6 perspectives for comparing a solar power plant before and after construction using drone surveying

In the construction of solar power plants, comparing the terrain conditions before site development with the finished condition after development is important for design verification, construction management, and maintenance management. Simply walking the site can miss overall elevation differences across a large site, slopes, drainage directions, and changes in the development area. Drone surveying, which can record terrain and construction conditions from the air, is useful for that.

However, the results obtained from drone surveys can vary in accuracy and the range of information that can be interpreted depending on flight planning, imaging conditions, the placement of control points and check points, the presence of vegetation or materials, and the processing methods. It is important to compare pre- and post-development conditions under similar circumstances and, when necessary, cross-check with ground surveys, on-site inspections, design documents, and construction records. By setting these premises, it becomes easier to confirm whether the grading has been carried out as planned, whether drainage or access routes are obstructed, and whether there are deviations that could affect subsequent work.

Table of Contents

• Clarify the purpose of comparing pre- and post-development.

• Observe terrain elevation differences and changes in cut-and-fill.

• Confirm consistency between the planned panel layout area and the site development area.

• Compare drainage plans and water flow

• Confirm the impact on slopes, retaining walls, and surrounding boundaries.

• Organize into a format that can be used as management materials.

• Summary

Clarify the purpose of comparing before and after land development

When comparing the before-and-after conditions of a solar power plant using drone surveys, the first thing you should decide is the purpose—what you are measuring to verify. Simply taking aerial photos often leads to a situation where, upon later review, “the appearance is clear but it is difficult to use as material for decision-making.” Whether you want to understand the pre-development topography, confirm the post-development as-built condition, compare the design drawings with the current situation, or check drainage and slope conditions, the required capture area and the way you organize the data will differ.

In pre-construction drone surveys, it is important to broadly record the site's overall topography, existing roads, the condition of wooded areas and farmland, existing structures, waterways, and conditions near boundaries. For solar power plants, you need to consider not only the surfaces where panels will be installed but also the locations of access paths, drainage facilities, fences, and substation and transformer equipment. Therefore, if pre-construction records are coarse, it becomes difficult later to determine whether a location was originally low or appears to have settled after construction.

Meanwhile, in post-construction drone surveys, we check whether the areas assumed in the design have been graded, whether the slopes deviate significantly from the planned gradients, and whether there are any points to confirm regarding changes in earthwork volumes or the shape of slopes. If pre- and post-construction data can be overlaid and compared, it becomes easier to identify which parts were cut and which parts were filled. This serves not only to verify the construction but also as a basis for tracking subsidence, rainwater ponding, and deterioration of access routes that may occur after the power plant begins operation.

If surveys are carried out without a clear purpose, photos may remain but information useful for making judgments tends to be lacking. For example, if you photograph a wide area before site preparation but only photograph the panel installation area afterward, it becomes difficult to compare relationships with nearby drainage channels and access roads. Conversely, even if you take detailed measurements only after site preparation, without baseline references from before preparation you cannot explain the amount of change. In drone surveying for solar power plants, the practical key is to record data before and after site preparation over as nearly the same area, using the same benchmarks and the same data-organization approach as possible.

The purpose of the comparison differs slightly among the contractor, the client, the management/maintenance company, and the design team. Contractors emphasize the as-built condition of earthworks and schedule control, while clients prioritize post-completion quality and future risks. Management companies sometimes want to use the condition immediately after completion as the initial baseline for maintenance management. Because each stakeholder focuses on different points, sharing in advance how the drone survey results will be used can reduce the need for retakes and missed checks.

Also, for before-and-after comparisons of site development, pre-flight safety checks and confirmation of relevant rules are essential. Drone surveying is an efficient way to inspect wide areas, but it should be conducted only after confirming the airspace where flights are permitted, nearby third parties, adjacent facilities, weather conditions, and takeoff and landing locations. When conducting public surveys or treating results as official deliverables, it is also safer to organize in advance the applicable standards, accuracy control, and the format of the deliverables.

