6 Perspectives to Reassess Earthwork Quantities at Solar Power Plants Using Drone Surveying

In solar power plant site development work, a wide range of terrain is handled, including cut-and-fill, slopes, drainage, maintenance access roads, and areas around racking foundations. If there is a discrepancy between the quantities estimated in planning and the actual on-site conditions, shortages or surpluses of earthwork, revisions to haulage plans, schedule adjustments, and additional consultations are likely to occur during construction. This is especially true for projects that make use of forestland, idle land, sloped sites, or previously developed land, where assumptions based solely on drawings can make it difficult to grasp actual undulations, existing drainage, or steps beneath vegetation.

Drone surveying is one effective means of capturing the entire power plant site as a surface and comparing the existing terrain with the planned surface. However, simply photographing and generating point clouds or terrain models does not necessarily make them directly usable for revising earthwork quantities. You need to clarify which area will be compared, how control points will be handled, how to exclude unnecessary objects, and how to separate design changes from as-built verification.

In this article, aimed at practitioners searching for information on "solar power plant drone surveying," we explain six perspectives to verify when reviewing site development quantities.

Table of Contents

• Why Drone Surveying Is Useful for Revising Earthwork Quantities

• Viewpoint 1: Align the reference standards of the existing topography and the planned topography

• Viewpoint 2 Confirm the boundary between cut and fill across the surface

• Perspective 3: Do not overlook differences in quantities around slopes and drainage.

• Viewpoint 4: Check the panel layout and its interface with maintenance roads

• Perspective 5: Organize quantities by excluding unwanted items and vegetation

• Viewpoint 6: Preserve outcomes as records usable in consultations

• Summary: When reviewing earthwork quantities, an early-stage, area-wide understanding is important.

Why Drone Surveying Is Useful for Reviewing Earthwork Quantities

The quantity of earthworks for a solar power plant is not simply a matter of how many cubic meters of soil are moved. In reality, it is determined by a combination of factors: the overall elevation differences across the site, racking layout, drainage directions, slope gradients, maintenance road gradients, boundaries with surrounding land, and interfaces with existing structures. Even if the drawings are well organized, once you get on site you may find the ground is more undulating than anticipated, existing embankments are uneven, watercourses have shifted, or there are level changes near the boundaries.

In conventional surveying, methods that set representative points and cross-section lines to capture the terrain are often used. While this approach is easy to manage, it can fail to pick up local undulations between cross-section lines or small steps at the edges of the development area. At solar power plants, where sites are large and many rows of panels are laid out, even small elevation differences can affect mounting heights, drainage slopes, and vehicle access routes.

In drone surveying, the entire site is photographed from the air and point clouds and surface models are created through photo analysis and similar methods. This makes it easier to compare the terrain before, during, and after earthworks on a surface basis. By viewing the terrain as surfaces rather than points, it becomes easier to identify early the areas where excavation is concentrated, where fill may be insufficient, and where drainage slopes may become unnatural.

When reviewing earthwork quantities, the important point is not to否定 the design quantities but to confirm the assumptions behind those quantities against actual site conditions. If the terrain data used at the design stage is outdated, if clearing or temporary works have progressed before earthworks, or if the terrain has changed near the boundary with adjacent land, the original quantities can deviate from the actual construction conditions. Using drone surveying allows you to organize those deviations as data with positional information rather than as a feeling based on drawings.

In solar power plant construction, multiple stakeholders handle the same site, including earthworks, foundations, racking, electrical work, fencing, and drainage. When sharing the results of a review of earthwork quantities, explaining only with a plan view can make it difficult to convey which location is being discussed. Using orthophotos and terrain models obtained from drone surveys allows sharing the area in a view similar to on-site photographs, making it easier to align the understanding of clients, designers, contractors, and managers.

