Four Basic Principles for Using Drone Surveying to Assess Soil Volumes at Solar Power Plants

In the construction of solar power plants, understanding the volumes of cut and fill at an early stage affects construction planning, schedule management, surplus soil disposal, drainage planning, and post-completion maintenance. Especially on large sites or land with elevation differences, simply walking the site makes it difficult to grasp the overall picture, and assumptions on drawings can differ from the actual terrain. One method used in such cases is drone surveying. By capturing the current conditions from above and organizing earthwork quantities based on topographic data, it becomes easier to compare changes before and after development.

This article explains the fundamentals that practitioners should know when using drone surveying to assess earthwork volumes at solar power plants, presented from four perspectives.

Table of Contents

• Why drone surveying is useful for quantifying soil volumes at solar power plants

• Basic 1 Grasp the current topography in planimetric terms

• Basic 2 Compare the site development plan with existing condition data

• Clarify the three basic concepts of cut, fill, and surplus soil

• Basic 4 Utilize earthwork volume data for construction management and sharing with stakeholders

• Considerations when using drone surveying to estimate earthwork volumes

• Summary

Why Drone Surveying Is Useful for Measuring Earthwork Volumes at Solar Power Plants

When developing a solar power plant, it is not enough to simply level the land. The entire site must be shaped while taking into account the racking layout for solar panels, maintenance access routes, drainage channels, retention basins, slopes, fences, access roads, and locations for electrical equipment. In particular, in forests, miscellaneous land, farmland-converted sites, and sloped terrain, existing topography can be complex, and it can be difficult to accurately visualize the volume of earthwork required for site formation from drawings and cadastral maps alone.

If construction begins without adequately understanding soil volumes, unexpected excavations may occur on site, fill materials may be insufficient, and temporary storage for surplus soil may be difficult to arrange. At solar power plants the sites are often large, and when soil transport distances are long this can affect the overall efficiency of the construction. Moreover, because it relates to drainage planning and slope stability, soil volumes are not merely a matter of quantity management but an important factor affecting the overall construction quality of the plant.

Even with conventional surveying methods, it is possible to understand the terrain through control point surveys and cross-section surveys. However, when you need to inspect a wide area quickly and comprehensively, or visually compare changes before and after site development, drone surveying can be an effective option. By organizing images and positioning data captured by drones into point clouds or terrain models of the current terrain, you can create materials that are easy to use for evaluating cut and fill.

However, drone surveying does not automatically produce accurate volume measurements just by flying. If site conditions, survey objectives, control point placement, flight and imaging plan, data processing, and the assumptions for volume calculations are not organized, the results can become difficult to use in practice. When using it to determine earthwork quantities for solar power plants, it is important to be aware of the plant’s specific site conditions and construction procedures and to clearly define the required accuracy and intended use.

Basic 1 Understand the existing terrain as a surface

The starting point for assessing earthwork quantities is to accurately understand the existing topography before site development. At a planned solar power plant site, the property may contain gentle slopes, valley-like depressions, existing embankments, traces of tree rootstocks, farmland ridges, existing roads, drainage ditches, and slope faces. Walking the site lets you observe local conditions, but there are limits to intuitively grasping which areas are high and which are low across the entire site.

In drone surveying, photographing the entire site from above and analyzing multiple images enables a surface-based understanding of the terrain. This makes it easier to detect terrain undulations, localized steps, and changes in drainage direction that are easily overlooked by point- or line-based surveys alone. At solar power plants, because the panel layout covers a wide area and the effects of earthworks tend to extend across the whole site, a surface-based understanding of the terrain is important.

When assessing the existing topography, first clearly define the survey area. Consider whether to include not only the area where the power generation equipment will be installed but also construction roads, temporary yards, temporary spoil storage areas, drainage facilities, retention ponds, maintenance access routes, and the areas around the fence line. Because earthwork volumes may not be confined to the interior of the development area, it is important to take a somewhat wider view of the area where soil is likely to be moved.

