4 Drone Survey Checks Used for Pre-Estimate Surveys of Solar Power Plants

In pre-estimate surveys for solar power plants, it is important to quickly grasp the site and organize the scope of construction and inspection, site access and delivery conditions, terrain undulations, the layout of existing equipment, and the relationship with the surrounding environment. If estimates are prepared using only drawings and past documents, level differences, poor drainage, overgrown weeds, restrictions on access routes, and narrowness around racking or fences that are only discovered on site may later come to light, leading to additional checks and rework. A pre-estimate survey using drone surveying is useful for this purpose. By confirming the current conditions from the air and combining photos, videos, data usable for generating plan views, and point clouds and elevation information that help understand the topography, it becomes easier to concretize the estimating conditions.

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Table of Contents

• Purpose of using drone surveying in pre-estimate surveys

• Check 1: Organize on-site conditions and the flight area in advance

• Check 2: Verify that terrain and site development conditions can be incorporated into the estimate.

• Check 3: Comprehensively identify existing equipment and surrounding obstacles

• Check 4 Align assumptions for deliverables and estimate conditions

• Summary

Purpose of Using Drone Surveying in Pre-Estimate Investigations

The purpose of using drone surveying in pre-estimate inspections for solar power plants is not simply to take attractive aerial photographs. The real objective is to organize the on-site conditions required for the estimate as objectively as possible. Solar power plants often cover large sites where changes in terrain and equipment layout are difficult to grasp at a glance. Walking the site for visual checks alone can make it hard to understand the overall slope of the site, the connectivity of access paths, drainage routes, the relationship to surrounding roads, and the extent of slopes and embankments. By combining drone surveying, you can obtain an overview of wide-area relationships that are easy to overlook during ground checks, increasing the amount of decision-making material available for the estimate.

At the pre-estimate stage, detailed design and construction planning may not yet be finalized. Therefore, the role of surveys is not simply to definitively determine exact quantities, but to clarify the assumptions behind the estimate and to identify locations that require additional investigation as early as possible. For example, even if only a portion of a power plant is scheduled for renovation, you cannot produce a realistic estimate unless you check access routes, temporary storage/staging areas, turning space for work vehicles, and potential interferences with surrounding equipment. Capturing the overall picture with drone surveying makes it easier to consider not only the target scope but also the surrounding conditions that will affect the work.

In a solar power plant, multiple elements are scattered across the site, including panels, mounting structures, junction boxes, collection equipment, maintenance walkways, drainage facilities, fences, gates, slopes, and regulating ponds. During pre-estimate surveys, it is necessary to grasp their positional relationships and to check ease of construction and inspection, whether materials can be delivered, worker movement routes, and access conditions for heavy machinery and vehicles. If the site is organized using aerial photographs and survey data, stakeholders can hold meetings while looking at the same current conditions, reducing misunderstandings.

However, drone surveying is not without limitations. In areas with dense vegetation the ground surface can be difficult to see, and details beneath panels, in building shadows, under trees, or alongside fences may not be fully discernible from aerial photographs alone. Surveying accuracy also varies depending on the flight plan, imaging conditions, how control points and check points are handled, processing methods, and site conditions. Even when using it for pre-estimate surveys, it is important to understand the limits of the acquired data and, where necessary, carry out ground verification and cross-checks with existing drawings.

What you should be especially careful about is treating drone surveying results as finalized quantities as they are. At the pre-estimate stage, you need to decide the required accuracy and the scope of verification according to the purpose. Whether you use it for a rough estimate, to check the scope of repairs, for purposes close to determining earthwork quantities, or to review a maintenance management plan, the required data granularity will vary. If you proceed with only capturing images while the purpose is unclear, problems are likely to occur later, such as “the necessary areas were not photographed,” “elevation information is insufficient,” or “there is no basis for comparison.”

Additionally, the operation of unmanned aircraft requires prior confirmation of conditions such as airspace, flight methods, consent from facility managers, and consideration for nearby residents and stakeholders. Even if a power plant is located in mountainous or suburban areas, whether flight is permitted and what procedures are necessary can vary depending on conditions such as overhead lines, roads, houses, critical facilities, areas around airports, densely populated areas, and airspace reserved for emergency use. In pre-estimate surveys, it is essential to plan not only the survey content but also to confirm the conditions that will allow safe flight.

