5 Checks to Carry Out Slope Verification of Solar Power Plants Using Drone Surveys

At solar power plants, it is important to continuously grasp the terrain conditions of the entire site, not just the power generation equipment itself. In particular, slope affects site development planning, drainage planning, the safety of access routes, slope stability, rainwater flow, and the susceptibility to sediment inflow and erosion, making it fundamental information for on-site management. Even in areas that appear to show little visible change, cumulative local variations in gradient and biased watercourses can lead to scouring around mounting racks, muddy access routes, slope deformation, and sediment accumulation in drainage facilities.

Conventional on-site inspections mainly involved personnel walking the site to check steep areas and locations prone to water accumulation. However, solar power plants cover large sites and include many areas to inspect—panel rows, maintenance paths, slopes, balancing ponds, drainage channels, and along fences—so visual inspection alone can make it difficult to grasp the overall picture. One practical and easy-to-use method is drone surveying. By using photos taken from above and the point clouds, digital elevation models, and orthophotos generated from them, it becomes easier to organize and analyze the site’s slopes across the surface.

This article explains five checks to keep in mind when conducting slope verification of solar power plants using drone surveying, aimed at practitioners searching for information on "solar power plant drone surveying". It organizes not only how to derive the tilt angle, but also which reference points to use, how to divide the area for inspection, and how to connect the results to on-site management.

Table of Contents

• Why slope verification is important at solar power plants

• Check 1: Align the on-site reference plane and coordinate conditions first

• Check 2: View the slopes of the area around the panels and the maintenance aisle separately

• Check 3: Read changes in slope and drainage direction

• Check 4: Confirm the roughness and missing data of the point cloud and elevation model

• Check 5: Organize risks based on differences before and after construction and before and after disasters

• Precautions when verifying slopes in drone surveying

• Summary

Why slope verification is important for solar power plants

Checking the slope of a solar power plant site is not simply a matter of seeing whether the site is tilted. The local slope is fundamental information for determining where rainwater will flow, where sediment is likely to accumulate, whether maintenance vehicles and workers can move safely, and whether slopes or developed surfaces are prone to deformation. In particular, on sites in mountainous areas, engineered fill or development sites, land converted from agricultural use, landfill sites, or valley topographies, even if the site appears generally gentle, there may be localized steep gradients or places where water concentrates.

In solar power plants, the area over which panels are installed is extensive, and terrain conditions can vary greatly even within the same site. The area near the entrance may be flat, but slopes can increase toward the back. While the ground between rows of panels may be gentle, there can be sudden changes in elevation along fences or near embankments. Also, even if the site was graded according to design during construction, after commissioning the surface shape can gradually change due to rainfall, uneven drainage, changes in grassland, sediment inflow, or ruts caused by vehicle traffic.

If you don't understand the slopes, you may misidentify the causes of problems occurring on site. For example, when puddles form on an access walkway, it may not be just a clogged drain; topographical factors can also be involved, such as an insufficient cross slope of the walkway, water tending to collect from surrounding areas, or flow concentrating from beneath the rows of panels. When soil is being washed away on part of a slope face, you need to consider not only the slope's gradient itself but also the direction of inflow from above and the surrounding microtopography.

Using drone surveying makes it easier to check slopes over an area rather than at individual points. Instead of measuring only a few points on site, photographing the entire site and understanding elevation differences and slope distribution makes it easier to clarify the relationship between the problem area and the surrounding terrain. In particular, when sharing the situation among clients, management companies, contractors, and maintenance personnel, explanations using aerial images and elevation models convey the situation more effectively than explanations that rely solely on on-site impressions.

However, data obtained from drone surveying can vary in accuracy and interpretation depending on imaging conditions, ground control and calibration conditions, processing methods, vegetation conditions, sunlight conditions, and how the ground surface appears. Therefore, for slope verification it is important not only to produce survey data but also to decide in advance which area will be examined and for what purpose. Whether the purpose of slope verification is design confirmation, post-disaster inspection, consideration of drainage improvements, or safety checks for maintenance paths will change which locations should be examined and how the deliverables are used.

