6 Points to Check Steep Slopes at Solar Power Plants with Drone Surveying

On planned or existing solar power plant sites, there are locations that are difficult to grasp by walking inspection alone—not only reclaimed land but also former forest land, sites that include slopes, land close to valley topography, and sites with many level differences. Especially on steep slopes, if surface undulations, drainage flow, the arrangement of mounting structures and access routes, and safe work movement lines are not understood in advance, rework tends to occur during the design, construction, and maintenance stages. Drone surveying is a method to check these steep-slope conditions from above across an area and support on-site decision-making using photos, orthophotos, elevation data, and point cloud data. However, shooting with a drone does not reveal everything. It is important to organize the viewpoints that need to be confirmed while taking into account slope-specific shadows, vegetation, poor visibility of the ground surface, changes in flight altitude, and how ground control points are established.

Table of Contents

• Reasons why drone surveying becomes necessary on steep slopes

• Perspective 1: Identify areas of steep slope and terrain changes

• Aspect 2: Confirm the impact on panel placement and racking plan

• Aspect 3: Identify drainage routes and rainwater concentration points

• Consideration 4: Confirm the risks of slopes and areas prone to collapse

• Perspective 5: Organize the safety of workflows and inspection routes

• Aspect 6: Separate the accuracy of survey data from the role of on-site verification

• Operational points to watch for in drone surveys on steep slopes

• Summary

Why drone surveying is necessary on steep slopes

In steeply sloped areas of solar power plants, on-site inspections are more difficult than on flat land. Maps and design drawings alone may not capture the slope’s subtle undulations, steps, boundaries between cut and fill, existing waterways, the condition and distribution of remaining trees, or the directions in which soil and debris are likely to flow. Walking the site to verify conditions is also important, but on steep slopes movement is limited and securing safe footing can take time. For this reason, a gap can arise between the area that site staff can observe and the areas across the entire property where problems are actually likely to occur.

Using drone surveying, you can continuously inspect steep slopes from above. It becomes easier to organize the overall shape of the slope, gullies, ridge-like elevations, low areas where drainage tends to collect, and the spatial relationships of flat areas and cut-and-fill slopes created by development as areal data. By combining not only photographs but also elevation data, it is easier to examine where water is likely to flow, which areas have large elevation differences, and which paths are likely to become steep. However, accuracy and the way features appear vary with shooting conditions, analysis methods, the placement of ground control points and check points, and vegetation conditions, so verification according to the intended purpose is necessary.

Also, on steep slopes, post-construction operation and maintenance are important. A solar power plant is not finished once installed; ongoing tasks such as weeding, inspections, checking drainage facilities, checking mounting structures and areas around foundations, and patrols after heavy rain are continuously required. If the effects of the slope are not sufficiently considered during the planning stage, there may be locations that are difficult to inspect after completion, rainwater may flow in unexpected directions, and managing vegetation and soil can become difficult. Drone surveying is also effective because it makes it easy to compare conditions before, during, and after construction and to keep a record of site changes.

However, drone surveying on steep slopes is not sufficient if you simply take photos from the air. The appearance of a slope changes with the viewing angle, and in vegetated areas the ground surface may not be visible. Even when the flight altitude appears constant, the distance to the ground can vary by location, which can affect photo overlap and resolution. For that reason, it is essential to decide in advance from which perspective you will analyze the data and, when necessary, combine it with site inspections and ground surveying.

Consideration 1 Understand areas with steep slopes and terrain changes

The first consideration when inspecting steep terrain with drone surveying is to identify which areas are steep and where the topography changes significantly. A solar power plant site may appear as a single contiguous parcel on drawings, but in reality it can include gentle slopes, steep slopes, steps, level areas, and valley-like depressions. Treating these as a single unit can easily reduce the accuracy of layout and construction planning.