However, there are also caveats to drone surveying. If capture conditions are poor, reference points are insufficient, vegetation or materials are treated as the ground surface, or the analysis scope is unclear, it becomes difficult to use the results as a basis for quantity calculations. If you intend to use it to review earthwork quantities, it is important to decide from the outset the accuracy, scope, reference standards, and recording methods required for quantity comparison.

Perspective 1 Align the reference levels of the existing terrain and the planned terrain

When reviewing earthwork quantities, the first thing to check is whether the reference standards for the existing terrain and the planned terrain are aligned. No matter how detailed the point clouds you create are, if the coordinate system or vertical datum are mismatched, the comparison results for cut and fill volumes become hard to trust. In solar power plant site development, the overall elevation differences across the site directly affect quantities, so the task of aligning the numerical assumptions is indispensable.

Existing terrain refers to the ground surface before or during earthworks captured by drone surveying. In contrast, planned terrain is the intended finished surface created based on design drawings and development plans. By comparing the two, you determine where to cut and where to fill. However, if the existing data are managed in on-site coordinates and the planned data are created using a different reference, horizontal positions and elevations can be misaligned when overlaid.

If quantities are calculated while the reference controls are misaligned, areas that are not actually cuts may be displayed as cuts, and fill volumes may appear overstated. Positional offsets are especially likely to show up as quantity differences in places with slopes—such as site edges, slope faces, drainage ditches, and maintenance roads. When reviewing earthwork quantities, you should first check the control points, elevation datum, coordinate orientation, drawing units, and data origin, and bring them into a comparable, aligned state.

When acquiring current conditions with drone surveying, installing clear ground control points and check points on the ground makes it easier to verify the positional accuracy of the deliverables. Control points are used for analysis, while check points are used to confirm how well the analysis results match the field. If everything is given the same role, verification of the deliverables can become insufficient. When using the data for earthwork quantities, it is important to separate analysis points and verification points and to inspect whether the deliverables conform to the on-site references.

Also, attention must be paid to the planned terrain data. At the design stage, quantities are sometimes calculated based on simplified terrain surfaces or representative cross-sections. In such cases, simply comparing them with the detailed existing surface obtained from drone surveys can produce differences caused by the coarseness of the planned surface. This is not a problem with the site, but a difference in how the comparison targets were created. When reviewing quantities, you need to check not only the level of detail of the existing data but also the degree of granularity at which the planned data were produced.

The boundary of the development area is also part of the criteria. In solar power plants, the areas where power generation equipment is sited, the areas to be developed, remaining green spaces, regulating ponds, drainage channels, temporary yards, and material storage areas may coexist. If it is ambiguous which areas are included in quantity calculations and which are excluded, the meaning of the numbers can become unclear later. Before starting comparisons, it is important to clearly identify on the drawings the areas subject to development, the areas to be excluded, and the areas assigned to separate construction works.

At this stage what is important is not to immediately look at the quantity results, but to put the premises for comparison in order. Confirm whether the criteria for the current conditions and the plan align, and if they do not, identify the causes before proceeding to quantity calculations. When the criteria are aligned, it will be easier in later discussions to explain which data, which scope, and which criteria were used for the comparison.

Viewpoint 2: Verify the boundary between cut and fill on a surface

When reviewing earthwork quantities, the next important task is to verify the cut-and-fill boundary across the surface. Cut and fill volumes affect the cost and schedule of site formation work, but the boundary cannot always be understood from the lines on the drawings alone. The distribution of areas that should be cut or filled can change depending on the site's topography, existing level differences, drainage direction, and the elevation relationships with the surrounding ground.

In solar power plants, it may be necessary to shape the terrain within a certain slope range to arrange panels efficiently. However, this does not mean making everything perfectly flat. Gentle slopes may be retained to allow natural drainage, or existing topography may be used to reduce the amount of grading. Therefore, the boundary between cut and fill cannot be determined simply by cutting high spots and filling low spots.