Check 1 First align the site's reference plane and coordinate conditions

When carrying out slope verification with drone surveying, the first thing to confirm is which reference you will use to assess heights and inclinations. Because slope is calculated from elevation differences and distances, if the elevation datum or coordinate conditions are ambiguous, judgments can vary when comparing data later. In particular, when comparing survey data from different times—such as before and after construction, before and after repairs, or before and after disasters—if the references are not aligned, it becomes difficult to determine whether the observed differences are actual terrain changes or discrepancies introduced during data creation.

In solar power plants, past site-formation drawings, as‑built drawings, design drawings, drainage plans, and existing survey results may be retained. Before conducting a drone survey, checking the coordinate system and the way elevations are handled in these materials will make post-survey processing easier. When overlaying existing drawings with drone survey results, you need to confirm not only the horizontal positions but also how elevation is treated. Comparing them without accounting for differences in elevation reference can affect judgments about slopes and level changes.

When placing reference points or known points on site, a layout that minimizes bias across the entire site is desirable. If reference points are concentrated only near the entrance, alignment for the far side or near slopes may remain uncertain. In a large power plant, it is important to distribute reference points according to the areas you want to verify—for example, at the four corners of the site, near the center, at locations with elevation differences, and around important drainage facilities. Of course, the places where they can be installed are limited by site conditions, so you should choose locations that are safely accessible, clearly visible in photographs, and easy to check later.

When checking slopes, it is often necessary to look not only at the overall average inclination but also at local variations in slope. Therefore, it is important to decide in advance on the concept of a reference plane. For example, whether you are looking at the overall trend across the site, the tilt differences between panel rows, the longitudinal gradient of maintenance walkways, or the slope of embankments will change how cross-sections should be taken and the ranges used for comparison. Calculating slope with a reference that does not match the on-site purpose will produce deliverables that are difficult to use for practical decision-making.

Also, when checking slope, it is necessary to distinguish whether the target is the ground surface or whether to include the upper surfaces of structures. In solar power plants, images and point clouds include panels, racking, fences, drainage structures, vegetation, and so on. If the purpose of slope verification is to assess the condition of the ground, using height information that includes panel surfaces and racking as-is may produce results that differ from the ground surface slope. During processing, define the extent to be treated as ground surface and do your best to minimize the influence of unnecessary elements.

If you standardize the reference conditions first, explanations in subsequent processes will be more consistent. If you can explain to the client and stakeholders "what range this slope was calculated from," "whether it is being compared under the same conditions as past data," and "whether the height reference is aligned," the scope of use for the surveying results becomes clear. Drone surveying can produce results that are easy to understand visually, but if the explanation of the references is insufficient, judgments may be made based solely on the impression of the images. When checking slopes, organizing the reference conditions is as important as producing easy-to-read deliverables.

Check 2: View the slopes around the panels and the access aisles separately

When verifying slopes at a solar power plant, it is insufficient to view the entire site as a single plane. In practice, the meaning of slope changes depending on functionally different areas such as around the panels, maintenance access routes, along fences, embankments, and around drainage facilities. In particular, the areas around the panels and the maintenance access routes should be assessed separately, because the inspection perspectives differ even within the same plant.

Around the panels, check the mounting condition of the racking, the ground around the foundations, drainage between panel rows, the flow of rainwater, and the occurrence of scour and sedimentation. Areas beneath panel rows are often shaded, so vegetation growth and how the surface dries can differ from the surrounding ground. Also, rainwater falling from the panels can concentrate at certain spots, forming narrow water channels under or between the rows. On steep slopes, these channels can extend downstream, leading to soil movement and surface erosion.

For maintenance access paths, it is important that workers and maintenance vehicles can move safely. If the longitudinal slope is steep, it can become slippery after rain or when surfaces freeze. If the transverse slope is insufficient, rainwater can pond on the path, making mud and ruts more likely. Conversely, if the transverse slope is too great, it can affect vehicle stability and pedestrian safety. Therefore, for maintenance access paths it is necessary to consider slope not just as a simple elevation difference but with awareness of the practical safety implications when the path is in use.