By using photos and elevation data obtained from drone surveying, you can spatially organize differences in slope aspect and gradient. For example, even within the same site, a south-facing slope and slopes that tilt in an east–west direction will require different panel layouts, solar exposure conditions, and drainage flow patterns. Also, slopes that appear continuous can contain small steps or traces of past earthworks. Such change points affect the installation of racking foundations, access for heavy equipment, drainage planning, and the design of inspection walkways.

In steep terrain, it is important not to judge by the average gradient alone. Even if the overall site average appears gentle, a locally extremely steep spot can become a constraint on construction. Conversely, land described as steep may include relatively manageable areas, such as ridge lines or pre-graded flat platforms. Using drone surveying to subdivide and inspect the slope in detail makes it easier to identify usable areas, areas requiring caution, and areas that need separate countermeasures.

When conducting an inspection, it is important to consider the slope’s direction, the degree of the gradient, the location of any steps or ledges, the condition of the ground surface, and the relationship with existing structures together as a whole. In a solar power plant, not only the area where panels will be installed but also equipment locations such as power conditioners, maintenance access routes, collector routes, drainage facilities, and fence positions should be considered. Examining only a part of a steep slope in detail will not lead to practical results if its relationship with the surrounding area is not understood.

Also, understanding changes in topography is useful not only before construction but also for post-construction comparisons. When the shape of slopes changes due to land development, excavation, embankment, or pathway maintenance, the flow of rainwater and the movement of sediment also change. If you retain pre-construction drone survey data, you can compare it to post-construction conditions to see which areas have changed and more easily identify locations that require attention for maintenance.

Aspect 2: Confirm the impact on panel layout and mounting structure planning

For solar power plants on steep slopes, terrain has a major impact on panel layout and racking plans. On flat land it can be easier to plan with consistent row spacing and similar racking conditions, but on steep slopes the slope orientation, elevation differences, and surface irregularities change the areas that can be used for installation and the construction methods. Drone surveying helps to grasp the current site conditions that form the basis for these layout considerations.

First, what you should check is the relationship between the direction in which the panels are arranged and the aspect of the slope. In places where the slope is steep, the height difference between panel rows increases, affecting how shadows are cast and how maintenance access paths are laid out. If adjacent rows differ greatly in height, even if the apparent spacing seems sufficient, they can be more susceptible to shading depending on the season and time of day. Using drone surveying to understand the terrain makes it easier to determine which areas require more careful consideration of inter-row spacing.

Next, it is necessary to confirm the constructability of the racking foundations. On steep slopes, elevation differences between foundation locations can be large, making it difficult for pile and foundation installation conditions to be uniform. In places where the ground surface is hard, soft, contains boulders, or may have been previously backfilled, construction exactly as shown on the drawings may not be possible. While drone surveys alone cannot determine ground strength, checking the slope shape and surface irregularities makes it easier to narrow down the areas that require additional investigation.

Also, on steeply sloped sites, prioritizing only the efficiency of panel layout can make maintenance difficult. Placing panels deep into steep slopes to increase the number of installations can make it hard to secure access routes for inspections, weeding, and repairs. Because solar power plants are facilities operated over the long term, you need to consider not only construction but also whether people can safely approach during operation, whether materials can be transported, and whether checks can be carried out in emergencies. Aerial surveys with drones allow an overview of the relationship between slopes and equipment layout, making it easier to review impractical arrangements at an early stage.

Furthermore, on steep slopes, shadow and reflection conditions from the surrounding terrain should also be checked. If ridgelines, trees, slope faces, or existing structures are nearby, some panels may be shaded depending on the time of day. You cannot accurately determine power output from drone survey photos alone, but it is possible to grasp the positional relationships of site features that could cause shading and use that information to inform separate simulations or on-site inspections.

What matters from this perspective is not to let drone survey results end up merely as site photos. By overlaying and verifying panel layout proposals, racking plans, foundation locations, access route plans, and drainage plans, you can reduce the discrepancies between design and the site that commonly occur on steep slopes. In particular, using terrain data early in the planning stage makes it easier to lower the risk of having to significantly revise layouts later.