Next, decide how finely you need to capture the terrain. For initial earthwork planning, the main objective is to understand the overall elevation differences and the general balance of cut-and-fill. On the other hand, if you plan to use the data for pre-construction quantity checks or as-built comparisons, you will need more practical control point setup and data organization. If you carry out surveying with an unclear purpose, you may find after capture that the coverage is insufficient or that the reference points needed for comparison are not consistent, so take care.

When assessing the existing topography, how accurately the ground surface can be captured is also important. In areas with tall grass, many trees, stored materials, or standing water, surface data obtained from images can deviate from the actual ground surface. Because planned sites for solar power plants often still have vegetation before site preparation, it is necessary to either cut the grass and check for obstructions before drone surveying or handle the acquired data with care.

Also, at the edges of a site and in areas with large elevation differences, images may be insufficient or the shape of slopes may be reproduced unstably. In earthwork volume calculations, differences near site boundaries and slope areas can affect quantities, so it is desirable to combine ground surveying and on-site verification as needed. Drone surveying is strong for capturing wide areas, but in practice it is important not to complete everything with a single method; instead, supplement methods according to site conditions.

Being able to grasp the existing topography as a surface makes pre-construction planning more concrete. For example, it becomes easier to see where large fills will be required on the site, where extensive cuts are likely, and how existing drainage routes should be handled. In solar power plants, because it is necessary to consider not only generation efficiency but also constructability, maintainability, drainage, and safety comprehensively, visualizing the existing conditions at an early stage is the first step in earthwork quantity management.

Basic 2 Comparing the site development plan and current condition data

After obtaining the existing terrain through drone surveying, comparing it with the site development plan becomes important. Earthwork quantities are calculated from the difference between the existing ground surface and the planned ground surface, so looking at only one of them cannot lead to a correct judgement. By overlaying the current terrain condition with the planned finished ground elevations, the extents of cut and fill become clear.

In solar power plant site development planning, the planned ground level is set by considering panel layout, racking height, pile installation conditions, access path slopes, drainage direction, slope inclinations, and other factors. Rather than making the entire site perfectly flat, some plans reduce earthwork by taking advantage of the natural terrain. Therefore, when estimating earthwork quantities, it is necessary to understand not only simple elevation differences but also the design intent regarding which areas will be developed and to what extent.

When comparing current-condition data with site development plans, it is essential to align the coordinate system and the vertical datum being used. If the datum of the field survey data differs from that of the design drawings, they may appear to overlap visually but actually have discrepancies in position and elevation. For wide sites such as solar power plants, small datum shifts can affect overall earthwork volume calculations. Therefore, it is important to confirm in advance the handling of control points, known points, vertical datum, and local coordinates.

It is also necessary to confirm the format of the data used for comparison. If the design drawings consist only of plan views, the planned surface may not be adequately reproduced. For use in earthwork quantity calculations, information that shows the planned elevations, slopes, slope shapes, and the boundaries of the development area is required. Grading lines that look simple on drawings can result in larger cuts or fills than expected when overlaid on the actual site topography. Using current-condition data obtained from drone surveys makes it easier to verify the validity of the plan at an early stage.

Before site development, it is sometimes useful to compare multiple design options. For example, by slightly changing the panel layout, relocating maintenance access routes, or altering slope treatment, you can examine how cut and fill volumes change. If you can identify in advance areas where earthwork volumes will change significantly, it becomes easier to consider not only construction costs but also the schedule, heavy-equipment planning, surplus soil disposal, and coordination with nearby residents.

However, when comparing current-condition data obtained from drone surveys with a site development plan, it is important not to take the calculation results at face value. Earthwork volume calculations can vary depending on the data extent, point cloud density, the method used to extract the ground surface, the interpolation method, and how boundaries are defined. In particular, using surface data that includes vegetation and structures as-is may result in calculations that are positioned higher than the actual ground surface. When comparing with the development plan, confirm the assumptions used in data processing and, if necessary, carry out on-site verification.

Also, at solar power plants, the site development area and the area for installing generation equipment do not necessarily coincide. Drainage facilities, maintenance access paths, temporary roads, material storage areas, and other locations used only during construction also affect soil volumes. When comparing with the plan, considering not only the terrain at completion but also the temporary soil movements required during construction leads to more realistic soil volume management.