When using elevation models created by drone surveys, if you view the area around panels and the maintenance paths with the same color coding or evaluation criteria, you may overlook important changes. Around the panels, fine water channels and localized scouring are important, whereas on maintenance paths continuous slopes and obstacles to passage are important. Instead of evaluating areas with different purposes by a single standard, dividing the items to check by area will provide information that leads to on-site action.

Also, whether the arrangement of the panel rows matches the orientation of the terrain is important. How the panel rows are arranged relative to the slope changes the way rainwater flows. When rows are aligned along the slope versus when they run across it, the places where water collects between rows and the routes by which it flows into drainage facilities differ. In drone survey outputs, checking the panel row layout on orthophotos and overlaying the terrain orientation with an elevation model or a slope map makes it easier to clarify the relationship with the flow direction.

When conducting on-site inspections, it is important not only to record the slope around the panels and the slope of the maintenance walkways separately, but also to examine the points where the two connect. If water is flowing from the maintenance walkway toward the panel rows, or if sediment is moving from the panel rows onto the walkway, subtle differences in elevation or changes in gradient at the boundary may be the cause. Such locations can be difficult to discern from the ground, but combining aerial imagery with elevation data makes it easier to understand the direction of flow and the paths of sediment movement.

To put the results of slope checks to practical use, it is not enough to describe them simply as "steep" or "gentle"; you need to organize where the slope is, in which direction, how severe it is, and what it might affect. For example, around panels consider ground conditions and drainage, and for maintenance walkways consider accessibility and safety. By separating the purposes of the checks in this way, inspection reports and improvement proposals become more specific.

Check 3: Read changes in slope and drainage direction

When checking slopes at a solar power plant, particular attention should be paid to slope faces and drainage directions. The plant site may include slope faces formed by earthworks, slopes that make use of existing terrain, valley topography receiving inflow from surrounding areas, and constructed surfaces that channel water toward drainage channels. In such locations, it is necessary to check not only the steepness of the slope but also the direction in which water will flow and where it will concentrate.

On slopes, pay attention to steep sections, rough surface areas, sparsely vegetated areas, visible rainwater streaks, and places where sediment has accumulated at the base. Aerial photography from drone surveys makes it easier to grasp the overall shape of the slope and its connections to the surroundings. Even slopes that are only partially visible from the ground become clearer in aerial images, making the positional relationships between the slope crest, slope toe, drainage channels, and nearby pathways easier to understand. Using an elevation model also helps organize which parts of the slope have changes in gradient.

When assessing drainage direction, simply confirming the location of drainage channels is not sufficient. Rainwater flows not only along the designed drainage routes but also following the actual microtopography. If there are slight undulations on the developed surface, rainwater can collect in unintended locations. Under rows of panels, at the ends of access aisles, along fences, at the top of slopes, or just before drains, spots where water tends to concentrate can occur. When checking slope, it is important to examine how water flows from higher to lower elevations across the surface.

Using drone survey results makes it easier to estimate drainage directions. By viewing on orthophotos the layout of on-site facilities and the state of vegetation, checking elevation differences with an elevation model, and taking cross-sections as necessary, you can clarify the directions in which rainwater is likely to flow. In particular, it is important to distinguish between areas that naturally flow toward drainage channels and areas where water is likely to pond along the way. Even if drainage channels are installed, if the surrounding surface gradients are not appropriate, water may not collect sufficiently.

By examining the slope together with the drainage direction, it becomes easier to link this to post-disaster inspections and preventive maintenance. If sediment has accumulated at the base of a slope after heavy rain, it is necessary to estimate where that sediment came from. Checking the upper slope gradient, the surface runoff direction, the extent of bare ground, and the locations of drainage facilities together makes it easier to narrow down the source. Even if you only observe sediment deposits on site, the origin can be difficult to identify, but with topographic data from drone surveys you can get a broad view of upstream conditions.