Perspective 3: Identify drainage routes and areas where rainwater concentrates

What is particularly important at solar power plants on steep slopes is the flow of rainwater. On slopes, rainwater tends to collect toward lower areas and its flow velocity tends to increase, so if drainage planning is not thoroughly considered, problems such as washout of access paths, sediment runoff around foundations, degradation of slope faces, and sediment accumulation in drainage channels may occur. Drone surveys are effective for checking elevation differences and terrain depressions across the entire site and for identifying locations where rainwater is likely to concentrate.

When assessing drainage routes, it's important to check not only existing channels and ditches but also the flows created by the terrain itself. On steep slopes, even without clear channels, rainwater can run along slight gullies on the slope, footpaths, or deposits of sediment. In aerial images, variations in vegetation density, bare-soil streaks, differences in soil color, and traces of gravel flow can provide clues to areas where water is likely to travel.

Using elevation data together makes it easier to assess which direction rainwater is likely to flow. It does not simply run from higher to lower ground; flow can be altered by engineered level areas, pathways, rows of mounting racks, fence lines, and existing drainage facilities. In completed solar power plants, rainwater that falls on panel surfaces can also concentrate in specific locations. On steep slopes, such concentration can lead to slope erosion, so care is needed at the planning stage.

When using drone surveying to assess drainage, it's important not to rely solely on data collected in fair weather. Images taken on clear days make the typical terrain and facility layout easy to see, but they do not show actual rainwater flow. It can be useful to check immediately after rain or when signs of past runoff remain. However, you should not attempt to fly in bad weather or in situations where safety is a concern. Prioritize on-site safety, and make assessments in combination with ground-level photos, patrol records, and drainage equipment inspection results as needed.

Understanding drainage routes is relevant not only to pre-construction design but also to maintenance and management planning. On steep slopes, fallen leaves, sediment, and vegetation can easily enter drainage channels and reduce their capacity. Regular aerial checks with drones make it easier to determine whether sediment is accumulating around drainage facilities, whether rainwater is flowing to unexpected locations, or whether new streaks of erosion are appearing on slopes.

Also, downstream impacts must not be overlooked. Even if it appears that rainwater is being managed within the site of a solar power plant, if flows concentrate onto off-site roads, farmland, forestland, residential areas, or waterways, it may become necessary to respond to neighboring stakeholders. Reviewing the drone survey results up to the site boundary and clarifying the direction of the downstream flow and its connection points will provide practical reassurance.

Consideration 4: Check risks of slopes and areas prone to collapse

On steep slopes, checking slope faces and areas prone to collapse is essential. At solar power plants, the site may include cut slopes and fill slopes created by land development, existing natural slopes, and slopes beside maintenance paths. If these conditions are not fully understood, they can impede ensuring safety during construction and inspections during operation.

In drone surveying, you can check the overall shape of a slope and changes on its surface from above. Even on steep slopes that are difficult for people to approach, viewing from above makes it easier to identify areas of exposed ground, zones with sparse vegetation, streaks that look like cracks, locations where sediment has accumulated, and spots that may indicate past failures. However, you cannot determine the risk of collapse from images alone. Information obtained by drone surveying should be treated as an initial check to be followed by specialized on-site confirmation and any necessary investigations.

When inspecting slope faces on steep terrain, it is important to view the upper slope, the mid-slope, and the lower slope separately. If drainage concentrates at the upper slope, water can enter there and cause sediment to be washed out at the bottom. If there are small steps or minor collapses on the mid-slope, they can expand over time. If sediment has accumulated at the lower slope, you need to consider the possibility that it flowed down from above. By surveying the entire slope with a drone, it becomes easier to understand these upstream–downstream relationships.

