6 Precautions for Drone Surveys to Avoid GCP Placement Failures at Solar Power Plants

When conducting drone surveys at solar power plants, it becomes easier to efficiently understand the terrain of large sites, site grading and earthworks, drainage flow, elevation differences around panel mounting structures, and the condition of access roads and slopes. However, processing the captured images does not always produce results with sufficient accuracy. In particular, if GCP installation, measurement, or recording is carried out poorly, orthophotos, point clouds, and 3D models can become misaligned, producing data that are difficult to use for design documents, construction management, and maintenance. This article organizes the common failure points when installing GCPs in drone surveys of solar power plants in a way that makes it easy for field personnel to make judgments on site.

Table of Contents

• Why GCPs are important in drone surveys of solar power plants

• Note 1: Clarify the purpose of the GCPs before on-site work

• Note 2: Place them evenly so they surround the entire solar power plant to avoid bias

• Note 3: Choose locations that are not obscured by panels or shadows

• Note 4: Ensure both that the GCPs can be surveyed and that they are visible in the imagery

• Note 5: Arrange them in three dimensions, taking into account elevation differences and slopes

• Note 6: Keep installation records and validation points to confirm the reliability of the results

• Field procedures to ensure stable GCP installation

• Summary

Why GCPs Are Important in Drone Surveying of Solar Power Plants

Solar power plants are one of the sites best suited to drone surveying. This is because their sites are large, it is easy to check changes in the ground surface from above, and they are well suited for before-and-after comparisons of earthworks and for records of operation and maintenance. In particular, a power plant contains many items to manage, such as panel rows, mounting structures, access roads, drainage channels, balancing ponds, slopes, fences, and earthwork boundaries. Even when it is difficult to grasp the overall picture by walking the site on the ground alone, using orthophotos and point clouds acquired by drone surveys makes it easier to understand the situation while getting an overview of the entire site.

On the other hand, in drone surveying of solar power plants, even if the images look good, attention must be paid to positional accuracy and vertical consistency. Data created from captured images may be used in conjunction with the site’s coordinate system, existing drawings, and construction management documents. For that reason, it is important not only that the appearance is good but that the data properly indicates where things are located. This is where GCPs become important. GCPs, sometimes called ground control points, are used to align images captured by drones and the three-dimensional data derived from them with local coordinates and reference standards.

Even with only the drone's onboard positioning information, it is possible to create image data with approximate positioning. Also, when using positioning methods such as RTK or PPK, the way GCPs are used and their necessity can vary depending on the project.

However, for projects such as solar power plants where you want to check the site's overall slope, drainage direction, positions near boundaries, racking layout, and the width of maintenance roads, positional offsets in deliverables can lead to practical problems. For example, problems can occur such as the fence appearing misaligned when overlaying an orthophoto with design drawings, the drainage channel alignment not matching the actual conditions, and difficulty distinguishing between measurement error and actual deformation when comparing changes in slopes.

GCPs are not something you can just place markers on the site. The usability of the deliverables can vary greatly depending on where you place them, how many you place, how you measure them, whether they can be clearly identified in the captured images, and whether you keep records that allow later verification. In photovoltaic power plants in particular, conditions that make GCP installation difficult often coincide, such as shading by panels, shadows from rows of panels, repetitive similar shapes, large graded areas, slopes and steps, and access routes separated by fences.

Therefore, GCP installation should not be regarded only as a task for the day of capture; it needs to be planned in advance as part of the survey plan. After confirming the flight route, flight altitude, intended use of the deliverables, required accuracy, on-site safety conditions, and areas that can be accessed, it is important to decide on the placement of the GCPs. The six points covered in this article are basic guidelines for improving the reliability of deliverables when conducting drone surveys at solar power plants.