Drone surveying should not leave all decisions to aerial data alone. The degree of ground compaction, the softness of topsoil, spring water, the details of drainage manholes, and cracks in structures become easier to assess when combined with on-site checks and other surveys or inspections. Use drone surveying as the "axis" for grasping the overall picture, and establish a workflow that focuses ground checks on necessary locations; this makes pre- and post-development comparisons more useful in practice.

Viewing terrain elevation differences and changes in cut-and-fill

When comparing a site before and after earthworks, what’s especially important are changes in elevation and cut-and-fill. At solar power plants, the ground is sometimes graded within a certain area to allow stable installation of the panel racking. However, the entire site is not necessarily made completely flat; slopes may be retained to suit terrain conditions and drainage plans. Therefore, what should be checked after earthworks is not merely whether the site has been flattened, but whether it is in a state close to the planned shape.

In drone surveying, comparing the ground surface before and after site development makes it easier to determine which areas were cut and which areas were filled. Simply walking the site can only capture changes in elevation across a wide area qualitatively. Especially on gentle slopes or large development sites, small undulations or changes in gradient can be difficult to detect by eye. By inspecting the terrain as a continuous surface using data acquired from above, it becomes easier to find local depressions or bulges and biases in slope that differ from the design.

When comparing the locations of cut and fill, it is important not to focus too much on earthwork volumes alone. For example, if an area that was designed to be handled as a cut actually contains a large amount of fill, it can affect drainage gradients and ground treatment. Conversely, if the height is insufficient where fill is required, it may affect the installation height of panel racking and the gradient of maintenance access paths. At solar power plants, because panel orientation, the relationships between rows, and the drivability of maintenance vehicles are also factors, comparisons need to consider not only changes in soil quantity but also how the site will be used after completion.

When checking elevation differences after site development, localized puddles and unnatural depressions must not be overlooked. Even when they are difficult to discern from photos taken in fair weather, viewing terrain data can reveal areas that are continuously lower than their surroundings. Such locations tend to collect water during heavy rain and can lead to muddy walkways, slope scouring, and erosion around cable routes. Confirming how the positions of low-lying areas have changed by comparing before and after development helps in considering drainage measures and additional maintenance.

Also, in areas where the natural terrain was significantly altered before development, attention must be paid to future subsidence and surface runoff. While drone surveys alone cannot determine ground strength, recording shape changes before and after development makes it easier to identify the starting point of any changes during later inspections. For example, if part of a pathway appears to have settled after operations begin, you can compare whether it was low immediately after development or whether it subsided afterward. This is useful when assessing the need for repairs and sorting out causes.

When comparing pre- and post-construction data, it is important to standardize the handling of reference points and coordinates. If the reference is shifted, areas that have not actually changed can appear to show differences. For large sites such as solar power plants, you need to organize the extent of comparison, reference points, coordinate system, and how elevation is handled before conducting surveys. If possible, sharing how control points and check points are treated and the methods for verifying the accuracy of survey results among stakeholders will make it easier to explain the reliability of the comparison results.

Note that terrain data generated by drones can be affected by vegetation, stockpiled soil, temporarily stored materials, heavy equipment, shadows, and similar factors. In areas with dense vegetation before earthworks, or where materials are placed after earthworks, it may not be possible to accurately compare the actual ground surface. Do not judge based solely on numerical differences; it is important to cross-check with on-site photos and construction records and to separate and organize areas that are suitable for comparison from those that require caution.

Confirm consistency between the planned panel layout area and the site development area

When comparing the site before and after preparation of a solar power plant, it is essential to verify that the planned panel placement area matches the actual area that has been prepared. Site preparation work is not just about leveling the land; it creates the groundwork for subsequent stages to install racking, panels, access paths, electrical equipment, fences, and other components. If the prepared area is insufficient, it can lead to problems such as panel rows not fitting as planned, access aisles becoming too narrow, and inadequate working space around equipment.

Using drone surveying, you can confirm from the air the site conditions before site development and the area that has been graded afterward. In particular, the perimeter of the planned panel layout, the ends of rows, the equipment yard, and the connection points to the access road may look fine on drawings but can require adjustments in the field due to topography, boundaries, or existing structures. By comparing the data from before and after development, it becomes easier to verify how much of the site has been prepared and whether any unprepared areas remaining could affect subsequent work.