In addition, understanding changes in slopes over time is also important. Even slopes that were stable at the time they entered service can have their surface conditions altered by rainfall or vegetation management. By conducting regular drone surveys and comparing the elevations and slopes of the same area, it becomes easier to identify locations where changes have occurred. However, because vegetation height and differences in imaging conditions can affect elevation data, when assessing changes it is necessary to interpret them together with how features appear in the images and the results of on-site inspections.

It is important to organize the results of slope and drainage direction checks in a form that is easy to explain to stakeholders. For example, a statement like “part of the slope is steep” alone can make it difficult to judge the priority of responses. If you explain by linking slope angle and observed phenomena—such as “rainwater collects at the top of the slope and sediment may be accumulating near the toe of the slope” or “the cross slope of the maintenance path is insufficient, creating sections where water does not flow easily into the drainage channel”—it becomes easier to translate into on-site measures.

Check 4: Verify roughness and missing data in point clouds and elevation models

When checking slope with drone surveying, point clouds, orthophotos, elevation models, cross-sections, and so on are often used as deliverables. However, these datasets do not automatically show the correct slope just by being captured. Because slope depends heavily on height information, it is important to check point clouds and elevation models for coarseness, missing data, noise, and the inclusion of non-ground elements.

At solar power plants, because panels cover large areas, there are locations where the ground surface is difficult to capture in images. Under the panels, in areas shaded by mounting racks, where grass is overgrown, or where trees and weeds are tall, point clouds representing the ground surface may not be obtained sufficiently. If slope is calculated in such places, the result may be based on the tops of grass or parts of structures rather than the actual ground. When checking slope, you must first confirm that the point cloud for the target area appropriately represents the ground surface.

The resolution and processing method of the elevation model also affect how slopes appear. If produced with a fine grid, local bumps and hollows become easier to see, but the model is also more susceptible to noise. If produced with a coarse grid, the overall trend is easier to discern, but you may miss small flow paths or scours. In practice, it is important to choose the scale you want to examine according to the purpose. The level of detail required differs when looking at drainage directions or the overall trend of a slope versus inspecting local scour around a support foundation.

Where data are missing, shapes interpolated from surrounding points may be displayed. Even if they look like a smooth surface, there may actually not be enough measurement points. Judging slope in such areas can differ from the actual on-site conditions. It is especially important to check data reliability under panels, in deep shadows, in areas with dense vegetation, on surfaces with little reflectance, and in areas with repetitive patterns. When reviewing deliverables, do not rely solely on neat color coding; also check point cloud density and how the images appear.

When creating slope maps and hypsometric (elevation color) maps, careful attention must be paid to color settings. If the color classification ranges are too wide, important slope differences can become hard to see. Conversely, if the ranges are made too fine, minute changes that are not practically significant can be exaggerated, making interpretation difficult. Tailoring the maps to the purpose of slope verification—separating maps that show overall trends from maps that examine key locations in detail—makes it easier to explain the results to stakeholders.

Also, drone survey results lead to safer decisions when combined with on-the-ground verification. If aerial data shows a location with a steep slope, it is advisable to confirm with site photographs and ground checks whether the cause is the actual terrain, vegetation or structures, or processing noise. In particular, when using drone survey results to make decisions about repair work or drainage improvements, do not draw conclusions based solely on the drone survey results; handle them together with on-site inspections for safety.

Quality checking of point clouds and elevation models affects the credibility of deliverables. When clients or stakeholders ask, "Does this slope really represent the ground?", being able to explain acquisition conditions, control points, point cloud density, areas of missing data, and the policy for excluding non-ground surfaces clarifies what the deliverable can be used for. Drone surveying is a convenient method, but it is not a panacea. By clarifying how far the data can be trusted and where on-site verification is required, slope checks become more practical for operational use.