Also, attention must be paid to the placement of equipment around slopes. If mounting racks, foundations, cable routes, fences, access walkways, or drainage facilities are close to a slope, slope deformation may affect the equipment. For example, a walkway installed at the bottom of a slope could become blocked by soil and debris, debris could accumulate along a fence, or the ground around a foundation could be washed away. Confirming the distance between equipment and the slope with drone surveying makes it easier to determine inspection frequency and repair priorities.

In existing solar power plants, comparing past photographs and survey data is effective. By comparing the condition immediately after construction with the current condition, you can check whether slope vegetation has changed, whether soil erosion has progressed, and whether drainage flows have altered. On steep slopes, it is important to notice changes while they are still small. Rather than responding after a major collapse occurs, it is easier to reduce site-management burdens by considering measures such as drainage, vegetation, access paths, and slope protection when signs are first detected.

When checking slope risks, it is also important to keep drone survey data in a format that is easy to share among stakeholders. Rather than verbally saying "the north slope is rough," showing it on an orthophoto or on a map that indicates the slope's location makes it easier to align the understanding of design, construction, and maintenance teams. Visualization with data is particularly helpful on steep slopes, where the gap in understanding between those who have seen the site and those who have not tends to be larger.

Perspective 5 Organize the safety of work flow lines and inspection routes

At solar power plants on steep slopes, it is important not only to verify whether equipment can be installed but also to confirm that people can work safely. During construction, material delivery, foundation work, racking assembly, panel installation, and wiring are required, and after operations begin there are inspections, cleaning, weeding, repairs, and emergency checks. On steep slopes, these work routes are easily constrained, and impractical plans can lead to accidents and reduced work efficiency.

Using drone surveying, you can get an overview of potential pathways across the entire site and areas that are difficult to traverse. When walking the site, it's easy to focus on the slopes and pathway conditions directly in front of you, making it hard to judge which routes are most reasonable for the site as a whole. By using aerial imagery and elevation data, you can more easily organize the distances from the entrance to each facility, elevation differences, the continuity of pathways, and areas unsuitable for vehicle or pedestrian movement.

When assessing work routes, it is important not to judge solely by the shortest distance. On steep slopes, even a short distance with a steep grade can significantly increase the workload. You also need to consider situations such as carrying materials, ground becoming slippery after rain, or tall grass reducing visibility. Use drone survey results to identify slopes and terrain changes, and classify routes into those that can be walked without difficulty, those to avoid during inspections, and those that require supplementary safety measures.

The safety of inspection routes directly affects the quality of post-completion maintenance. Locations that are difficult to inspect tend to be checked less frequently, which can delay the detection of abnormalities. When equipment is located on the far side of steep slopes or close to slope faces, it is necessary to confirm in advance whether access paths that allow inspectors to approach safely are secured, whether the paths are prone to damage from rainwater, and whether vegetation will obstruct passage. By surveying the whole area with drones, it becomes easier to find places where inspection routes are interrupted or where crossing a slope is unavoidable.

You also need to plan emergency access routes. For example, after heavy rain when you want to inspect drainage facilities, after strong winds when you want to check around panels, or when you need to respond to equipment abnormalities, you may have to enter the site under worse-than-normal conditions. On steep slopes, routes that are normally passable can become more dangerous due to mud, fallen branches, earth and debris, and overgrown vegetation. If you set up a system to perform initial remote inspections using drone surveying, it will be easier to identify hazardous areas before entering the site.

When organizing work flow routes, it is important to consider the construction phase and the operational phase separately. During construction, routes for heavy machinery and material delivery are required, whereas in the operational phase the focus is on inspection routes that people can walk along safely. If temporary access paths are created solely for construction convenience but are difficult to use for post-completion management, they will leave a long-term burden. Based on topographic information obtained from drone surveying, examining short-term constructability and long-term maintainability simultaneously is especially important on steep slopes.

