dark, making it difficult to confirm terrain undulations and sediment accumulation. Also, when slopes run continuously along the site perimeter, fences and trees can be captured in images, making it hard to inspect the lower parts of the slope. Therefore, before shooting, it is necessary to check the panels’ orientation, the slopes’ aspect, sun exposure, and the positions of obstacles, and to consider the conditions under which the areas you want to see will appear in the images.

Site safety should also be checked before filming. Areas near slopes often have unstable footing and can be difficult to approach on foot, so inspection by drone is effective. However, if there are power transmission facilities, communications equipment, trees, fences, buildings, or workers near the flight route, you need to establish safe separation distances and a monitoring system. Because there are also places within the power plant with poor visibility, deciding the takeoff and landing locations and the pilot’s position in advance will make decision‑making during filming more reliable.

In slope inspections, site drawings and past inspection records are also useful. If you incorporate development planning drawings, drainage plans, past repair histories, locations where soil runoff has occurred, and areas of concern noted during patrols into the imaging plan, the survey becomes directly tied to maintenance management rather than a mere record. Since the slope as shown on drawings can appear different from the actual site, it is important to verify the images obtained from drone photography together with on-site records.

Careful preparation before shooting makes it easier, when you later review the deliverables, to explain how slope changes relate to the layout of facilities within the solar power plant. Slopes at solar power plants change due to overlapping factors such as rainwater, site shaping, equipment layout, and maintenance conditions. When using drone surveying, it is important not to treat slopes in isolation but to have the awareness to read the relationship between water and terrain across the entire plant.

Establish criteria for photographing under consistent conditions to make changes easier to see

A major advantage of using drone surveying for slope inspections is that you can repeatedly record the same area and compare changes. However, for comparison purposes you need to keep the capture conditions as consistent as possible each time. If there are large differences in flight altitude, shooting direction, image overlap, flight area, control points, time of capture, or weather conditions, it becomes difficult when overlaying the outputs to determine whether observed differences are actual terrain changes or merely differences in appearance caused by the capture conditions.

First, decide the scope of the periodic photography. You can photograph the entire solar power plant site each time, but if the purpose is slope inspection, it is more efficient to predefine an area that includes the perimeter slopes, slope shoulders, slope toes, drainage facilities, and the junctions with maintenance access paths. In particular, make sure that locations which previously showed deterioration or abnormalities and places where rainwater tends to collect are covered consistently so they can be compared in subsequent inspections.

Next, record the imaging conditions. In addition to flight altitude and capture interval, record the date and time of capture, weather, ground surface conditions, whether it was before or after mowing, and the number of days since rainfall; these details will help you interpret changes later. For example, images taken immediately after mowing and those taken when the grass has grown can look very different even on the same slope. Immediately after rainfall it's easier to identify water flow paths and wetness, but you may also see effects from mud and reflections. Recording differences in conditions makes it easier to avoid overinterpreting differences when comparing images.

Reference points for alignment are also important. When checking changes to slopes, images and point clouds are sometimes overlaid on past data. In such cases, aligning positions using fixed objects within the power plant or survey control points increases the reliability of the comparison. When installing reference points, choose locations that are unlikely to move, easy to see during imaging, and can be used continuously in the future. In power plant operation and maintenance, it is important not only to store each survey as a separate record but also to organize them chronologically using the same coordinates and the same reference.

To create comparable results, it is also important not to leave the shooting plan entirely up to on‑site staff. Even if the person in charge changes each time, it is useful to document the flight route, shooting altitude, key inspection points, and types of deliverables as simple procedures so the same area can be photographed with the same approach. This reduces variation caused by differences in personnel experience and makes it easier to maintain continuity in slope inspections.

Also, determine the photographing frequency according to the purpose. The points to check differ for regular inspections during normal conditions, ad hoc inspections after typhoons or heavy rain, and comparative inspections before and after repairs. During normal times, track overall trends in change; after heavy rain, focus on checking for scour or sediment outflow; and for before-and-after repair inspections, check the work area and impacts on the surroundings. Organizing the shooting conditions for each purpose makes it easier to make decisions about slope management.