Check 5 Organize risks by the differences between pre- and post-construction and pre- and post-disaster conditions

Slope verification is not something you do once and finish; it becomes much more meaningful when you compare data from different times. At solar power plants, there are multiple occasions to check the terrain, such as before and after construction, at completion, during routine inspections, after heavy rain or typhoons, and before and after repair work. By conducting drone surveys under conditions as similar as possible, changes in slope and elevation can be captured, making it easier to organize locations that pose risks.

Comparing pre- and post-construction conditions allows you to confirm how the designed graded surfaces and drainage directions are reflected on site. Even if the design drawings appear to provide sufficient drainage gradients, local irregularities can remain on the actual constructed surface. Conducting drone surveys after grading and checking elevation models and cross sections makes it easier to identify locations where water is likely to pool and where gradients differ from the plan.

Comparing before and after a disaster allows you to spatially confirm whether terrain changes have occurred. After heavy rain, the surface of slopes may be washed away, sediment may accumulate around drainage ditches, and ruts or scouring may form on maintenance paths. Simply walking the site can make it difficult to grasp the extent of damage and its relationship to upstream areas. If you have drone survey results from before the disaster, comparing them with post-disaster results lets you determine where elevations and slopes have changed.

When examining differences, it's important not to rely too heavily on the numerical values alone. Different surveying conditions can produce differences even where no actual change has occurred. Grass height, the timing of image capture, lighting conditions, the arrangement of control points, and differences in processing methods can all influence the observed differences. Therefore, locations that appear to have changed on a difference map should be checked against orthoimages, on-site photographs, and the condition of the point cloud, and be carefully evaluated to determine whether they represent real terrain change.

In risk assessment, it is important to consider not only the magnitude of changes but also their impact on surrounding facilities. For example, even with the same degree of slope change, the level of importance varies if it is near a panel foundation, just before a drainage ditch, at the upper part of a slope, around the perimeter of a fence, or on a steep section of a maintenance walkway. Locations that could affect power generation equipment, traffic routes, drainage facilities, or adjacent land should be prioritized for inspection. The results of drone surveys can show areas of change on a map, making it easier to organize the priority of on-site responses.

Also, by conducting periodic slope checks, you can notice not only sudden abnormalities but also gradual changes. Even if each change is small, when you line up multiple sets of data you may see trends such as the drainage path at the same location deepening, sediment accumulating at the toe of the slope, or a section of a walkway beginning to settle. If such trends are identified early, it becomes easier to consider countermeasures before major repairs become necessary.

When reporting differences, it is important to state the verification conditions as well as whether changes occurred. Organizing information about when the surveyed datasets were compared, whether the coverage area was the same, whether the reference points were the same, whether vegetation had any effect, and whether on-site verification was carried out clarifies the assumptions behind the report. Although difference maps produced by drone surveys are visually easy to understand, failing to explain differences in conditions can lead to overreaching conclusions or misunderstandings.

Precautions when proceeding with slope verification in drone surveying

To make practical use of slope verification using drone surveying, it is necessary to consider the survey plan, on-site safety, data processing, and how the deliverables will be used as an integrated whole. The first important step is to clarify the purpose of the slope verification. The required capture area and deliverables vary depending on whether it is for evaluating drainage improvements, inspecting slopes, confirming the safety of maintenance/access paths, or verifying as-built conditions after construction. If you collect imagery while the purpose is unclear, problems are likely to arise later, such as "insufficient resolution for the areas you want to see," "insufficient control points," or "weak data in the direction you want to extract cross sections."

In an imaging plan, it is important not only to capture a wide area but also to ensure that locations where you want to check slopes are adequately covered. Areas with large terrain changes or that are prone to problems—such as between panel rows, access/maintenance paths, embankments, drainage channels, and along fences—should be photographed with attention to image overlap and lines of sight. During times when shadows are heavy or when vegetation is tall and the ground surface is difficult to see, it may be hard to acquire ground-surface data. If possible, checking local mowing/vegetation status, weather, and sunlight conditions before shooting will help stabilize the quality of the deliverables.

