6 items to confirm site grading irregularities at solar power plants using drone surveying

In site preparation work for solar power plants, it is necessary to ensure proper ground elevation, slope, and drainage flow to match site conditions and design requirements. However, solar power plant sites are large, and gentle elevation changes and localized unevenness can be difficult to detect from drawings or visual inspection at ground level alone. Small surface irregularities remaining after site preparation can affect drainage, racking and foundations, maintenance access paths, and inspection routes, so it is important to verify conditions at an early stage.

Unevenness refers to a condition in which the ground surface has bumps and hollows, undulations, or localized differences in elevation. In solar power plants, even slight unevenness can affect rainwater flow, muddy conditions, the drivability of access paths, and the ease of maintenance work. Drone surveying acquires wide-area images and terrain information from the air and can be utilized as a means to grasp the condition of the graded surface across an area. However, surveying results alone cannot conclusively determine the condition of the ground. It is necessary to make judgments by combining on-site inspections and construction records while checking the shooting conditions, the placement of ground control points and check points, analysis conditions, and how the ground surface appears.

This article organizes six points that field practitioners should keep in mind when using drone surveys to check for site grading irregularities at solar power plants.

Table of Contents

• Clarify the purpose of checking for earthwork irregularities.

• Determine topographic changes before and after land development.

• Check the drainage slope and the risk of standing water.

• Verify the ground elevation around the mounting frames and foundations.

• Inspect the management passages and work flow paths for surface unevenness.

• Compile as survey results that can be used for reporting and repair decisions.

• Summary

Clarify the purpose of checking for unevenness in earthworks

When checking for grading irregularities at a solar power plant, the first thing to clarify is the purpose of the inspection. If your goal is simply to find obvious bumps and dips, you can walk the site and perform a visual inspection. However, if you want to assess the construction condition of the entire plant, compare changes before and after grading, investigate the causes of poor drainage, or verify the installation conditions of mounting structures and foundations, the scope to be inspected and the required deliverables will differ.

Surface unevenness can be difficult to judge by appearance alone. Even areas that look fine in sunny weather can have low spots where water tends to pool after rain. In large power plants, not only local bumps and hollows but also gentle overall elevation differences and irregularities in slope across the site can affect operation and maintenance. An advantage of drone surveying is that it makes it easy to check such wide-area conditions across the surface. Rather than checking only points, you can view the terrain flow, the continuity of low spots, and how fills and cuts blend together, thereby increasing the information available for on-site decision making.

However, conducting drone surveys does not automatically reveal all issues with surface unevenness. If imaging conditions, the placement of ground control points or check points, the assumptions used in analysis, or the way outputs are interpreted are insufficient, evaluations of surface irregularities can be incorrect. In particular at solar power plants, weeds, crushed stone, mud, temporarily stored materials, tracks left by heavy machinery, and shadows from racking or panels affect how the ground surface appears. When viewing terrain data generated from images, it is important to confirm to what extent it represents the actual ground surface.

If you clarify the objectives, on-site verification work can be made more efficient. For example, whether the confirmation documents will be used after completion of earthworks, to check the flow of rainwater, as material for discussions with the contractor, or to review maintenance management plans will change the level of detail required in the explanations. Checking for grading irregularities is not merely a surveying task but site management aimed at making the power plant easier to operate safely. Therefore, before conducting drone surveying, it is important to organize the inspection targets, the judgment criteria, and how the deliverables will be used.

Also, in the assessment of grading irregularities, it is necessary to consider not only the design drawings but also actual site conditions. Even if the design provides the required drainage gradient, in reality the flow of water can change depending on the conditions at the drainage outlet, the locations of side ditches and catch basins, the surrounding topography, and how the access roads interface. Because solar power plants lay out panel rows, mounting structures, walkways, cable routes, and drainage facilities across a wide site, localized irregularities can affect other equipment or circulation routes. To organize these complex situations, terrain information obtained from drone surveys should be used while cross-referencing on-site photographs and design information.

Assess topographic differences before and after land development

A useful way to check for earthwork irregularities is to determine the terrain differences before and after the earthworks. If pre-construction terrain data is still available, you can compare, on an areal basis, which parts were cut and which parts were filled by the works. In solar power plants the sites are large and the original terrain is not uniform. The points to watch after development vary depending on pre-development conditions—forests, unused land, former farmland, slopes, and so on. By comparing the site before and after the earthworks, it becomes easier to identify terrain changes that are hard to notice by simply walking the site.

When checking differences, it is important not to simply look at whether levels have gone up or down, but to verify whether those changes align with the design intent and construction conditions. In embankment areas, check for places where settlement or insufficient compaction may be suspected; in cut sections, check whether they have been excavated more than planned; and near boundaries, check whether any steps remain. This is especially true for solar power plants, where rows of panels are laid out over wide areas, because irregularities in the ground surface can affect the amount of adjustment required during construction and the ease of operation and maintenance.