Comparing the existing conditions with the plan is not just for calculating quantities. It is also a process for identifying pre-construction risks. In locations that require large cut excavations, slope stability and drainage treatment become issues. In locations that require large fills, attention must be paid to compaction, settlement, and rainwater flow. By visualizing the existing conditions with drone surveying and overlaying them with the plan, you can increase the information available for pre-construction decision-making.

Basic 3 Organizing the Concepts of Cut, Fill, and Surplus Soil

An important factor in estimating earthwork volumes is the relationship between cut, fill, and surplus soil. “Cut” is the amount of soil removed from the existing ground, “fill” is the amount of soil placed in low areas, and “surplus soil” is the soil left over that cannot be used on site. In the construction of solar photovoltaic power plants, it is often desirable to balance cuts and fills on site as much as possible, but in practice this is not determined simply and depends on factors such as soil type, moisture content, compaction conditions, construction sequence, and temporary storage locations.

By using drone surveying, you can spatially identify areas with predominantly cut and areas with predominantly fill by comparing the existing surface with the planned surface. This allows you to visually organize where soil will be excavated and where soil will be required. In large solar power plants, construction efficiency decreases as the distance over which soil must be moved increases, so not only the volume of soil but also the soil-movement plan becomes important.

When comparing cut and fill volumes, you should not simply subtract the numbers; you need to consider the properties of the soil. Soil in its natural (in-situ) state can change in volume when excavated. Also, when used as fill, compaction can alter its volume. Furthermore, if the material contains stones, roots, debris, soft soils, or high-moisture soils, not all of it may be usable as fill as-is. The volumes obtained from drone surveys are quantities calculated from terrain differences and should be treated separately from the amounts actually available for construction use.

In site development for solar power plants, the extent to which the terrain beneath the panels is leveled also affects the volume of earthwork. Depending on the racking specifications and pile installation methods, it may be possible to install without significantly altering the terrain. On the other hand, access paths, vehicle routes, drainage channels, and areas around equipment foundations may require specific slopes or flatness. Therefore, when assessing earthwork quantities, it is clearer to consider separately the site preparation required for the power generation equipment and that required for access, drainage, and safety.

Handling surplus soil is also important. When cut exceeds fill, it is necessary to consider whether the excess soil will be used on site, temporarily stockpiled, or transported off-site. Even when leaving surplus soil on site, you must choose locations that do not impede drainage, do not place undue load on slopes, and do not hinder future inspections or mowing. Because solar power plants require long-term operation and maintenance after completion, arranging surplus soil solely for temporary convenience during site formation can cause problems for later management.

Conversely, if there is a large amount of fill, the cut material on site may not be sufficient, and it may be necessary to bring soil in from off-site. In such cases, the properties of the imported soil, compaction, drainage, and the potential for settlement must also be considered. If drone surveying can provide an early estimate of the fill volume, it becomes easier to plan material procurement and construction scheduling.

In earthwork volume calculations, the setting of boundary limits has a large impact on the results. For example, whether you target the entire site, only the panel installation area, or include slopes and drainage facilities will change the cut and fill volumes. Also, if data near the boundary is lacking, the calculation results can become unstable. While drone surveys make it easier to capture wide areas, if it is not made clear which area was used as the calculation target, stakeholders may end up with different understandings of the quantities.

In practice, it is important not to calculate earthwork volumes once and be done with them, but to review them in response to changes in the development plan or site conditions. Because the terrain changes at each stage—after clearing, after topsoil removal, after rough grading, and after final grading—conducting drone surveys multiple times as needed makes it easier to track the movement of soil. On particularly large sites, it is difficult to grasp daily changes in earth volumes by visual inspection alone, so the approach of acquiring data from fixed points is effective.

The purpose of organizing cut, fill, and surplus soil is not just to know the quantities. It is to reduce unnecessary movement of soil during construction, prevent drainage and slope problems, and minimize delays to the schedule. Drone surveying is a means of producing the information needed for those decisions. In the site development of solar power plants, organizing earthwork quantity data with attention to how it will be used on site, rather than focusing only on the numbers, is crucial.