Regarding on-site safety, in addition to ensuring the safety of the drone flight itself, attention must also be paid to equipment specific to solar power plants. Confirm the locations and relationships of panels, mounting structures, electrical equipment, power transmission equipment, fences, work vehicles, surrounding roads, and adjacent properties, and carefully determine the flight area and takeoff/landing sites. On slopes or in muddy areas, pay attention to the movement of ground personnel. Even when surveying to check slope angles, rather than forcibly entering hazardous areas, it is more practical to use a drone to obtain a wide-area overview and to secure safety and perform ground verification only where necessary.

In data processing, being conscious of the distinction between the ground surface and structures is indispensable. The point cloud of a solar power plant includes panel surfaces, racking, vegetation, fences, drainage structures, and so on. If the purpose of slope verification is the ground surface, it is necessary to create an elevation model that takes these influences into account. Especially under panels and in grassy areas the ground is hard to see, so indicating on deliverables where data is insufficient will make subsequent decision-making safer.

The way deliverables are compiled is also important. To make deliverables easy for practitioners to use, it is effective to combine an overview map, key areas, cross-sections, site photographs, and verification comments. An overview map alone makes it difficult to judge details, and cross-sections alone make it hard to understand relationships with the entire site. By separately indicating areas with steep slopes, areas where drainage tends to concentrate, and areas that require on-site verification, stakeholders can more easily share the same understanding.

Furthermore, the results of slope checks should not be used to make immediate repair decisions; it is important to evaluate them in stages together with on-site conditions. Even where drone surveys make a slope appear steep, the slope face may be stable and pose no problem. Conversely, even if the numerical slope values are not large, locations where rainwater concentrates and soil is being washed away should be prioritized for inspection. By comprehensively assessing slope values, how features appear in images, on-site deterioration, drainage routes, and impacts on equipment, inspections become practical and aligned with field work.

Note that when using deliverables for contract specifications, public surveying, construction inspections, insurance claims, or similar purposes, the required accuracy, surveying methods, deliverable formats, involvement of qualified personnel, and scope of on-site verification may be specified. When drone survey deliverables are to be treated as official documents, it is important to confirm the intended use and the required conditions with stakeholders in advance.

Summary

Slope verification at a solar power plant is an important task related to plant maintenance, drainage measures, slope inspections, safety checks of maintenance paths, and post-disaster inspections. Although slope is basic terrain information, it can be difficult to grasp the overall picture by walking the site alone. By utilizing drone surveying, you can comprehensively capture a large site and more easily clarify the relationships around panels, maintenance paths, slopes, and drainage facilities.

To make slope verification results usable in practice, first align the reference plane and coordinate conditions and prepare them so they can be compared with past data and drawings. Then look at the area around the panels and the maintenance walkway separately, and check the slopes according to each purpose. For slopes and drainage directions, you need to interpret not only the gradient itself but also where water will flow from and to, and where sediment is likely to accumulate.

Also, verifying the quality of point clouds and elevation models is essential. Panels, vegetation, shadows, missing data, and differences in processing methods can alter the way slopes appear. Even if the maps look clean, if you do not confirm that they accurately represent the ground surface, they become difficult to use for practical decision-making. When using differences from before and after construction or before and after disasters, it is important to organize the comparison conditions and separate actual terrain changes from discrepancies in the data.

When checking slopes at solar power plants, it is necessary not only to identify where tilting is occurring but also to organize how that tilting affects things. If you can explain the on-site issues—poor drainage, sediment inflow, slope deformation, walkway safety, and scouring around equipment—in relation to the slopes, the quality of inspection reports and improvement planning will be enhanced.

If you plan to use drone surveying of solar power plants to advance slope verification, monitoring of terrain changes, and streamlining inspection records, it is important to organize in advance the survey objectives, reference conditions, flight plan, handling of ground surface data, and the intended scope of use for deliverables. Efficiently surveying large plants and organizing slope and drainage directions into a format that can be used in practice will facilitate on-site management and communication with stakeholders.