When you overlay the as-built topography produced by a drone survey onto the planned topography, you can spatially identify where the existing surface is higher than the plan and where it is lower. This reveals areas where cut sections continue in bands, where localized fills are required, and where the planned and existing surfaces largely coincide. Undulations in the terrain that are hard to discern from cross sections alone become easier to understand when viewed as surfaces.

Near the boundary between cut and fill, reviewing quantities is particularly important. Even a small shift in the boundary can change the balance between cut and fill. When planning to reuse soil on-site, the difference between cut and fill volumes affects whether material must be hauled off-site or brought in from off-site. Changes in the balance of earthwork volumes also impact hauling plans, temporary stockpiles, construction sequencing, and heavy-equipment movement paths, so it is advisable to confirm them early.

Also, the boundary between cut and fill can change during construction. Areas that were planned as fill in pre-development plans may prove to be higher than expected when the ground surface is checked after clearing or topsoil stripping. Conversely, if existing fill is found to be loose and needs to be removed, the amount of fill required may increase. Conducting drone surveys not only before development but also during the development allows you to check differences from the plan in stages.

When reviewing quantities, it is important not to judge solely by the simple total. Even if the combined value of cut and fill volumes does not change significantly, the construction plan will change if the locations where they occur change. If cut increases on the north side of the site and fill increases on the south side, the hauling distance for soil may become longer. On sloping ground, whether soil generated at the upper part is moved to the lower part or temporarily stored for use elsewhere affects safety and efficiency.

When using drone survey outputs, your understanding deepens if you review representative cross-sections as well as color-coded cut-and-fill maps and difference maps. By grasping overall trends from surface data and confirming specific elevation differences on cross-sections, it becomes easier to explain the causes of quantity discrepancies. In particular, when discussing matters with the client or designers, it is important not only to state that quantities have increased or decreased, but also to show the locations and reasons—for example, that the existing ground is higher than planned in this area, resulting in increased cutting here.

Viewpoint 3: Do Not Overlook Quantity Differences Around Slopes and Drainage

When reviewing earthwork quantities for a solar power plant, if you only check the flat areas you can overlook quantity differences around slopes and drainage. The flat areas where the power generation equipment is arranged tend to attract attention, whereas the site perimeter, slopes, drainage ditches, detention ponds, and the sides of maintenance roads are places where quantities are easily split into many small items, and differences between the design and the actual conditions are likely to become problems later.

Slopes are related to the stability of earthworks and the impact on adjacent land. If slopes become higher than planned, gradients steeper, or edge treatments change, it affects not only the volume of earthworks but also safety measures and maintenance. In solar power plants, widening flat areas to secure the installation surface for generation equipment can result in the perimeter slope being larger than expected. This change is difficult to discern from plan views alone, and areal verification by drone surveying is effective.

Drainage is also important. On a power plant site, to properly convey stormwater, plan on-site drainage, perimeter drainage, catch basins, detention basins, and the like. When reviewing earthwork quantities, if you overlook the excavation volumes of drainage structures or the surrounding grading, additional excavation or fill may be required later. Insufficient slope in drainage channels, or adjusting drainage directions to match the existing topography, will also affect quantities.

With drone surveying, because the entire site’s terrain can be viewed as a continuous surface, it becomes easier to understand the relationship between slopes and drainage. For example, you can confirm as elevation differences the flow of water from flat areas toward the perimeter, places where water tends to collect at the toe of slopes, locations where drainage crosses maintenance roads, and spots that are lower toward adjacent properties. When reassessing earthwork quantities, you need not only the soil volume figures but also the perspective of where the water will flow.

Especially for solar power plants installed on forested or sloped land, insufficient drainage planning after earthworks can lead to sediment runoff and erosion. Utilizing the existing terrain to reduce earthwork quantities is effective, but if there is no outlet for drainage, the maintenance burden can increase. By using drone surveys to confirm existing gullies, waterways, and low-lying areas and comparing them with the planned topography, you can reassess the project from both the quantities and drainage perspectives.