Basic 4 Use earthwork volume data for construction management and sharing with stakeholders

Earthwork volume data obtained from drone surveying can be used not only for design and quantity verification but also for construction management and sharing among stakeholders. In solar power plant construction, multiple stakeholders are involved, including the client, designers, contractors, surveyors, land developers, electrical contractors, and management companies. If stakeholders have differing understandings of earthwork volumes, it can affect schedules, costs, the scope of work, and decisions about the final form.

Topographic data obtained from drone surveying has the advantage of making it easy to visually share site conditions that are difficult to convey with plan drawings or quantity tables alone. For example, areas where earth cuts are concentrated, lowlands that require fill, valley terrain that demands attention to drainage, and locations that need slope treatment can be more easily explained using drawings or color‑coded materials. Because the overall picture can be confirmed without everyone walking the site, this also helps improve the accuracy of meetings.

In construction management, it is effective to check changes in earthwork quantities in line with the progress of the work. By acquiring data at stages such as the pre-development existing conditions, after clearing and topsoil treatment, after rough grading, and after final grading, you can compare how much and over what area the terrain has changed. This makes it easier to confirm whether the grading is proceeding according to the design, whether unexpected excavation or fill has occurred, and whether increases or decreases in surplus soil match the plan.

Earthwork quantity data also helps in determining the placement of heavy machinery and the sequence of construction. If you understand the relationship between areas with a lot of cut and areas that require fill, it becomes easier to plan construction routes that minimize the distance soil must be transported. At solar power plant sites, where the site area is large and access roads may be limited, deciding where to move soil from and to is directly tied to construction schedule management. Using terrain information obtained from drone surveys makes it easier to develop work plans while viewing the entire site.

When sharing with stakeholders, how survey data is presented is also important. Specialized point cloud data and numerical values alone do not guarantee that everyone will have the same understanding. In practice, it is effective to organize materials suited to the recipient, such as plan drawings, elevation color-shaded maps, cross-sections, comparison diagrams, and earthwork volume summaries. For the client, clearly show overall quantities and risks; for construction personnel, indicate specific construction extents and elevation differences; for design personnel, provide the information needed for plan changes—materials should be prepared according to their intended use.

It is important to clearly state the assumptions in any documents to be shared. If the survey date, survey area, reference points, vertical datum used, calculation scope, structures or vegetation that were excluded, or the conditions of the planned surface are unclear, it will be difficult to verify the basis for the quantities later. Because earthwork volume data are used for construction decisions, recording when and under what conditions the figures were calculated helps prevent disputes.

At solar power plants, the terrain after site development affects future operation and maintenance. Areas with poor drainage, places where sediment easily accumulates, sections where access routes are difficult to use, and locations where mowing and inspections are hard to carry out can become problems after operations begin. If the terrain is recorded by drone surveys during and after site development, those records can be readily used for post-completion maintenance inspections and retrofit planning.

Furthermore, before-and-after comparison materials are also effective as explanatory materials for the construction. Because they can objectively show what the terrain looked like prior to earthworks and which areas were altered by the construction, they become convenient for internal reporting and stakeholder consultations. Of course, it is assumed that surveying accuracy and the assumptions behind earthwork volume calculations will not be asserted excessively, and that on-site verification and cross-checking with design documents will be conducted as needed.

What matters when leveraging earthwork volume data is converting survey results into information that can be used on site. Even highly accurate data has limited effect if it is presented in a way that site personnel find difficult to understand. Conversely, if the necessary scope, quantities, risks, and construction decision points are clearly organized, drone surveying can help not only with earthwork volume management but also with schedule coordination and quality control.

Precautions When Determining Earthwork Volumes Using Drone Surveys

When using drone surveying to determine earthwork volumes at solar power plants, you should not focus only on convenience but also understand the points to watch for. The first important thing is to clarify the purpose of the survey. Depending on whether you want a rough estimate of earth volumes, to use it for comparing construction plans, for progress management during construction, or for uses close to as-built verification, the required accuracy and the way data are produced will differ.