Note 1 Clarify the purpose of GCP before fieldwork

The most common initial mistake when installing GCPs is going into the field without being clear about why the GCPs are being placed. Even for drone surveying of solar power plants, there is not a single purpose. Depending on the objective—ascertaining current site conditions before development, monitoring progress during construction, recording as-built conditions at completion, preparing ledgers for operation and maintenance, checking for drainage problems or slope deformations, preparing management documentation for areas around panels, etc.—the required deliverables will vary.

For example, if you only want to get an overall sense of the site, relative positional relationships and overall visual clarity are prioritized. On the other hand, when comparing by overlaying with existing plan drawings or construction drawings, coordinate consistency becomes important. Furthermore, when checking slope elevation differences or drainage gradients, stability in the vertical direction cannot be ignored. If GCPs are installed without clarifying the purpose, problems may arise later such as "the images are easy to view but do not match the drawings," "the overall alignment seems correct but the edges are shifted," or "difficult to use for checking vertical information."

Before entering the site, you first need to decide how the deliverables will be used. Whether you will produce orthophotos, point clouds, 3D models, perform area or distance checks, or compare with past data will affect the necessity of GCPs and their placement strategy. Rather than using the same layout for every project, it is important to determine the role of GCPs according to the intended use of the deliverables.

At a solar power plant, multiple parties—such as the construction company, design staff, the client, and maintenance personnel—may be responsible for reviewing the deliverables. Therefore, it is important to organize the purpose of the GCP in a way that anyone can explain, rather than using criteria understood only by on-site staff. For example, if you share in advance whether you prioritize alignment of the entire plant, verification near boundaries, or height checks around drainage facilities, it will be easier to judge installation positions.

Also, you should decide in advance not only to install GCPs but also whether to designate validation points. GCPs are reference points used in processing, and validation points are points used to verify the accuracy of the deliverables. If you use all points for alignment, it becomes difficult to objectively confirm how accurate the deliverables actually are. In professional situations where you need to explain deliverables, how you leave validation points affects your ability to explain their reliability.

Clarifying the purpose of GCPs also contributes to the efficiency of field work. If the number of points to install, their locations, the surveying method, and the post-capture processing procedures are decided, you can reduce the time spent hesitating on site. Conversely, if decisions are made in the field while the purpose remains unclear, there is a higher risk of forcing installations in areas that are difficult to access for safety reasons or placing them where they are hard to identify in the captured images. In drone surveying, it is important to consider the entire process—from pre-planning through results verification—not just the data capture.

Note 2 Arrange evenly around the entire power plant

A common mistake when placing GCPs at solar power plants is that GCPs end up concentrated only in locations that are easy to install. Areas along maintenance roads and around the entrance are easy to work in and convenient for setting up surveying equipment, so points tend to cluster there. However, if GCPs are biased toward one area, positional errors in the products generated by image processing can become large at the edges or on the far side. In large plants, areas farther from the entrance and at the site boundaries are more likely to receive less thorough checks, so special care should be taken to avoid biased placement.

As a general rule, arranging GCPs so they bound the entire survey area from the outside is effective. Consider a balanced layout that surrounds the survey area, such as at the four corners of the site, near the perimeter, near the center, and at locations where the shape bends. Solar power plants are not always on near-rectangular sites; in mountainous or sloped terrain they can occupy irregularly shaped plots. If a site is long and narrow, changes width partway, or is divided into multiple grading terraces, simply placing GCPs at the four corners may be insufficient.

When planning GCP placement, look at the entire power plant on the site plan and confirm which area you want to use as the survey deliverable. If there are no control points near the outer edge of the capture area, distortions are more likely to occur at the edges during image processing. In particular, if locations you may want to check later—such as the fence perimeter, the top or bottom of slopes, around retention ponds, or outside access roads—fall at the edges, plan so those areas are not treated as margins of the deliverable.

However, it’s not simply a matter of placing them around the perimeter. GCPs are of little use unless they can be clearly identified in the captured images. Locations such as alongside fences, immediately beside panels, in the shadow of trees, or partway down a steep slope may look like good positions on a plan but can be hard to see in the images. You need to consider ease of installation, visibility in the imagery, safety, and the overall balance across the survey area simultaneously.