Therefore, when using drone surveying for pre-estimate surveys of solar power plants, organizing the work around four main checkpoints—site conditions, terrain, existing equipment, and deliverables—makes it easier to put into practice. Below, the four checks to be confirmed in the pre-estimate survey are explained in a workflow that is easy for field personnel to use.

Check 1 Organize local conditions and the flight area in advance

The first things to check are the on-site conditions and flight area that will allow drone surveying to be conducted safely and effectively. Solar power plants are installed in a variety of locations such as suburban and mountainous areas, reclaimed land, idle land, and coastal areas. If there are houses, roads, railways, power lines, communications equipment, woodlands, farmland, waterways, etc. nearby, they will affect flight plans and the photography/survey coverage. In pre-estimate surveys, it is important to first organize the site boundaries, the area to be surveyed, obstacles that may interfere with flight, and locations that can be used for takeoff and landing.

When checking site conditions, it's common to assume that photographing the entire site is sufficient, but the information needed for estimates is not limited to the site itself. When considering material delivery and work planning, access roads, gates, off-site staging areas, road width, curves, gradients, and surrounding traffic conditions are also relevant. Even for renovation work within a power plant, the required personnel and sequencing change depending on where materials are brought in, where vehicles are turned for maneuvering, and which passageways workers use. When determining the scope of a drone survey, you need to consider not only the work area but also the surrounding areas related to the work.

When organizing the flight area, it is important to align the survey objectives with the imaging coverage. For example, for estimates of panel replacement or racking repair, you will need the location of the target section, surrounding access paths, areas suitable for temporary staging, and access routes for work vehicles. For estimates of drainage improvement, you need to check low-lying areas, places where water tends to accumulate, ditches and drainage inlets/manholes, the slope of embankments, and the direction of runoff to adjacent land. For estimates of mowing or weed-control measures, it is necessary to understand the extent of vegetation growth, the conditions beneath the panels, along fences, on slopes, and the condition of maintenance access paths. Because the information required differs by purpose, it is essential to clarify before flight "what the survey is intended to determine."

On-site safety checks are also important. At solar power plants, overhead lines and guy wires, tall trees around the site, steep slopes, fences, monitoring equipment, and communications equipment can become obstacles to flight. Especially during pre-estimate surveys, many sites are being visited for the first time, so you may have to significantly change your flight route after arriving on site. By checking maps and existing documents in advance and visually confirming obstacles after arrival before flying, you can more easily avoid missed shots and hazardous flights.

In flight planning, confirm relevant laws such as the Aviation Law, the facility manager's rules, and the safety management standards within the power plant. The vicinity of airports, airspace at or above 150 m (492.1 ft) from the ground or water surface, airspace over densely populated areas, emergency-use airspace, night flights, beyond-visual-line-of-sight flights, and distances to people or property affect whether flight is permitted and whether permissions or approvals are required. Even within the power plant site, there may be restrictions depending on surrounding conditions, so confirming these at the pre-estimate survey stage leads to safe operations.

Also, at solar power plants, reflections on panel surfaces and the repetition of identical equipment can make it easy to misidentify locations from photos alone. To make organization easier and to allow later verification of which section was photographed, which row was the target, and which aisle was used as a reference, decide in advance how to divide the photographic coverage. When on-site images are shared among estimators, construction personnel, management companies, and clients, mapping section names, aisle names, and equipment numbers to the photos will reduce confusion during meetings.

Weather and lighting as flight conditions also affect the quality of pre-estimate surveys. Strong winds or rain make flying difficult and cause instability in the quality of captured images. When the sun is at a low altitude, shadows lengthen and areas beneath panels and walkways can become hard to see. Conversely, during periods of strong sunlight, reflections can be prominent and affect image interpretation. In pre-estimate surveys, it is important to be mindful not only of how easy it is to shoot but also of conditions that make it easy to interpret the site later.