When checking quantities related to slopes and drainage, it is also important to clearly separate the areas in question. If you mix the quantities for site leveling, slope shaping, excavation for drainage facilities, and backfilling around structures, it becomes difficult to tell where discrepancies arose. Even when using terrain data obtained from drone surveys, dividing the areas for comparison and, when necessary, organizing quantities by zone will make negotiations and decision-making easier.

Furthermore, slopes and areas around drainage are locations that are easily affected by vegetation and temporary materials. If weeds are tall at the time of imaging, analysis may make the ground surface appear higher. Likewise, if materials or sediment are temporarily placed in drainage ditches, they should not be treated as true terrain. For data used to review quantities, it is necessary to verify on-site conditions and exclude items that are not the ground surface.

Viewpoint 4: Check the panel layout and its interaction with maintenance roads

Earthwork quantities are not determined by topography alone. In a solar power plant, the required extent and height of earthworks change depending on panel layout, racking foundations, access roads for management, maintenance and inspection spaces, fences, electrical equipment, and drainage facilities. Therefore, when reviewing earthwork quantities, it is important not to look only at the terrain model but to check the relationship with equipment layout.

In the area where panels will be installed, it is necessary to distinguish between parts that can be accommodated by adjusting the mounting structure's height and parts that require ground leveling through earthworks. In locations with significant ground undulation, relying solely on adjustments on the racking side can affect constructability and maintainability. Conversely, trying to level everything through earthworks can lead to excessive cut-and-fill volumes. Using drone surveying to capture the current topography makes it easier to decide how far to address with earthworks and where to make adjustments on the equipment side.

The interface with access roads must not be overlooked. At solar power plants, internal roads and pathways are necessary for delivery of materials during construction, for maintenance inspections after completion, and for emergency response. If road gradients are too steep, they can impede vehicle traffic and affect drainage flow. Even if there appears to be no problem on the design drawings, when overlaid with the existing terrain the cutting and filling required for the road sections can be larger than anticipated.

By overlaying planned panel rows and road alignments onto orthomosaic images and terrain models obtained from drone surveys, you can verify how variations in grading quantities affect equipment layout. For example, it becomes easier to detect early issues such as panel row ends being too close to slopes, increased embankment required at road tie-in sections, drainage channels being too close to racking foundations, or fence perimeters being close to slope shoulders. This is a perspective that is difficult to obtain from quantity calculations alone.

Also, if you change the planned surface to reduce the amount of earthworks, it can affect the layout of the power generation equipment. Raising or lowering the ground surface can alter the appearance of the mounting racks, the impact of shadows, the slope of inspection walkways, and the direction of drainage. Making a decision to reduce earthwork based solely on quantities can cause construction or maintenance problems in other areas. Using drone survey results makes it easier to compare quantities, layouts, and access routes on the same screen.

It is also useful for checks during construction. If drone surveying is carried out once land development has progressed, you can verify whether the planned panel layout and maintenance access roads match the actual on-site condition. Discovering problems after completion can lead to significant rework, but if differences can be identified during intermediate stages, they may be resolved with localized adjustments. This is especially useful at large power plants, where walking the site to check all elevation differences takes time, so an aerial, area-wide overview is helpful.

Coordination checks also require information sharing among stakeholders. The site-development team examines earthwork quantities and slopes, the electrical team looks at wiring routes and equipment foundations, and the maintenance team assesses inspection routes and safety. By discussing based on the same topographic data, it becomes easier to prevent each party from making decisions on different assumptions. Drone surveying can be used not only to reassess earthwork quantities but also to verify the overall consistency of the layout plan.