If surveying is carried out without a clear purpose, the data may be insufficient when you later need to use it. For example, problems can arise such as only part of the earthworks area being captured, reference points being recorded inadequately, vertical references not matching the design drawings, or surface data that still includes vegetation being used as-is. For surveys intended for earthwork volume calculations, it is important to clarify how the deliverables will be used before capture.

Next, care must be taken in handling reference points. In drone surveying, terrain models are created from images and positioning data, but for practical use it is important that they are consistent with the on-site reference. By properly placing reference and check points and organizing them using the same coordinate system and elevation conventions as the design drawings, you will produce data that is easy to use for comparisons. Data with ambiguous references may look tidy but will be difficult to use as a basis for earthwork volume calculations.

Shooting conditions also affect the results. Strong winds, rain, fog, strong backlighting, or times of deep shadow can reduce image quality. Although planned sites for solar power plants are often open, slopes, wooded areas, existing structures, and temporary materials can also cast shadows. If the time of day for shooting, flight altitude, image overlap, and coverage area are not properly planned, the terrain model may have gaps or distortions.

The influence of vegetation should not be overlooked. On undeveloped land, grasses and low shrubs often cover the ground surface. Surface data generated from images may include the tops of that vegetation rather than the bare ground itself. As a result, earthwork volume calculations can differ from the actual ground surface. Because measured heights at the same location can change before and after mowing, it is important to record the site's condition at the time of the survey.

Also, the results of earthwork volume calculations may vary depending on the calculation methods and conditions. Grid size, point cloud processing methods, how boundary lines are defined, settings for excluded areas, and how the design surface is created all influence the results. When sharing earth volumes with stakeholders, you should not simply present the quantities; you must be able to explain which area was covered and according to what criteria the calculations were made.

Drone surveying excels at capturing wide areas, but it does not remove the need for detailed checks. The bottoms of drainage channels, areas around structures, slope edges, under trees, shadows cast by level changes, and narrow passages can be difficult to inspect with drones alone. For critical locations, combining drone data with ground surveying and on-site verification leads to more reliable decisions.

Confirmation of laws and safety management is also necessary. When flying a drone, you need to check the flight location, surrounding environment, third‑party access, power lines, roads, neighboring properties, weather, and so on, and take safety precautions. Planned sites for solar power plants are often in suburban or mountainous areas, but there may also be houses, roads, farmland, transmission facilities, and other structures nearby. It is important not to neglect on‑site checks and safety planning before flight.

Finally, pay attention to the storage and updating of deliverables. Data acquired by drone surveys can potentially be used to compare conditions before and after site development and for future operation and maintenance. If you organize and archive the survey date, coverage, coordinates, processing conditions, calculation results, and correspondence with drawings, it will be easier to verify the construction history later. Because solar power plants are facilities operated over the long term, keeping records of the terrain at the time of development will also assist future inspections and renovations.

Summary

When assessing earthwork quantities for solar power plants, it is important to accurately capture the existing topography before development, compare it with the development plan, and clarify the relationships among cut, fill, and surplus soil. By utilizing drone surveying, it becomes easier to capture wide sites and complex terrain across their surfaces, enabling more efficient earthwork assessment and information sharing among stakeholders.

Especially for solar power plants, not only the panel layout but also maintenance access roads, drainage channels, slopes, temporary construction yards, and spoil disposal areas affect site development planning. Therefore, understanding earthwork quantities should be treated not merely as a simple quantity calculation but as a basis for decision-making that includes constructability, safety, drainage, and maintainability.

On the other hand, drone surveying is not infallible. Vegetation effects, the placement of control points, shooting conditions, data processing, and how the calculation boundaries are defined can all cause the calculated earthwork volumes to vary. For practical use, it is important to clearly define the survey objectives and, while conducting on-site verification and cross-checking against design documents, organize the underlying assumptions before applying the results.

In the development of solar power plants, having an early understanding of topography and earthwork volumes makes it easier to improve the accuracy of construction planning. Using drone surveying can visualize the entire site and make it easier for stakeholders to make decisions based on the same information. If you want to streamline earthwork volume assessment and consistently utilize topographic data from site development planning through construction management, it is advisable to organize site conditions, surveying objectives, required accuracy, legal and safety management, and the intended use of deliverables before considering the use of drone surveying.