What is important in pre-estimate surveys is organizing site conditions as "information usable for estimating." Rather than merely finishing after photographing, you need to record which areas were flown, which areas could not be flown, and which locations require ground verification. For example, if there are areas obscured by tree shadows, areas under panels that could not be checked, or locations avoided for shooting due to flight restrictions or safety reasons, these must be reflected in the conditions for the estimate. Making clear which parts have not been confirmed will reduce additional work and misunderstandings after the estimate.

Check 2: Verify terrain and site-preparation conditions in a form that can be reflected in the estimate

The second check concerns terrain and site-development conditions. In estimates for solar power plants, local elevation differences, slope conditions, drainage flow, slope-face geometry, access-route gradients, and the roughness of the ground surface greatly affect the scope of work and workability. By using drone surveying, you can create data—not just aerial photographs but also analytical outputs—to capture terrain trends. In pre-estimate surveys, it is important to verify the terrain information while being mindful of how much of it will be reflected in the estimate.

At solar power plants, even if the original construction drawings are available, the current conditions may have changed after operations began due to rainwater runoff, sediment deposition, scour, overgrowth of weeds, ruts in access routes, slope deformations, and the like. This is especially true for plants on slopes or in mountainous areas, where the terrain as planned at construction may not fully match the present topography. If construction quantities and the scope of work are estimated without confirming current conditions in a pre-estimate survey, additional work may be required on site. By using drone surveying to capture the current terrain, it becomes easier to identify areas of concern at the estimating stage.

When checking the topography, the first thing you want to look at is the overall elevation difference of the power plant. Knowing which parts of the site are high and which are low makes it easier to consider water flow, access for work vehicles, the burden of transporting materials, and safety measures during construction. Even power plants that appear flat often have gentle slopes that concentrate rainwater in certain areas. Even when aerial photos make judgment difficult, having data that summarizes elevation differences makes it easier to identify low-lying areas and spots prone to water accumulation.

Next to check are the conditions of slopes and embankments. At solar power plants, there are often embankments along the site perimeter or at plot boundaries. When estimating mowing, repairs, drainage improvements, erosion countermeasures, fence repairs, and similar work, the slope height, gradient, length, and the ease of securing work scaffolding are important. Aerial surveying with drones provides an overview of the entire slope, making it easier to identify continuous deformations and watercourses that are hard to see from the ground. However, on slopes with dense vegetation it is difficult to accurately capture the ground surface, so ground inspections should be combined as needed.

The slope and surface condition of access routes also affect estimates. Conditions—such as whether work vehicles can enter, whether materials can be transported, whether the ground becomes muddy after rain, and whether the access width is sufficient—relate not only to construction costs but also to the schedule. Drone surveying to grasp the continuity of access routes and trends in slope makes it easier to consider delivery plans and how to divide work areas. This is especially true at large power plants, where access routes can branch complexly and information collected by walking the site alone can make it difficult to organize the overall spatial relationships. Aerial records are effective as shared reference material for estimators when rechecking the site.

Confirming drainage conditions is also essential. At solar power plants, places where rainwater accumulates, clogged drainage channels, insufficient slope in side ditches, inflow of sediment, and runoff from slopes all affect maintenance and repair work. If a pre-estimate survey overlooks the possibility of poor drainage, additional measures may be required after construction. Drone surveys can provide an aerial view of low-lying areas, differences in ground surface color, uneven vegetation, traces of sediment deposition, and the locations of drainage facilities. Combining this with on-the-ground checks makes it easier to organize the assumptions for drainage improvement estimates.

When using topographic data for estimates, it is necessary to make the assumptions about accuracy clear. Data produced by drone surveys vary in quality depending on flight altitude, image overlap, the placement of ground control points, processing conditions, and how the ground surface appears. In pre-estimate surveys, it is important to distinguish whether the data will be used for rough approximations or whether near-design-level accuracy is required. Especially when using the data for earthwork volume calculations or slope evaluations, you must confirm that the required accuracy is met, that it is consistent with on-site reference points, and how the effects of vegetation and structures will be handled.