If rows of panels are neatly aligned within a power plant, similar patterns repeat in image processing, making it difficult to identify feature points in some locations. GCPs support alignment within such repetitive shapes. Distributing them across the entire plant, rather than only in a portion of it, makes it easier to improve the stability of processing results. In particular, when photographing a large power plant by dividing it into multiple blocks, GCP placement that takes the connectivity between blocks into account is important.

Also, just because the survey area is large doesn't mean you should indiscriminately increase the number of GCPs. The more points you add, the more on-site work and verification during processing increase. What matters is placing them where they are needed for the objective, with a justifiable rationale. Placing too few leads to unstable results, but placing too many increases the on-site burden and can lead to mistakes in the management of point names and measurement records. It is important to plan a well-balanced distribution while considering the overall shape of the power plant and the intended use of the deliverables.

Note 3 Choose a position not obscured by panels or shadows

A major consideration unique to solar power plants is shading and shadows caused by the panels. GCPs must be not only surveyable on the ground but also clearly visible in aerial images. However, within a plant there are many places—under panels, near mounting structures, in narrow spaces between rows, and areas where shadows extend depending on the time of day—where shadows occur. Markers that were visible at installation can be buried in shadow or hidden behind the edges of panels in the captured images, making it difficult to accurately mark points during processing.

Shadows cast by the panels vary depending on the time of day and season. In the morning and evening, shadows tend to stretch longer and may fall between rows of panels or on parts of maintenance roads. Even near midday, the appearance of shadows changes depending on racking height, panel angle, and terrain conditions. When installing GCPs, it is important not only to consider how they look at the moment of installation but also to anticipate the shadows during the actual shooting times. When photographing a large site over an extended period, you also need to consider that shadow positions may change between the start and end of shooting.

Also, at solar power plants, because similar rows of panels repeat, if GCPs are too small or blend into the background they become difficult to locate in images. Markers should have shapes and sizes that are easily distinguishable from the ground and surrounding equipment, and be sufficiently identifiable given the image resolution. It is important that the center of the marker can be correctly specified during processing; it is not enough for something that merely looks like a marker to be visible. Markers whose centers are difficult to read lead to variability in point specification.

GCPs should be placed on as flat and stable a surface as possible. On gravel, grass, muddy ground, or steep slopes, markers can shift or tilt. In solar power plants, maintenance roads and some prepared surfaces are often covered with crushed stone, and unless markers are secured they may move due to wind or contact during work. If imaging or surveying is carried out after a marker has moved, it becomes unclear which position should be used as the reference.

Pay attention to weeds and vegetation. At power plants under maintenance, there are areas where grass has grown and areas where the ground surface is uneven immediately after weeding. Placing a marker on grass can cause parts of it to be obscured during imaging, or wind can move the grass so the center becomes difficult to read. Especially when acquiring detailed, low-altitude imagery, if the GCP center is not clear it will hinder accuracy verification. Before installation, tidy the surrounding area and ensure the marker is stably visible.

In terms of avoiding panels and shadows, it is also effective to check once from above how they appear after installation. Even if something seems fine from a person’s eye level on site, it can be surprisingly inconspicuous in drone images. By checking how the GCPs appear in pre‑flight verification images and adjusting their positions or the orientation of the markers as needed, you can reduce rework during the processing stage. Because GCPs are involved in both ground work and aerial photography, it is important not to judge solely from the ground‑level perspective.

Note 4 Reconcile surveyable conditions with those visible in photographs

When placing GCPs, locations that are easy to survey do not necessarily coincide with locations that are clearly visible in imagery. At solar power plants, areas along maintenance roads and open flat spaces are easy to survey, while areas important for deliverables may be behind rows of panels or near slopes. Conversely, locations that are clearly visible in aerial images may be places that cannot be safely approached from the ground or that have poor conditions for setting up surveying equipment. When planning GCPs, it is important to reconcile ground surveying conditions with visibility in aerial imagery.