Using three-dimensional data and elevation information produced by drone surveying, you can color-code and inspect the terrain after land development. Because you can visually grasp trends in elevation differences, it becomes easier to share locations to check among site stakeholders. However, while color-coded displays are easy to read, the impression they give can change depending on the display range and color settings. Small differences can appear as major issues, and important differences can become less noticeable. Therefore, you should not judge based solely on color appearance; it is necessary to evaluate in conjunction with comparisons to the design, on-site verification, and the effects on drainage and construction.

When dealing with before-and-after earthworks differences, attention must also be paid to survey timing and site conditions. If the coordinate or elevation references differ between pre- and post-development datasets, differences may appear in places that have not actually changed. To make a valid comparison, the coordinate system, vertical datum, handling of control points, and analysis conditions should be aligned as much as possible. Also, if materials, heavy machinery, or temporary structures remain on site during post-development imaging, they can affect the terrain data. It is essential to confirm that the data used for comparison have been prepared under the same assumptions.

Tracking differences is useful not only for post-construction verification but also for future maintenance. By recording which areas had large fill volumes, which locations were originally low, and which direction the terrain drains, it becomes easier to infer causes when puddling or settlement occurs in the future. Even if problems are hard to see immediately after site development, surface irregularities can become noticeable during the rainy season, after snowfall, following heavy rain, or after heavy machinery has passed. Keeping pre- and post-construction data makes it easier to monitor long-term changes in the power plant's condition.

Check the drainage slope and risk of water pooling

When checking for site grading irregularities, the drainage gradient and the risk of standing water are especially important. Solar power plants often occupy large sites, and if rainwater does not drain properly, it can lead to muddy conditions, soil runoff, slope erosion, deterioration of maintenance access paths, and weakening of the ground around the mounting structures. Even if surface undulations appear minor, a series of low spots where rainwater does not drain easily can affect the overall operation and maintenance of the plant.

Drone surveying uses images captured from above to compile terrain elevation information, providing the data needed to identify low-lying areas and spots where water is likely to collect. Simply walking the site can make it difficult to perceive subtle differences in slope. Especially on large sites, you can only check the areas immediately around where people walk, so it takes time to grasp the overall drainage direction. By using drone surveying, you can more easily get an overview of elevation differences across the entire site and confirm whether the terrain encourages water to flow toward drainage outlets.

When checking drainage slopes, simply looking for low spots is not enough. Even if there are places where water collects, it may not become a major problem if drainage facilities or natural outflow routes are secured beyond them. Conversely, a slight rise can block water flow and create localized puddling. When inspecting grading irregularities, you should look at connections with the surrounding area rather than the low point itself. It is important to read topographic information to determine where water comes from, where it is headed, and where it is likely to stop.

In solar power plants, areas beneath panel rows and around mounting racks are easily overlooked during routine inspections. Due to panel shadows, weeds, crushed stone, and height differences with access paths, rainwater ponding can be difficult to detect by visual inspection alone. Combining terrain checks from drone surveys with on-site photographs taken after rain makes it easier to link locations of standing water with topographic causes. Even when relying only on survey data collected in clear weather, identifying low-lying areas, gently sloping areas, and locations with unclear drainage outlets allows you to prioritize them for subsequent inspections.

When checking drainage gradients, the interface between the graded surface and drainage facilities is also important. Even if side gutters, catch basins, sedimentation ponds, or drainage channels are installed, water may not flow as expected if the surrounding ground surface is not properly connected toward those facilities. Also, even slight steps around maintenance paths or the foundations of panel rows can interrupt water flow. Using the results of drone surveys, you can check these interface areas across the surface and narrow down the locations to focus on in the field.

When assessing the risk of puddles, it's also important not to be too definitive. Even if low-lying areas can be inferred from terrain data, actual puddles vary depending on rainfall, soil type, permeability, clogging of drainage facilities, and the condition of weeds. Therefore, in reports, rather than asserting "puddles will form," use expressions such as "areas likely to pond," "locations where confirmation of drainage direction is needed," and "areas to prioritize for inspection after rain," which makes the report more practical to use.

Confirm the ground level around the mounting racks and foundations

Uneven ground at a solar power plant affects the installation of racking and foundations. If the ground surface where the racking will be installed is significantly disturbed, the amount of adjustment required during construction increases, and the finish around the foundations can become inconsistent. While the racking itself can be adjusted in height and tilt according to the design, large irregularities in the surrounding ground can impair workability, drainage, and walking safety during inspections.