When checking panel placement, not only the planar position but also the slope and elevation differences are important. At solar power plants, racking height adjustments can accommodate certain terrain changes, but if ground irregularities are too large they will affect construction and maintenance. If the terrain after land development varies greatly from one panel row to another, the amount of adjustment required during racking installation may increase, and passage between rows or mowing operations may be hindered. By checking the terrain as a surface with drone surveying, it becomes easier to identify in advance which row areas require attention.

When comparing the graded area and the design area, it is also necessary to check the clearances near the boundary. If panels or fences are planned close to the boundary, unstable edges of the earthworks or insufficient distance between the top of the slope and the equipment can affect post-construction safety and maintainability. Drone surveying provides an overall view that makes it easier to confirm how the edges of the earthworks are positioned relative to the boundary lines. However, legal determination of the boundary and judgments about rights relationships should be handled by cross-checking relevant documents and official survey results.

Also, when checking the planned panel placement area after site development, it is important not to confuse it with temporary staging areas or construction traffic routes. During the development work, temporary facilities such as material storage areas, heavy equipment access routes, temporary drainage, and soil stockpiles may be set up. If you look only at aerial photos, these can appear to be post-completion management spaces. When comparing the site before and after development, you need to separate and organize the equipment areas that will remain permanently from the temporary construction spaces, and identify the parts that will affect the final layout.

In practice, it is important to compare the post-development verification results with the design drawings and construction drawings, and to share any discrepancies with stakeholders as early as possible. If a shortage in the development area is noticed only after panel installation work has begun, rework and schedule adjustments can become substantial. Comparisons using drone surveying make it easier to visually explain the before-and-after conditions of construction, making them a useful tool for building consensus not only with site personnel but also with clients and designers. Discrepancies in extent that are difficult to convey with site photos and words alone can be more easily shared by combining aerial records with terrain data.

Compare drainage plans and water flow

In developing a solar power plant site, drainage planning is important. Installing panels across a large site can change how rainwater flows compared with pre-development conditions. Because grading changes the ground surface slope and establishes locations for maintenance access routes, slopes, channels, and catch basins, it is necessary to verify where water will collect and which direction it will flow. If drainage does not function properly, it can cause muddy areas, soil and sediment runoff, scouring of slopes, and standing water around equipment.

Comparing conditions before and after land development using drone surveys makes it easier to understand the relationships among the original valley topography, low-lying areas, and existing waterways. By observing how locations that tended to collect water before development were treated afterward, you can verify whether the drainage plan is appropriate for the site conditions. For example, if the direction in which water naturally flowed before development has been blocked after development, it is necessary to confirm that an alternative drainage route has been adequately provided. Aerial inspection of the terrain is well suited to capturing such changes in water flow over a wide area.

When checking drainage, it is important to verify that the post-construction slope is continuous. Even if adequate slope exists in parts, small depressions or reverse slopes along the way can cause water to accumulate. In particular, the areas between panel rows and beside maintenance access paths are places where people and vehicles enter for routine inspections, mowing, and fault response. If these areas are prone to becoming muddy after rain, the maintenance burden increases. Identifying low spots and locations where water is likely to collect through a before-and-after comparison of the site makes it easier to plan repairs or drainage measures before operations begin.

Also, attention must be paid to drainage near slopes. If rainwater concentrates at the top of a slope after site development, the surface can be eroded and soil may wash downhill. Drone surveying makes it easier to check the slope shape and surrounding gradients to determine whether the layout tends to concentrate water in one spot. Even when aerial photos look tidy, terrain data can reveal that flows are likely to concentrate at certain locations. Combined with on-site checks, it is important to confirm that drainage outlets and downstream discharge points are being properly handled.