When measuring the coordinates of GCPs, the stability of the reference surveying method is critical. When using satellite positioning, pay attention to the openness of the sky, surrounding obstructions, and reflective effects from mounting racks and fences. Solar power plants have many metal structures, and in some locations positioning can become unstable. Even if the sky appears open, measurements can be affected by rows of panels, mounting racks, nearby slopes, trees, electrical equipment, and similar factors. If survey measurements that are unstable are used as GCPs, the very reference for image processing becomes unstable.

On the other hand, if you prioritize surveying conditions too much, you may end up placing GCPs in locations that are difficult to see in the images. For example, spots next to fences or the roadside where equipment is easy to set up can have parts of the marker obscured by shooting angles or shadows. Near rows of panels, the marker can overlap with panels or mounts in the captured images, making the center difficult to read. GCPs require both that the surveyed measurements are appropriate and that the point can be accurately identified in the images.

On site, checking for each GCP whether it can be surveyed, is visible in the imagery, and can be installed safely helps reduce failures. Especially when workers split into multiple teams to install them at a large power plant, it is important to share the installation standards. If each person places markers or assigns point names differently, post-processing tends to become confused. Standardizing point names, marker orientation, measurement order, photographic records, and methods for confirming installation and removal stabilizes the quality of on-site work.

GCP markers should be ones that make it easy to determine the center on the captured images. You can substitute existing on-site objects as markers, but at solar power plants there are many similar pieces of equipment, gravel surfaces, and rows of panels, so they can become difficult to identify on images later. Even when using existing corners or equipment as reference points, you must confirm whether that point can be uniquely identified during processing and whether it appears in multiple images within the capture area. The more visually similar objects there are at a site, the more important it is to use a clear marker.

Also, GCPs must be installed, surveyed, and kept in the same position until the photography is completed. If a marker is moved after surveying, removed before photography, or shifted by vehicles or workers stepping on it, the surveyed coordinates will no longer match the positions in the images. At solar power plants, vehicles may use maintenance roads, so caution is necessary when placing markers along traffic routes. Operational procedures are required that prioritize safety, such as placing auxiliary markers around them as needed, sharing installation locations among workers, and confirming that they will not be removed until photography is completed.

Note 5 Consider elevation differences and slopes when arranging three-dimensionally

In GCP placement for solar power plants, balance in elevation is as important as horizontal positioning. While elevation changes may be small at flat plants, in practice plants are also installed in mountainous areas, hilly terrain, reclaimed land, and sites with level differences. When panel surfaces are divided into steps, or slopes, balancing ponds, drainage channels, and access roads are arranged in complex ways, placing GCPs only at similar elevations can weaken vertical consistency.

When you want to use point clouds and 3D models created by drone surveying to check drainage gradients, slope geometry, settlement or scour of prepared surfaces, and changes in fill areas, reliability in the vertical direction is particularly important. If GCPs are concentrated on flat maintenance roads, alignment around those roads may be good, but vertical offsets can become noticeable on the upper and lower parts of slopes, at site edges, and in areas with level changes. Even if the horizontal positions appear to match, if the elevations are not stable it can affect assessments of earthwork volumes and gradients.

In solar power plants, site development is carried out with panel layout prioritized, so to understand water flow and changes in topography it is necessary to view the entire site in three dimensions. Deliverables that include height information are effective for checking where rainwater is likely to collect, whether sediment has accumulated beneath slopes, and whether appropriate gradients toward drainage channels have been established. Therefore, it is important to consider GCPs not merely as planar markers but as references that support three-dimensional deliverables.

On sites with elevation differences, consider whether GCPs can be placed at positions that represent changes in the terrain, such as the upper, lower, and middle sections. However, do not force installation on the middle of slopes or on steep terrain because of safety risks. It is important to choose positions that reflect terrain changes within areas that can be safely accessed. Depending on site conditions, placing them on flat areas near the top or bottom edge of a slope, rather than on the slope itself, may be sufficient to improve the balance of placement.