Using drone surveying, you can create documentation that verifies ground elevation over a wide area before and after mounting structure installation. If surveyed before mounting the structures, you can check whether there are any extreme elevation differences around foundation locations and whether the prepared surface is stable along the direction of the panel rows. If surveyed after mounting the structures, it can be used as documentation to check for soil heave, subsidence, or steps that would obstruct drainage under the panel rows and around the foundations. However, after the mounting structures and panels are installed, the areas where the ground surface is difficult to see from aerial photos increase. Therefore, if you want to identify grading irregularities, conducting the survey before mounting the structures will make comparison easier.

What you want to check around the racking and foundations is the flow of the ground in the longitudinal and transverse directions. If gentle undulations continue along the panel rows, rainwater can tend to accumulate beneath the rows. If there are steps in the transverse direction, water can flow from the walkway side toward the foundation side. Also, areas that have been excavated and backfilled for foundation work may not blend well with the surrounding ground and can develop localized unevenness. These differences may not be noticeable immediately after completion but can emerge with rainfall or over time.

When checking ground elevation, you need to be aware of the relationship between the design values and the as‑built/current values. If a design ground elevation has been established, comparing it with the results of drone surveying can reveal areas that may deviate significantly from the design surface. However, during actual construction, fine adjustments may be made in response to site conditions. Therefore, rather than focusing solely on numerical differences, it is important to assess the situation after reviewing construction records, design changes, and the outcomes of on‑site consultations.

Uneven ground around foundations also affects the ease of maintenance. If there are sudden bumps or hollows in areas where inspectors walk, footing becomes unsafe after rain or during mowing. When mowing equipment or small service vehicles need access, steps in the ground surface or mud can reduce work efficiency. Because solar power plants require long-term management even after starting operation, it is important to identify trends in ground unevenness at the site development stage and reflect them in repair and inspection plans.

The results of drone surveying can also be used to direct on-site inspections. By extracting areas with significant surface irregularities from the terrain data and organizing them together with on-site photos and location information, it becomes easier to establish a shared understanding with construction and management personnel. Rather than verbally saying "the low spot at the back of the panel row," indicating the location on a drawing and showing its height relationship to the surroundings makes it easier to determine whether repairs are necessary and to set their priority.

Check for unevenness of maintenance passages and work flow paths

In solar power plants, not only the panels and electrical equipment but also the condition of access paths and work routes is important. If ground irregularities remain on access paths, they can impede the movement of inspection vehicles, mowing, inspection of drainage facilities, and movement during emergencies. In plant operations, because daily inspections and regular maintenance are carried out continuously, pathway unevenness is an inspection item that must not be overlooked.

Unevenness in maintenance walkways can be detected to some extent by walking the site, but it takes time to inspect all walkways across the site to the same standard. Using drone surveying makes it easier to check, over a wide area, walkway elevations, transverse slopes, and the continuity of low spots. In particular, the site perimeter, the areas between panel rows, routes leading to substations and collection facilities, and the connections to delivery/access roads are locations that will be used frequently after construction, so it is important to understand the condition of the ground surface there.

When it comes to unevenness in access pathways, the concern is not limited to level differences that impede vehicle passage. Even slight depressions can collect water, turn into muddy areas, and make the surface more prone to damage during traffic. Conversely, local bumps can prevent rainwater from draining laterally, causing it to flow into the edges of the pathway. At solar power plants, because pathways can either interrupt drainage flows or, conversely, become channels for water, it is important to consider not only the height of the pathway itself but also how it connects with the surrounding ground.

The key point in drone surveying is to view management access paths as surfaces rather than lines. Looking only at the elevation at the center of the path may not reveal the relationship with the shoulders or adjacent ground. If the shoulders are lower, they can be more prone to erosion or collapse from rainwater. If only one side of the path is higher and water tends to collect on the other side, water can flow into grassed areas or under the panel rows. To grasp these conditions, it is effective to inspect the terrain including the full width of the path and the surrounding area.

Even if an access road looks fine immediately after construction, ruts and settlement can occur from vehicle traffic and rainwater. By conducting drone surveys regularly, you can compare the initial condition with the current condition and more easily identify deterioration and settlement trends in the road. Rechecking surface irregularities is especially useful after works that involve heavy vehicles or after typhoons and heavy rains. Considering stable operation of a power plant, it is worthwhile to incorporate drone surveys not only at the completion of construction but also into inspections after the start of operations.

The results of checks on access routes and work-flow paths can be reflected in maintenance plans. By organizing locations that need repairs, places to inspect after rain, areas where vehicle entry should be avoided, and spots where caution is needed underfoot during grass cutting, you can improve the safety and efficiency of on-site work. Checking for site surface irregularities can be used not only to evaluate construction quality but also as practical information for the long-term management of a power plant.