When comparing site conditions before and after development, on-site conditions after rain also provide valuable information for assessment. While drone surveys in clear weather tend to have more stable shooting conditions, puddles and runoff traces can be difficult to see. If possible, keeping photos and ground inspection records taken after rain during post-development inspections can help supplement signs of poor drainage that terrain data alone cannot reveal. However, flying in bad weather or strong winds poses safety concerns, so avoid attempting flights under unsafe conditions and prioritize safety according to local site conditions.

Comparing the drainage plan with the actual terrain is useful not only immediately after completion but also for ongoing maintenance. Because solar power plants are operated over long periods, sediment and vegetation can accumulate in drainage channels over time, changing the flow of water. If the condition immediately after construction is recorded by drone surveying, you can compare where changes have occurred during inspections conducted years later. Clearly documenting the initial condition also aids later repair decisions and revisions to maintenance plans.

Confirm impacts on slopes, retaining walls, and surrounding boundaries

When comparing conditions before and after land development, it is necessary to check not only the interior of the panel installation area but also the slopes, retaining walls, and surrounding boundaries for impacts. Solar power plants are installed on various types of land, such as forests, sloped terrain, former farmland, and previously developed sites. When earthworks alter the topography, they can affect elevation differences at the site edges and with adjacent land, the flow of drainage, and the movement of soil and sediment. Even if the central area of the generation equipment is neatly arranged, problems remaining at the perimeter can lead to trouble after operations begin.

With drone surveying, you can obtain an aerial overview of the entire slope shape. When viewing a slope from the ground, poor footing can make it difficult to confirm overall continuity. Photographing from the air makes it easier to understand the slope’s length, curvature, changes in elevation, and relationship to drainage facilities. By comparing the slope shape before construction with the slope shape after construction, you can determine which areas have been altered, whether any abrupt changes have occurred, and whether there are locations where rainwater is likely to concentrate.

One thing to be careful about when inspecting slopes is not to judge safety solely by how neat they look. Drone surveying is effective for checking shape and surface condition, but it cannot directly determine subsurface conditions or stability calculations. Slope stability is determined by multiple factors such as soil properties, slope gradient, drainage, construction methods, and the condition of protective works. Therefore, if you find from above any abnormal deformation, traces of soil washout, lines that look like cracks, or disturbances in vegetation, it is important to follow up with on-site inspections and professional assessment.

Even when there are retaining walls or structures, it is reassuring to check the positional relationships before and after development. Confirm whether additional fill has been placed behind retaining walls, whether drainage is concentrating around structures, and whether pathways or equipment have come too close after construction. Aerial photos alone may not reveal fine cracks or the condition of drainage outlets, but they are effective for broadly understanding the relationship between structures and the surrounding developed surfaces. It is practical to extract areas of concern from drone surveys and then carry out focused on-site inspections.

Impacts on surrounding boundaries are also an important item to check. Verify whether site development has made it easier for soil and sediment to flow off-site, whether the differences in elevation with adjacent properties have become larger, and whether the placement of perimeter fences and maintenance access routes is reasonable. In particular, when adjacent properties include roads, waterways, farmland, houses, or forests, care must be taken to ensure that post-development water flow and sediment movement do not affect external areas. Continuously recording the perimeter with drone surveys makes it easier to use the footage as explanatory material later.

However, caution is necessary when handling boundaries. Features and fences visible in drone surveys do not necessarily indicate the official boundary. Boundary confirmation involves survey results, drawings, boundary markers, and verification by relevant parties. The outputs of drone surveying should be treated as materials for understanding the extent of development impacts and the existing conditions, rather than for determining the boundary itself. When a formal decision is required, it is necessary to combine the required procedures with professional surveying.

Organize into a format usable as management materials

Drone surveys before and after site development are not finished once the shooting is done. To be useful in practice, it is important to organize the data in a way that makes later comparisons easy. In the construction and management of solar power plants, documentation is required for various situations such as design, construction, inspection, maintenance, and client briefings. If data from before and after the site work are properly organized, it becomes easier to explain conditions objectively without relying on memories from the site.