Perspective 5: Organize quantities by excluding unwanted items and vegetation

When revising earthwork quantities using drone surveys, something to watch out for is including items that are not part of the ground surface in the quantity calculations. Candidate sites for solar power plants and sites under development may contain vegetation, logging residue, temporarily stockpiled soil, materials, heavy equipment, temporary roads, steel plates, drainage materials, etc. If these are included in the terrain data, they can make the ground appear higher than it actually is or be calculated as unnecessary volume.

The influence of vegetation is a particularly important point of caution. In areas with tall grasses or remaining shrubs, points obtained from photo analysis may indicate the surface of the vegetation rather than the ground. If compared with the planned terrain in that state, the existing ground surface can appear higher, resulting in an increased calculated cut volume. When using this for a pre-construction quantity review, it is advisable to check the timing of the imaging and the on-site vegetation conditions, and, if necessary, conduct the survey after clearing or mowing.

Temporary stockpiles also complicate quantity reconciliation. During site development, cut soil is sometimes temporarily placed on-site. If that condition is recorded by drone surveying, the temporary stockpiles are treated as terrain, which affects comparisons between the existing surface and the planned surface. While this is useful when you want to determine the quantity of the temporary stockpiles themselves, if you want to check the finished formation surface or verify surpluses or shortages of excavated material, the temporary stockpiles need to be handled separately.

Materials and heavy machinery are the same. Even if they look small from above, they appear as objects with height in point clouds and terrain models. If materials are included within the area used for quantity calculations, local discrepancies arise and the distribution of cut and fill becomes unnatural. Before capturing imagery, organize materials as much as possible, and record items that cannot be moved as exclusions when compiling the deliverables so they are easier to identify later.

Methods for extracting the ground surface are also important. Point clouds obtained from photo analysis contain a variety of points, such as ground, vegetation, structures, vehicles, and materials. When using them for earthwork quantity calculations, you must select the points to be treated as the ground surface, remove unnecessary points, and then construct the terrain surface. Because relying solely on automated processing can produce classifications that do not match actual site conditions, locations that significantly affect quantities require visual inspection and cross-checking with site photos.

Also, in the development of solar power plants, the handling of topsoil also affects quantities. When topsoil is stripped and managed separately, the difference between the existing terrain and the planned terrain alone may not accurately represent the actual earthwork quantities. The quantity expressed as a difference in surface elevation is a separate issue from the types of soil to be handled and the treatment methods used during construction. While drone survey results are effective for grasping terrain differences, judgments regarding soil properties, topsoil, disposal, and reuse need to be organized together with on-site inspections and construction planning.

To avoid the effects of unwanted objects and vegetation, it is important to verify at three stages: before data capture, during analysis, and during quantity reconciliation. Before data capture, tidy the site as much as possible; during analysis, exclude non-ground surfaces; and during quantity reconciliation, record the areas excluded and the reasons for those decisions. Keeping this workflow makes it easier to explain the basis for any quantity discrepancies later.

Perspective 6: Preserve results as records that can be used in consultations

Reviewing earthwork quantities does not end with verification conducted only within the site. If discrepancies in quantities arise, it may be necessary to hold consultations among the client, the designer, the contractor, and the manager. What is important in such cases is to be able to explain not only the numbers but also at which point in time, over what area, using which data, and how the comparison was made. The results of drone surveys become more valuable when organized as records that can be used in those consultations.

First, what you should record is the date of photography and the on-site conditions. Whether it was before site development, after tree felling, after topsoil stripping, or during site development changes the meaning of the quantities. Even on the same parcel, the terrain can change depending on the timing of the photography. If you record the work stage at the time of photography, the weather, the condition of the ground surface, and the presence of temporary soil stockpiles or materials, it will be easier to interpret the results later when you review them.

Next, clarify the survey scope and the excluded areas. On the site of a solar power plant, non-development green spaces, existing waterways, adjacent properties, temporary yards, access roads, and the like may appear in the same images. If it is unclear which areas were included in the quantity calculations and which were not, the numbers can be misleading on their own. In the deliverable maps, it is desirable to clearly indicate the quantity calculation area, the excluded areas, and the areas requiring separate verification.