Also, in locations with elevation differences, GCPs are more likely to end up in slope shadows or in the blind spots of panels. When a marker is placed on a slope, the imaging angle can distort the marker or make its center difficult to read. If a marker is installed in a tilted position, its appearance in the image becomes unclear, which can reduce the accuracy of point picking. Choose as level and stable a location as possible, and if placement on a slope is unavoidable, thoroughly check both visibility and safety.

When comparing changes using drone surveys conducted multiple times, the vertical reference is also important. When comparing data from different times—before and after site development, during construction and after completion, or before and after inspections—if the placement of GCPs or the survey reference changes significantly, it becomes difficult to determine whether differences are actual changes or just differences in surveying conditions. When regularly surveying the same power plant, consider reference points that are easy to reuse or locations that can be set up using the same approach each time, as this will make it easier to improve the reliability of comparisons.

Note 6 Retain installation records and verification points to confirm the reliability of results

What is easily overlooked when installing GCPs is recordkeeping and verification. Even if you place GCPs on site, measure their coordinates, and perform photography, if those records are not organized you can run into problems during post-processing or when explaining results. Especially on large sites such as solar power plants, there is a risk of mix-ups in point names, misunderstandings about installation locations, insufficient capture of markers in images, and data-entry errors for survey values. GCPs require quality control not only for the installation itself but also for the recording and checking.

At the site, it is reassuring to record for each GCP the point name, installation location, survey time, measurement method, the condition of the marker, and the surrounding conditions. Additionally, keeping ground-level photographs makes it easier to confirm the point's position later during image processing. Ground-level photos are useful not only as close-ups of the marker but also as wider shots that show the relationship to nearby equipment, rows of panels, and access roads. Photos that only show a large close-up of the marker can make it difficult to tell which location the point corresponds to.

Managing point names is also important. At large power plants, similar rows of panels and roads can repeat, so relying solely on on-site memory can easily cause mix-ups. Point names should be assigned according to consistent rules and managed so that survey data, field notes, photographs, and the designated points in the processing software all match. Errors in point names can lead to offsets in the entire deliverable, so they must always be checked before processing.

Leaving validation points is also important for confirming the reliability of the results. If all survey points are used as GCPs in processing, it becomes difficult to independently verify how accurate the processing results are. By setting aside validation points, you can check the position and elevation of the deliverables at points that were not used in processing. This makes it easier to explain not only that the processing was completed, but also how well the deliverables align with the site coordinates.

Validation points should be placed according to which locations are prioritized in the deliverables. If the horizontal positions across the power plant are the focus, consider placing validation points evenly along the perimeter and near the center. If vertical accuracy is the focus, include locations related to elevation differences such as steps, slopes, and areas around drainage facilities. However, validation points must also be clearly visible in the captured imagery and have stable survey measurements. As with GCPs, it is important not only that they are easy to install but that they provide the quality necessary for validation.

When checking deliverables, it is important not to be reassured by looking only at the numerical results of processing. Visually check whether the GCP-specified positions on the images are shifted, whether point names have been mixed up, whether there are unnatural distortions at the edges, and whether the shapes of panel rows or access roads are unnaturally curved. Because solar power plants contain many linear installations, unnatural bends or shifts are often relatively easy to spot. Combining numerical checks with visual inspections can improve the reliability of the deliverables.

Field operations to stabilize GCP installation

To successfully install GCPs, it is important not only to address individual points of caution but also to organize operations across the entire site. At a solar power plant site, various conditions affect surveying work, such as safety management of power generation equipment, restrictions on access areas, vehicle circulation routes, working hours, weather, wind, solar radiation, and weed-control status. GCP installation may appear to be a simple task that can be completed quickly, but it must be treated as a critical process that determines the quality of the deliverables.

First, before starting work, review the site drawings, aerial photographs, past survey results, and accessible areas, and determine the approximate candidate positions for GCPs. If you wait until you arrive on site to decide on placements, you will often be pressed for time and tend to favor locations that are easy to install. By deciding candidates in advance, you can choose alternative positions on site while maintaining the original intent of the layout if changes become necessary.