Perspective 6: Separate the accuracy of survey data from the role of on-site verification

When using drone surveying to assess steep slopes, it is important to clearly define how far the collected data can be relied upon and at what point on-site verification or alternative surveying becomes necessary. While drone surveying can efficiently capture large areas, vegetation, shadows, shooting conditions, the placement of control points, and abrupt changes in terrain can affect how results appear and their accuracy. In particular, steep slopes require more careful operations than flat ground.

First, there are different types of data obtainable from drone surveying. These include photographic data for confirming current conditions, orthoimages aligned to position, three-dimensional data that include elevation, and data representing terrain and structures as point clouds. What can be determined varies depending on which data you use. While photographs are suitable for checking what is visible, they do not reveal ground strength or subsurface conditions. Even if elevation data can confirm differences in height, it does not necessarily accurately represent the ground surface when it is covered by vegetation.

On steep terrain, the placement of reference points and verification points is also important. If reference points are concentrated in only part of the site, it can become difficult to verify positional accuracy on steep slopes or in recessed areas. Because the locations where they can be installed safely are limited, it is necessary to plan in advance where to place reference points and which points will be used to check the results. When using survey results for design or construction decisions, it is important to choose verification methods appropriate to the intended use.

Also, on steep slopes the effects of imaging conditions become more significant. Depending on the orientation of the slope, some areas may be strongly shadowed, making it difficult to identify the ground surface and structures. In areas with many trees or grasses, what is visible from above may be the vegetation surface rather than the actual ground. The appearance of the same location can change between periods when grass is tall and when it is short. For this reason, it is important to record survey results for steep slopes together with the time of capture and the site conditions.

When using the results of drone surveying in practice, it is necessary to differentiate the approach to accuracy according to the purpose. If the objective is to grasp the overview of a slope in the early planning stage, the ability to efficiently view a wide area is prioritized. On the other hand, when used for determining foundation positions, checking near boundaries, strict control of as-built conditions, or making structural safety judgments, more careful verification is required. By combining ground surveying, on-site inspections, and specialist investigations as needed, you can leverage the strengths of drone surveying while reducing the risk of incorrect judgments.

Clearly defining the division of responsibilities for data makes it easier to prevent misunderstandings among stakeholders. Even if an image from a drone survey looks "fine," if it is unclear whether that judgment is for design verification, construction verification, or safety verification, interpretations can diverge in later stages. On steep slopes, a single oversight can lead to significant rework, so it is important to decide in advance what you require from the survey results and to clearly document the items to be checked.

Operational Points to Be Aware of When Conducting Drone Surveys on Steep Slopes

When conducting drone surveys on steep slopes at solar power plants, you need to organize not only the inspection points but also operational precautions. Steep slopes feature complex terrain and require consideration of flight altitude, line of sight, wind effects, takeoff and landing locations, and safety management. Flying without sufficiently checking site conditions not only prevents you from obtaining the necessary data but also increases operational risks.

First, check the conditions around the site before flight. On steep slopes, the height above the ground can vary greatly from place to place, making it difficult to keep the distance to the subject consistent. If altitude settings are inappropriate, photo resolution and overlap may be insufficient, and adequate results may not be achieved in post-processing. When there is a large altitude difference between the upper and lower parts of a slope, it is important not to treat the flight plan uniformly, but to consider shooting conditions for each target area.

Next, it is necessary to secure the safety of takeoff and landing areas. On steep slopes, flat open areas may be limited. Locations where the ground is sloped, the grass is long, or gravel and loose soil are unstable can cause the aircraft to become unstable during takeoff and landing. Select a place where work can be performed safely in advance, and ensure operators do not have to operate in awkward postures. If necessary, consider operating from a safe location outside the site.