Organize as survey results usable for reporting and repair decisions

After confirming grading irregularities with a drone survey, it is important to organize the collected information into a format that can be used for reporting and repair decisions. Even if data is acquired on site, if the deliverables are hard to understand, stakeholders may not share the same understanding, making it difficult to determine whether repairs are necessary or to set priorities. Because grading irregularities at solar power plants are often reviewed by multiple parties—contractors, clients, designers, and management companies—the materials should be prepared so that anyone can easily understand the situation.

As deliverables, combining an overall terrain map, a color-coded elevation-difference map, a location map of spots requiring verification, on-site photographs, and confirmation comments will make the output practical for field work. However, adding too many materials can make the key points hard to discern, so it is important to organize them according to the purpose of checking ground irregularities. If the purpose is to check drainage, show the relationship between low spots where water tends to collect and the downstream end. If the check is prior to installation of support frames, indicate the ground heights around foundation locations and the tendencies of level differences. If the purpose is to verify maintenance access routes, show bumps or unevenness that could affect passage and areas at risk of becoming muddy.

One thing to be careful about in reports is to separate what can be concluded from the survey results and what requires on-site verification. Drone survey data can reveal elevation differences and tendencies in surface irregularities. However, the degree of soil compaction, soil composition, surface muddiness, clogging of drainage facilities, and subsurface conditions cannot be determined solely from images and topographic data. Therefore, in reports it is practical to use expressions such as "there is a tendency to be low in the survey data," "it may be depressed compared to the surroundings," and "on-site inspection after rain is desirable," thereby providing material for judgment while avoiding definitive statements.

When using it for repair decisions, organizing priorities can also be effective. If all irregularities are treated the same, the scope of response becomes too wide to be realistic. You need an approach that checks high-impact locations first—areas that affect drainage, places near mounting racks or foundations, locations on maintenance walkways, and spots near site boundaries or slopes. By using the results of drone surveys, you can narrow down priority inspection points across a large site and make it easier to plan on-site investigations and repair work.

When organizing survey deliverables, it is also important to ensure they can be referenced later. Keeping the date of photography, weather, site conditions, control points used, analysis parameters, creation date of the deliverables, and so on makes future comparisons easier. Because solar power plants are operated over the long term, recording the condition immediately after site development provides a baseline for judging later subsidence, erosion, and poor drainage. For locations suspected of unevenness, keeping location information, photos, and comments together will make confirmation at the next inspection smoother.

It is also important that report materials be presented in a way that is understandable not only to technical staff but also to site managers and owners. If explanations rely only on terrain data and technical terms, stakeholders who are not familiar with surveying may have difficulty grasping the intent. Including the location on drawings, on-site landmarks, the expected impacts, and the next actions to be confirmed makes a report more likely to lead to on-site responses. Drone surveying should be used not as an end in itself for data acquisition, but as a means to detect site grading irregularities early and to improve the construction quality and maintainability of power plants.

Summary

Unevenness of site grading at solar power plants is an important item to check because it relates to drainage, racking installation, access paths, and maintenance routes. On large sites, it is not easy to grasp the overall surface undulations and slope irregularities by visual inspection from the ground alone. By utilizing drone surveying, it becomes easier to confirm, across the site, terrain differences before and after grading, the distribution of low-lying areas, drainage directions, ground elevations around racking and foundations, and the condition of access paths.

On the other hand, the results of drone surveys are not foolproof. Because appearances can change depending on imaging conditions, analysis parameters, and site conditions, it is important not to draw conclusions based solely on survey results but to make judgments in combination with on-site verification and construction records. In particular, the risk of ponding and ground stability vary with rainfall, soil type, drainage facilities, and the passage of time. By extracting suspicious locations from terrain data and establishing a workflow to focus on those areas during on-site checks, you can make unevenness inspections practical and usable in the field.

When checking for ground unevenness after earthworks, make the purpose clear and proceed in the sequence of comparing before and after construction, checking drainage slopes, examining around racks and foundations, inspecting maintenance access paths, and organizing findings into reporting materials; this flow makes it easier to reduce missed checks. Drone surveying of solar power plants is not merely aerial photography but a process of organizing on-site information to enable long-term stable operation of the plant. By recording the condition immediately after earthworks and, when necessary, comparing it after operations begin, you can also facilitate early detection of settlement and poor drainage.

When planning to introduce drone surveying for earthwork verification and maintenance of solar power plants, it is important to organize flight planning, safety management, the handling of control points, and the intended uses of the deliverables in advance. Not only does this allow efficient inspection of wide-area terrain, but by compiling the data into materials that support on-site verification and repair decisions, it becomes easier to share information about surface irregularities and drainage risks among stakeholders.