Next, upon arrival at the site, reassess the candidate locations based on actual conditions. Even locations that appear acceptable on the drawings can be unsuitable for reasons such as overgrown grass, muddy ground, shadows cast by panels, vehicle traffic, or interference with equipment inspections. GCP placement requires both pre-planning and on-site judgment. Rather than placing them strictly according to the plan while ignoring site conditions, it is important to adjust to safe and reliable positions while preserving the intended purpose of the deliverables.

Division of roles among workers is also important. If the person responsible for installing GCPs, the person measuring coordinates, the person taking records, and the person checking the photography plan are different, insufficient information sharing can make mistakes more likely. By deciding on the basic flow—reading out point names, checking survey measurements, photographing markers, confirming removal, etc.—you can reduce confusion on site. In particular, when installing multiple points at the same time, it is necessary to make clear which points have been surveyed and which points should remain until photography is complete.

Pay attention to weather conditions. In strong winds, markers can move easily and this can affect imaging. On days with strong sunlight, shadows become pronounced and can affect the visibility of GCPs. Cloudy skies are better at reducing shadow effects, but they can also change image contrast and the appearance of the ground surface. In rainy conditions or immediately after rainfall, the ground can become muddy and markers may be unstable. At solar power plants, because electrical equipment is present, prioritize safety and consider deciding to refrain from carrying out risky tasks.

Also, it is important to treat post-shoot verification as part of the on-site work. If, after returning to the office and checking the images, you find that GCPs are not visible, are unreadable due to shadows, or the point names are unclear, you may need to re-shoot or re-install them. If possible, check representative images on-site and confirm that the required number of GCPs are visible and that their centers can be read before packing up. A brief on-site check can prevent major rework later.

When repeatedly surveying the same solar power plant, creating standard procedures for GCP placement is also effective. Rather than deciding from scratch each time, recording past layouts, positions that were easy to see, locations to avoid, times when shadows tend to appear, vehicle traffic routes, and points to coordinate with the site manager will make subsequent work more consistent. Because operations and maintenance of solar power plants require periodic comparison of the same locations, keeping surveying conditions as consistent as possible increases the usability of the results.

Summary

In drone surveying of solar power plants, the quality of GCP placement has a major impact on the reliability of the deliverables. Solar power plants are expansive, with repetitive rows of panels, and on-site conditions such as shadows and obstructions, slopes, elevation differences, and perimeter areas to be managed make GCP placement difficult. To produce deliverables that can be used not only for clean orthoimages and point clouds but also for design documentation, construction management, and maintenance, it is essential to plan and install GCPs systematically, survey them, record them, and verify them.

To prevent failures, it is important first to clarify the purpose of the GCPs. The approach to their placement varies depending on whether you prioritize overall positional alignment, verification in the vertical direction, or overlay with drawings. Next, arrange them evenly so they surround the entire power plant to prevent the resulting outputs from becoming unstable at the edges or on the far side. Additionally, choose positions not obscured by panels or shadows, and ensure they can be measured stably by ground surveying while also being clearly visible in the captured images.

In power plants with elevation differences and slopes, it is important to consider GCPs not only in plan but also in three dimensions. If vertical control is weak, it becomes difficult to judge drainage gradients, slope shapes, and changes to the developed surface when checking them. Additionally, keeping point names, installation photos, measurement records, and verification points makes it easier to later explain the reliability of the deliverables. GCPs are not items whose role ends the moment they are placed; they are the reference that supports processing, verification, and explanation.

In practical work at solar power plants, you are required to inspect large sites within limited time and compile them into materials that are easy for stakeholders to understand. For that, it is important not only to make shooting efficient but also to ensure the deliverables can be used with confidence. By carefully planning GCP placement and operating according to site conditions, the results of drone surveying become more than mere photographic records and are easier to use as practical data for construction management and maintenance.