Comparing with existing drawings is also important. If grading plans, as-built drawings, past survey maps, or equipment layout drawings are available, cross-checking them with the conditions obtained from a drone survey makes it easier to find discrepancies. Discoveries such as a passage shown on the drawings being difficult to use on site due to vegetation, a drainage channel on the drawings that is nearly filled with sediment, or locations outside the planned grading area that are likely to be affected by work will improve the accuracy of estimates. In pre-estimate surveys, it is important not to rely solely on current-condition data, but to make judgments by overlaying it with existing materials.

When checking topography and site development conditions, it is also necessary to articulate in words the risks that could affect estimates. For example, phrase them so they can be reflected in the estimating conditions, such as: "Part of the target area has elevation differences, which may create a significant burden for manual carrying," "Slope faces are difficult to verify at the ground surface due to vegetation, so detailed quantities will be finalized after on-site confirmation," and "There may be sediment accumulation around drainage facilities, requiring additional checks for cleaning and repair." To leverage drone surveying results in estimates, organizing findings into forms usable for estimation decisions is more important than the data itself.

Check 3 Thoroughly identify existing equipment and surrounding obstacles

The third check is existing equipment and surrounding obstructions. In a pre-estimate survey for a solar power plant, it is necessary to comprehensively verify not only the panels and racking but also cables, junction boxes, collection equipment, substation equipment, monitoring equipment, fences, gates, drainage facilities, maintenance walkways, perimeter roads, surrounding trees, and overhead lines. Using drone surveying allows an overhead view of equipment layouts and the positional relationships of obstructions, making it easier to organize the construction scope and working conditions.

When checking existing equipment, it is important not only to look at the target equipment but to include surrounding equipment that may interfere with the work. For example, even when estimating a partial replacement of panels, you need to check whether there is sufficient workspace around the target row, whether it is close to an aisle, whether there is a place to store materials, and whether there are obstacles under the mounting frames. For work involving junction boxes or wiring routes, the distances between equipment, the location of aisles, the routing of existing cables, and the positional relationships of equipment that must not be touched during work are important. Organizing the overall layout with a drone survey makes it easier to concretely envision the scope of work on site.

Because solar power plants consist of consecutive rows of similarly shaped panels, it can be difficult to identify the target locations using only ground-level photographs. Having planar materials such as aerial photos or orthophotos makes it easier to mark the target area or assign numbers to each section. In pre-estimate meetings, it is important to clarify "which row and which range will be the target" and "how far the scope of this estimate will extend." Overhead imagery obtained from drone surveys allows stakeholders to point to the same locations while reviewing, reducing discrepancies in understanding the target scope.

Understanding surrounding obstacles also affects the accuracy of estimates. If there are tall trees around the perimeter of a power plant, this can affect whether felling or pruning is required, the impact of shadows, safety during flight, and the access conditions for work vehicles. If overhead lines or utility poles are nearby, you need to confirm the proximity risk during operations and any height restrictions during deliveries. If fences or gates are narrow, there may be constraints on material delivery or heavy-equipment entry. Conducting a drone survey to get an overview of the surrounding environment allows on-site constraints to be identified early and makes it easier to reflect them in the estimate conditions.

Particular attention should be paid to distinguishing information that can be seen in aerial photographs from information that cannot. From above it is easy to grasp equipment layout and the connectivity of pathways, whereas wiring under panels, fine corrosion under supports, the condition of bolts, details of connection points, and the condition inside control panels cannot be confirmed. When using drone surveying for pre-estimate investigations, the range that can be confirmed from the air should be used for broad situational awareness, while detailed deterioration checks and electrical verifications need to be left to ground inspections and specialist confirmation. If this distinction is blurred, there is a risk that items not checked by the drone survey will be mistakenly assumed to have been confirmed.

The extent of weed and tree growth is also closely related to checking existing equipment. When vegetation is overgrown, pathways can appear narrower, the condition of drainage channels and fence lines can be hidden, and the situation under panels can be difficult to discern. Overgrowth affects workability not only when estimating mowing and weed-prevention measures but also when estimating refurbishment work and inspection tasks. By using drone surveys to confirm the extent of overgrowth from above and organizing that information together with ground-level photos, it becomes easier to determine which areas require pre-clearing and which areas need passage secured before work.