Drone surveying becomes more valuable the more records you accumulate. Rather than looking for anomalies in a single flight, continuing to capture imagery to the same standards makes it easier to explain the progression of small changes and the post-repair stability. For slope inspections at solar power plants, keeping records in a comparable state is more important than the act of photographing itself.

Understand potential anomalies in 3D using point clouds and orthoimages

In slope inspections at solar power plants, using point clouds and orthophotos in addition to aerial photographs makes it possible to grasp site conditions in a more three-dimensional way. Orthophotos are well suited to organizing the spatial relationships of the entire plant from above. Because slopes, panel rows, drainage channels, pathways, fences, and surrounding terrain can be checked on a single planar image, they are easy to use for explaining the locations of potential anomalies and for sharing with stakeholders.

On the other hand, point clouds are effective for checking the undulations and changes in elevation of slopes. Conditions such as part of a slope bulging, collapsed soil accumulating at the slope toe, sediment gathering around drainage ditches, or the shoulder of a pathway being eroded can be difficult to judge from photographs alone. Confirming these as point clouds or terrain models makes it easier to understand the three-dimensional changes.

However, when using point clouds, it is important not to judge solely by the visual appearance of the results. On slopes with dense vegetation, point clouds may capture the surface of the vegetation rather than the ground. In areas where panels, racks, fences, and trees coexist, unnecessary points may be included. Therefore, do not assume that differences in point cloud elevation necessarily indicate terrain changes; assess them in conjunction with imagery, on-site inspection, and historical data.

What you first want to check in an orthophoto are traces of water flow. If thin streaks of discoloration or traces of sediment flow appear on slopes, rainwater may be concentrating and flowing. If sediment has accumulated around drains or catch basins, scouring may be progressing upstream. If streak-like marks continue from beneath panels toward the slope, surface drainage within the power plant may be affecting the slope. These positional relationships are easy to confirm from aerial images.

What we want to check with point clouds is changes in slope geometry. When you can compare previous and current data, it becomes easier to determine whether parts of the slope have been cut away or whether soil and sediment have accumulated. It also helps to check whether the extent of a collapse has expanded, whether deposition at the slope toe has increased, or whether repaired sections have blended into the surroundings. In particular, when site photographs alone do not convey the extent well, having three-dimensional data makes it easier to explain to stakeholders.

At solar power plants, slope abnormalities do not necessarily affect panels or mounting structures immediately. However, if slope changes progress, they can lead to reduced drainage function, damage to maintenance access paths, scouring around fence foundations, and inflow of soil and sediment around power generation equipment. If potential changes can be identified at an early stage using point clouds and ortho images, it becomes easier to carry out on-site inspections and plan repairs.

When sharing deliverables with stakeholders, it is important not to provide only technical data but to organize the information so that the points to be reviewed are clear. Mark inspection points on the orthophoto, supplement shapes with point clouds and cross-sections, and include site photographs so that the actual conditions can be confirmed; this makes it easier to align understanding among managers, contractors, and inspectors. The value of drone surveying is determined not by the quantity of data but by whether it can be organized into a form that supports decision-making.

Stabilize verification accuracy by accounting for the effects of vegetation, shadows, and weather

When using drone surveying to inspect slopes, what is often overlooked is the influence of vegetation, shadows, and weather. Slopes at solar power plants see grass height change with the seasons, soil color and moisture change before and after rain, and shadow positions shift depending on the time of day. When these conditions change, photographing the same location can make it appear different. To consistently confirm changes in slopes, you need to anticipate differences in shooting conditions in advance.