Be aware of wind effects. In mountainous areas and on slopes, terrain can easily alter wind flow, and winds that feel weak at ground level can be stronger aloft or in valley channels. Pay particular attention to wind direction and turbulence on the upper parts of steep slopes, near ridgelines, and in valley-like terrain. Conducting surveys under conditions that do not permit safe flight can affect not only aircraft stability but also image quality. It is important to assess the weather, wind, sunlight, and on-site safety, and to be prepared to adjust the schedule if necessary.

The choice of shooting period also affects the outcomes. When vegetation is dense, the ground surface and traces of drainage become difficult to see. Conversely, if you want to assess the condition of vegetation, photographing during periods when plants are present is effective. On steep slopes, it may be appropriate to schedule different shooting times depending on the objective. If land development planning or topographic assessment is prioritized, periods when the ground surface is more visible may be preferable. If maintenance or weed-control planning is emphasized, data from periods that show how vegetation grows will be useful.

Data management is also important. In drone surveys of steep slopes, multiple sets of imagery tend to be generated—before, during, and after construction, and during inspections. If you do not record the shooting date, coverage, purpose, weather, reference points used, processing conditions, and items confirmed on site, it becomes difficult to judge when comparing data later. Organizing file names and storage locations so that all relevant personnel can view the same data and make decisions has great practical importance.

Furthermore, it is important to treat drone surveying as part of safety management. If hazardous spots on steep slopes can be preliminarily checked from above before people directly inspect them, it becomes easier to narrow down the areas that need to be entered on site. However, just because something was seen by drone does not mean on-site confirmation is unnecessary. By using drones to obtain a broad overview, confirming necessary locations on the ground, and supplementing the information needed for decision-making, you can create a workflow that makes surveying practical while reducing the risks associated with steep slopes.

Summary

When inspecting the steep slopes of a solar power plant with drone surveying, it is important not just to take aerial photos but to organize findings from multiple perspectives: topography, equipment layout, drainage, slope faces, work access routes, and survey accuracy. On steep slopes, small elevation differences and water flows that are unlikely to be problematic on flat land can significantly affect constructability and maintainability. If you can grasp the entire site early on in a planar manner, you can reduce redesigns and rework during design and construction and more easily create a plant that is simple to inspect.

From Perspective 1, it is important to identify areas of steep slope and changes in terrain. Rather than judging only by the average gradient, separating steep sections, gentle sections, steps, gullies, and flat areas allows you to clarify areas that can be used and areas that require caution.

From Perspective 2, confirm the impact on panel layout and racking plans. Since slope orientation and elevation differences affect row spacing, shading, foundation construction, and ease of inspection, decisions must consider not only layout efficiency but also long-term operation.

In Viewpoint 3, assess drainage paths and points where rainwater concentrates. On steep slopes, rainwater tends to accumulate and can lead to scouring of pathways and sediment runoff. Use drone surveying to confirm the relationship between terrain and drainage facilities and to organize this information including the downstream end. In Viewpoint 4, check the risks to slopes and areas prone to collapse. Do not determine safety from images alone; if you find signs of deterioration, follow up with on-site inspections and professional investigations.

In Aspect 5, we organize the safety of work flow and inspection routes. On steep slopes, it is important not only to consider material delivery during construction but also the ease of inspection, weeding, and repairs during operation. By using drone surveying to obtain an overall view, it becomes easier to identify impractical routes and locations that are difficult to inspect. In Aspect 6, we separate the accuracy of survey data from the role of on-site verification. While drone surveying is strong for capturing wide areas, it is affected by vegetation, shadows, the placement of control points, and shooting conditions, so it is important to combine it with ground checks or other surveys depending on the purpose.

At solar power plants on steep slopes, data must be used not only for pre-construction planning but also with an eye toward post-construction operation and maintenance. By using drone surveying to clearly record site conditions and share them among stakeholders, variability in decision-making can be reduced and long-term management quality more easily improved. To streamline steep-slope inspections and leverage topographic data in the planning and management of solar power plants, it is important to advance flight planning, control-point management, on-site verification, and data organization as an integrated process.