Quantitative results should be organized so that trends by location are clear, not just the total amounts. Overall cut volume, fill volume, and net volume are of course important, but those alone can sometimes make it difficult to reach construction decisions. If you classify and organize which areas are experiencing increased cutting, which areas are lacking fill, and how much difference exists on slopes and around roads, it becomes easier to consider appropriate countermeasures.

In consultation materials, combining orthophotos, difference maps, representative cross-sections, and the approach to quantities makes the message easier to convey. Orthophotos help intuitively show on-site positional relationships, and difference maps help indicate elevation differences compared to the plan. Representative cross-sections are effective for explaining specific height differences. By combining these elements, you can share not just tables of numbers but where and what is happening on site.

Also, the way deliverable data are managed is important. When reviewing earthwork quantities, multiple datasets are generated: existing-condition data, planned data, comparison results, revised plans, and meeting materials. If file names and storage locations are not organized, it becomes unclear which of these is the latest deliverable. It is important to manage files with names that indicate the capture date, project stage, coverage area, and purpose so that stakeholders can all refer to the same data.

The results of drone surveying also pertain to later accountability. Once site development work progresses, the pre-development terrain can no longer be verified on site. That is precisely why it is important to keep records from before and during the work. Even if it later becomes necessary to check quantity discrepancies or the scope of construction, having a chronological record makes it easier to determine at which stage the terrain changed.

When compiling results for consultations, it is also important to avoid overly definitive wording. The quantities obtained from drone surveys can vary depending on flight conditions, analysis methods, ground surface extraction, and the handling of control points. Therefore, rather than presenting these figures as the single correct answer, it is more appropriate in practice to explain that they are the results of comparing the current terrain and the planned terrain obtained under these conditions. By clarifying the conditions and then presenting the comparison results, stakeholders will find it easier to make judgments.

Summary Revising earthwork quantities requires an early-stage, area-wide understanding.

When reviewing earthwork quantities for a solar power plant, it is important to accurately understand the differences between the existing terrain and the planned terrain. For plants on large sites, drawings and a few cross-sections alone may not fully capture terrain undulations, the boundaries between cut and fill, or subtle differences around slopes and drainage. By using drone surveying, you can record the entire site as a surface and more easily identify, at an early stage, locations where quantity discrepancies are likely to occur.

However, drone surveying does not automatically produce correct earthwork quantities simply by capturing images. It is necessary to align the reference standards between the existing conditions and the design, clarify the comparison area, confirm the distribution of cut and fill, treat slopes and drainage areas separately, verify the interfaces with panel layouts and maintenance roads, and exclude unwanted objects and vegetation. Only by organizing these items will the results become usable for reviewing earthwork quantities.

What's particularly important is not to view quantities simply as numbers. Total cut and fill volumes are important, but in practice you need to check where the variances occur, how they affect the construction sequence, whether there are issues with drainage or slope stability, and whether they conflict with equipment layout. By using drone survey results as terrain differentials, orthophotos, cross-sections, and area-by-area organization, it becomes easier for stakeholders to share an understanding of on-site conditions.

Also, keeping chronological records of before, during, and after site preparation is useful for later consultations and maintenance management. At solar power plants, once site preparation progresses, it becomes difficult to verify the original terrain on site. That is precisely why conducting drone surveys at an early stage and recording the current terrain helps improve the accuracy and explanatory power of quantity reviews.

To smoothly revise site preparation quantities, a surveying system that is easy to use on site and a way of organizing results that can be easily shared with stakeholders are indispensable. When carrying out solar power plant site preparation, point cloud acquisition, terrain comparison, and pre- and post-construction records as a single workflow, it is important to organize in advance the site conditions, required accuracy, comparison scope, and intended use of the results, and to consider an approach to drone surveying that can be used in quantity negotiations.