The effects of vegetation are particularly significant. When grass is overgrown, surface scour, small cracks, and sediment accumulation on the ground can become difficult to see. When creating point clouds, the scanner may capture the top of the grass rather than the ground surface, resulting in heights that differ from the actual terrain. Comparing before and after mowing can make differences caused by the presence or absence of grass appear to be changes in topography. Therefore, when the purpose is periodic comparison, it is advisable to record the timing of mowing and the condition of vegetation and to capture images under as similar conditions as possible.

However, the presence of grass does not mean you cannot photograph. The way the grass lies and differences in color can sometimes reveal patterns of water flow and areas prone to moisture. The important thing is to distinguish what can and cannot be confirmed when the surface is covered with grass. If you want to check fine surface deformations, shooting after mowing is appropriate. If you want to observe water flow or moisture tendencies, conditions after rainfall can be informative. It is important to choose the timing of photography according to your objective.

The effects of shadows cannot be ignored. At solar power plants, shadows from panels and mounting structures fall on the ground. Depending on the slope orientation, the appearance of a slope can change significantly between morning and afternoon. In images with strong shadows, differences in soil color and fine surface undulations become difficult to discern. Conversely, when sunlight is too strong, the contrast between bright and dark areas increases, making it difficult to assess the entire image. Choosing times of day that make comparisons easier and ensuring conditions do not differ greatly from previous imagery will make it easier to detect changes.

Clear skies are not always the optimal condition. Overcast skies that provide uniform brightness can sometimes make it easier to reduce the impact of shadows. However, strong winds or rain can affect both safety and image quality. If wind makes the aircraft unstable, images may blur and capture positions may vary. Rain, fog, a wet lens, and ground reflections also reduce inspection accuracy. For slope inspection, you need to choose conditions that balance flight safety and the clarity of the results.

Photographing after rainfall is effective if you decide in advance how the images will be used. After heavy rain, features that are not visible under normal conditions—such as flow paths, sediment runoff, deposition at the base of slopes, and clogged drainage channels—can become easier to detect. For power plant maintenance, operating routine periodic imaging separately from ad hoc imaging after heavy rain makes it easier to identify the causes of slope changes. However, immediately after rainfall the ground is wet and appears darker, and puddles can obscure surfaces, so these images should be treated as serving a different purpose than normal topographic comparisons.

To stabilize inspection accuracy, it is important to record imaging conditions and reflect them in the deliverables. Recording not only the capture date and weather but also whether grass was mowed, the time since the last rainfall, the slope’s moisture, how shadows fall, and the time of capture will make it easier to judge when reviewing images later. The results of drone surveying are not complete with images and point clouds alone. Combining them with notes on site conditions increases the reliability of slope inspections.

Retain as a record for the report and the next inspection

After using drone surveys to inspect slopes, how you document and retain the results is important. Simply storing the captured data alone is unlikely to translate into practical maintenance work. At solar power plants, multiple stakeholders — managers, inspection personnel, contractors, repair companies, and landowners — may view the information. Therefore, it is important to organize the findings not just as technical data but as reports that clearly show the inspection points and the status of any follow-up actions to anyone who looks at them.

In the report, first clarify the imaging scope and the purpose of the inspection. Depending on whether it is a routine inspection of the entire power plant, an ad hoc inspection after heavy rain, or a before-and-after comparison for repairs, the points the reader should focus on will differ. Organize the date of imaging, imaging conditions, inspection scope, and the types of deliverables used, and make it clear which parts of the slope were examined. If orthophotos or overall maps are available, indicating the locations of suspected anomalies makes it easier to follow up with field verification.

Next, link the inspection results to on-site response priorities. For example, sort and organize whether an observation is a minor surface flow trace, scour that requires follow-up monitoring, or a slope failure that should be inspected on site promptly. However, avoid over-determining the level of hazard based on images alone. Drone surveys can only show changes visible from above. Subsurface conditions, soil type, detailed drainage function, and the internal stability of slopes cannot always be assessed from images alone. Therefore, in reports it is more practical to use action-oriented expressions such as "on-site confirmation required," "follow-up observation recommended," or "verification of drainage conditions necessary."