First, be sure to clearly record the date of capture, the area covered, the survey conditions, the reference standards used, and the name of the target area. Even if only photos and terrain data remain, their value as management documents is reduced if it is not clear when, over what area, and for what purpose they were acquired. In particular, when comparing before and after development, conditions can change even on the same site depending on construction sections or work phases, so it is important to keep them organized chronologically.

Next, it is important to make pre- and post-earthworks data viewable side by side. By changing the presentation according to purpose—such as aerial photographs, terrain models, materials showing elevation differences, and annotated verification drawings—you can communicate more effectively to stakeholders. Documents that show changes in elevation and slope are useful for construction personnel, while documents that show the overall picture and areas of concern may be more useful for clients and management companies. By organizing the necessary information to suit the recipient, the results of drone surveying are more likely to inform on-site decision-making.

When preparing documentation, be careful not to use overly definitive language. What drone surveys can confirm is mainly the shape of the ground surface and visible changes. For example, rather than declaring "it is safe," separating confirmed facts from the scope of judgments—such as "no major deformation was confirmed in this area," "organized as locations where drainage is likely to concentrate," and "extracted as locations requiring on-site inspection"—increases the credibility of the documentation. Even when specialized verification is required in later stages, the results of a drone survey are easier to use as a starting point.

Also, data management methods are important in practice. When keeping records for each stage—before site preparation, during site preparation, after site preparation, after panel installation, and after the start of operation—if file names and folder structures are inconsistent, it can take time just to find the necessary documents. Organizing them so that the date, construction section, surveying purpose, and type of deliverable are clear makes later comparison easier. Since solar power plants are intended for long-term operation, it is important to keep documents in a state that can be handed over even if the person in charge during construction changes.

When used as management documentation, linking on-site photographs and ground inspection records is also effective. For low-lying areas, slopes, drainage channels, and locations near boundaries identified by drone surveys, attaching photos and notes taken on the ground makes the basis for decisions clearer. Combining an aerial overview with ground-level close-up verification increases the persuasiveness of the report. In particular, for post-construction repair instructions and handover to a management company, materials that make it clear where to look are required.

Additionally, comparative materials from before and after site development can be used in future maintenance planning. If, after operations begin, changes in the ground, poor drainage, deterioration of access routes, or slope degradation are discovered, having documentation that can be compared with the condition immediately after development makes it easier to determine whether changes have occurred. In managing a solar power plant, how clearly the initial condition is documented affects the quality of subsequent inspections. Drone surveying is an effective means of preserving initial records for large sites.

Summary

Comparing drone surveys before and after the earthworks of a solar power plant is not merely a record-keeping task but an important process that supports design verification, construction management, drainage checks, perimeter safety inspections, and the preparation of maintenance documentation. By understanding the terrain before development and comparing it with the finished condition afterward, changes at the site can be grasped spatially. On large sites, elevation differences, low-lying areas, slopes, drainage directions, and shifts in the extent of the earthworks—items that are easy to miss from ground-level inspections alone—become much easier to identify and organize through drone surveying.

What's particularly important is to establish conditions that allow comparison before and after site development. If the imaging coverage, reference points, recorded items, and methods of organization are inconsistent, the data you've acquired will be difficult to use for decision-making. Before development, broadly record the existing terrain and the surrounding environment; after development, verify the design area, grades, drainage, slopes, and the conditions near boundaries. By combining aerial data with ground verification, you can make more practical judgments.

Moreover, it is important to use the results of drone surveying not only as something handled solely by on-site personnel, but as materials to ensure a common understanding among the client, designers, contractors, and management company. If changes before and after site development can be shown visually, it becomes easier to share issues and points of caution that are difficult to convey by verbal explanation alone. For inspections and repair decisions after operations begin, being able to compare against the condition immediately after development as a baseline helps improve the quality of long-term management.

When incorporating drone surveying for solar power plants into practical operations, it is important to consider not only how easily sites can be viewed from above but also how survey results will be compared and how they will be incorporated into management documents. At the same time, you need to organize rules for flight operations, safety management, accuracy control of survey results, and the division of responsibilities for on-site verification. If you can properly organize the differences before and after site development, you can reduce rework during construction and make it easier to utilize the data for post-completion maintenance.