When organizing the positions of existing equipment, making the data easy to read is also important. Even if there is a large amount of photographic data, it becomes difficult to use for estimating if you can't tell what is shown where. If you organize drawings so that the target equipment, walkways, fences, gates, drainage facilities, prospective work locations, and so on can be identified, estimators can quickly confirm the information they need. Especially when multiple people prepare an estimate, they may focus on different points even when looking at the same data, so having shared organized materials helps reduce variability in judgment.

In pre-estimate surveys, it is necessary to check not only the quantity and location of equipment but also the work risks. For example, conditions that significantly affect estimates include narrow passageways that may delay material transport; level differences around the target equipment that require securing work platforms; proximity to fences that limit the work area; and nearby trees or overhead lines that require caution during flight or work. Based on information obtained from drone surveying, it is important to specifically record how easy the work will be and any constraints.

Also, at solar power plants, multiple stakeholders—owners, management companies, construction firms, maintenance personnel, and so on—may share information. When conducting drone surveys for pre-estimate investigations, you should also consider who will view the deliverables and how much explanation will be required. Photos that are clear to staff familiar with the site can be difficult for stakeholders seeing the site for the first time to interpret spatially. Organizing overall photos, close-up photos, explanations of the target area, and correspondence with ground-level photos will make it easier to explain the estimated contents.

Check 4 Align assumptions for deliverables and estimation conditions

The fourth check is to align the assumptions underlying the deliverables and the estimate conditions. In drone surveying, simply capturing images does not automatically make an estimate accurate. For an estimate, you need to clarify which deliverables are required, what level of accuracy is being sought, what area is to be covered, and what will be considered already verified. One common failure in pre-estimate investigations is that the survey objectives and the contents of the deliverables do not match.

Possible deliverables include aerial photographs, oblique photographs, videos, orthophotos, point cloud data, elevation data, simple plan drawings, current condition explanatory materials, and survey area maps. However, it is not necessary to produce all deliverables every time. The deliverables required differ depending on whether you want to grasp the overall situation for a rough estimate or to depict the renovation area in detail. At the pre-estimate stage, it is important to select deliverables that are necessary and sufficient for the purpose and to avoid excessive work or omissions.

For example, if the primary objective is to verify delivery routes and work areas, overview photographs, plan-view materials that show the locations of passageways, photos of gates and access roads, and documents showing the locations of obstacles are important. If the main objective is to check terrain and drainage, materials that indicate elevation differences and slope trends are necessary. For an equipment renovation estimate, you need materials that show the location of the equipment in question, the surrounding work space, material delivery routes, and any potential interference with existing equipment. Deliverables must be designed to match the intended purpose.

To ensure consistent assumptions for an estimate, it is important to clarify the survey scope and the areas excluded from the survey. Even if the entire power plant is photographed, if only part of it is subject to analysis or verification, that area must be specified. Conversely, if only certain sections are photographed, it must be made clear that no estimation judgment has been made for areas that were not photographed. If such ambiguities are reflected in the estimate, later misunderstandings—such as “I thought the whole plant had been checked” or “That area was supposed to be excluded”—can occur.

You should always clarify the assumptions regarding accuracy. The required accuracy of drone surveying results varies depending on the purpose. For a preliminary overview before preparing an estimate, it may be sufficient to understand the overall positional relationships, approximate areas, and terrain trends. On the other hand, if the data will be used for decisions closer to quantity calculations or construction planning, a more careful surveying plan and verification of consistency with on-site reference benchmarks are necessary. When incorporating results into estimates or survey reports, it is important to make clear to what extent the deliverables can be used for decision-making and to avoid making overly definitive claims.