If you include comparisons with past data, explain carefully how the changes appear. If the imaging conditions differed between the previous and current captures, it is important to clearly state those differences. Factors such as before-and-after mowing, post-rainfall conditions, and differences in shadows can cause changes seen in imagery to be mistaken for actual terrain change. In comparative materials, not only showing the differences but also listing conditions that require caution in interpretation will help prevent readers’ misinterpretation.

To link records to the next inspection, it is important to keep the locations of inspection points in a reusable form. Manage potential abnormal locations by number or name so the same spots can be checked when photographing them during the next inspection. For example, using site-friendly names such as the north-facing outer slope, the area near the drainage channel confluence, the slope beneath the maintenance walkway, and the toe accumulation area makes them easier to share among stakeholders. If the same locations can be checked at the next inspection, it becomes easier to determine whether changes are progressing or whether they are stable.

In terms of data management, it is useful to organize captured data, processed images, point clouds, reports, on-site photos, and repair records as a single project. Maintenance of power plants is long-term, so it is important to keep records in a state that allows the history to be traced even if personnel change. Slopes in particular may change significantly over a short period or may evolve gradually over several years. If records of regular drone surveys are kept, it becomes easier to determine when changes started if an anomaly occurs.

A report being detailed is not enough. What matters is that stakeholders know what to do next. If it is clear which slopes were inspected, where potential changes may be occurring, whether an on-site inspection is necessary or monitoring will suffice, and when and under what conditions the next imaging should be carried out, the results of drone surveying can be more easily used to inform maintenance and management decisions.

Summary

When using drone surveying for slope inspections at solar power plants, it's important not to focus solely on capturing clean aerial images. Whether you want to observe slope topography changes, search for potential maintenance anomalies, or compare conditions before and after repairs, the shooting plan and the way you produce deliverables will differ. By first clarifying your objectives and understanding the relationships among slopes, drainage, panel layout, and maintenance access paths before shooting, you will obtain information that is more useful in practice.

Especially on the slope surfaces of solar power plants, water flow is important. By checking where rainwater collects, junctions of drainage channels, the slope crest and slope toe, surface flow from under panels, and interfaces with access paths, it becomes easier to detect signs of sediment runoff and scour. In drone surveys, organizing positional relationships using orthophotos and supplementing three-dimensional changes with point clouds and terrain models makes it easier to grasp the extent and direction of changes that are difficult to discern from on-foot patrols alone.

On the other hand, vegetation, shadows, weather, and differences in capture conditions can greatly change how results appear. During periods when grass is overgrown versus after it has been cut, what can be observed on the same slope differs. After rainfall, watercourses and sediment runoff are easier to find, but care is needed when comparing with normal conditions. Recording the date of capture, weather, mowing status, time elapsed since rainfall, and time of capture will make later judgments more consistent.

Moreover, the value of drone surveys is determined by whether their deliverables can be linked to subsequent inspections and repair decisions. It is important not only to store the captured data but also to organize the locations to check, suspected anomalies, the need for on-site verification, and policies for ongoing monitoring, and to retain this information so the same locations can be compared next time. Ensuring that past conditions can be tracked even when personnel change will aid the long-term maintenance of the power plant.

The slopes at a solar power plant are an important part of the surrounding environment that support the generating equipment. By detecting small changes early and continuously monitoring drainage and terrain conditions, it becomes easier to make decisions about inspections and repairs. When incorporating drone surveying into slope inspections, it is important to design it as an integrated management process covering image capture, analysis, reporting, and comparison with subsequent surveys.

If you want to make onsite slope inspections more efficient, it is best to start by establishing drone surveying procedures tailored to the power plant’s conditions. By combining aerial records, geotagged images, terrain verification with point clouds, and organization into reports, it becomes easier to share changes that are hard to notice through patrols alone. If you are considering using drones for inspection and maintenance of solar power plants, clarify your site conditions, inspection objectives, safety management system, and how deliverables will be used, and consider operational methods that allow for continuous comparison.