Also, in pre-estimate surveys, it is clearer to organize separately what was interpreted from photographs and survey data and what was visually confirmed on site. Items inferred from drone images should be described, as appropriate, as "possible" or "requires confirmation" and distinguished from what has been confirmed on the ground. For example, even if an aerial photo shows a location that appears likely to collect water, actual drainage problems may not be assessable without checking conditions during rainfall and the state of the ground surface. In pre-estimate surveys, avoiding mixing inferences and confirmed information contributes to reliability.

When organizing deliverables, it is also important that they are easy to use when reviewed later. Recording the capture date, survey scope, capture conditions, target sections, standards used, and on-site considerations confirmed can be useful not only when preparing estimates but also for later meetings and additional surveys. Pre-estimate survey data is not something that is used up only for the initial estimate. It can also contribute to construction planning, stakeholder briefings, schedule coordination, additional surveys, and revisions to maintenance plans. For that reason, it is important to make deliverables easy to view, easy to explain, and easy to reuse.

When reflecting items in the estimate conditions, not only what was confirmed but also what could not be confirmed becomes important information. For example: the ground surface under the panels cannot be confirmed from the air; parts of drainage channels are obscured by weeds; portions of the site perimeter were outside the flight range; and the detailed ground condition of slopes requires on‑the‑ground inspection. If these are organized as estimate conditions, it becomes easier to explain the need for additional surveys or separate measures. Conversely, issuing a quote while leaving unconfirmed items ambiguous makes it difficult to justify later changes in conditions or additional costs.

The way information is shared with stakeholders also affects the value of deliverables. Even if you only provide technical data, it will not be used if those involved in pricing decisions cannot understand it. By combining materials that show the overall picture, materials that indicate the specific locations, and documents that organize points of caution in writing, it becomes easier to communicate not only to estimators but also to clients and property managers. In pre-estimate surveys for solar power plants, it is important not only that the technical aspects are correct but also that they can be explained as estimating conditions.

Finally, when reflecting the results of drone surveys in estimates, it is necessary to clarify the purpose and scope of responsibility of the on-site survey. A pre-estimate survey is not for post-construction quality assurance or for detailed design itself, but a survey to organize the assumptions required for estimating. Of course, increasing the survey’s accuracy will improve the reliability of the estimate, but it cannot completely eliminate all risks. For that reason, it is important to organize the deliverables, scope of verification, unconfirmed items, and the need for additional surveys, and to incorporate them into the estimate conditions in a reasonable manner.

Summary

Using drone surveying for preliminary estimates of solar power plants allows an aerial overview of the entire site and makes it easier to organize the information needed for estimating. Because large sites, complex access routes, slopes and embankments, drainage facilities, existing equipment, and surrounding obstacles can be inspected from above, it becomes easier to understand the spatial relationships and working conditions that are difficult to grasp by ground inspections alone. In particular, drone-survey materials are useful when first inspecting a site and when sharing estimating conditions among stakeholders.

On the other hand, drone surveying does not reveal everything just by capturing images. There are parts that cannot be fully checked from the air, such as under panels, beneath trees, drainage channels hidden by weeds, equipment details, and electrical conditions. What is important in pre-estimate surveys is to separate and organize the areas that can be confirmed by drone surveying from those that require ground inspection or additional investigation. By not mixing confirmed information with estimated information, the reliability of the estimate conditions is enhanced.

By sequentially checking the four items—site conditions and flight area, topography and land development conditions, existing facilities and surrounding obstacles, and deliverables and the assumptions for estimation—you can more easily organize the information needed for pre-estimate surveys. In particular, for solar power plants, access routes, pathways, drainage, slopes, and the relationships with surrounding facilities often affect estimates more than the work area itself. When using drone surveying, it is essential to use it not as mere record photography but as current-condition documentation that can be used for estimation decisions.

Reducing on-site uncertainties during the pre-estimate survey makes it easier to explain the scope of work, decide on additional investigations, and build consensus among stakeholders. If you want to efficiently grasp the current condition of a solar power plant and organize the estimate conditions clearly, it is important to plan the entire process—from confirming flight conditions and on-site safety checks to combining with ground surveys and organizing deliverables. When using drone surveying for pre-estimate surveys, be clear about what can and cannot be confirmed, and present it in a form that can be explained as the estimate conditions.