Dramatically Reduced Construction Errors with High-Accuracy Preliminary Quantity Design

Introduction: Why Do Quantity Discrepancies Frequently Occur on Construction Sites?

On construction sites, it is not uncommon for there to be discrepancies between the quantities estimated during the planning stage and the actual quantities used during construction. For example, a site manager may have experienced situations such as “the amount of ready-mixed concrete estimated in the design turned out to be insufficient on site” or “the earthwork estimate was too optimistic and additional truck trips had to be arranged.” Behind the frequent occurrence of such quantity discrepancies are the information shortages and reliance on heuristics inherent in conventional preliminary quantity estimation methods. Pre-construction site surveys and drawings are limited to two-dimensional cross-sections and plan information, making it difficult to fully grasp detailed terrain or structural undulations. As a result, quantity estimates made during design can be inaccurate, leading to material shortages or surpluses at the site.

In addition, preliminary quantity design often incorporates safety margins or judgment based on experience. If a designer decides “let’s estimate on the higher side” or “let’s leave some allowance just in case,” this can lead to obvious surpluses in quantities during construction, or conversely to shortages when the design quantities were barely sufficient. Assuming “a little extra is better than not enough” can cause unnecessary cost increases, while shortages can lead to schedule delays and additional procurement costs.

In short, conventional analog preliminary quantity design contains unavoidable errors and uncertainties, which become a source of construction mistakes and rework on site. This article discusses the approach and benefits of high-accuracy preliminary quantity design, which is attracting attention as a key solution to these quantity discrepancies.

Correlation Between the Accuracy of Preliminary Quantity Design and On-Site Troubles

The accuracy of preliminary quantities calculated during the design phase directly affects the frequency and severity of troubles that occur on site. Preliminary quantity design is the process of estimating in advance the volumes of earthworks, concrete, and materials required for construction. If these quantity estimates are inaccurate, various problems will arise during construction.

For example, if the estimate of excavated soil is too low, the number of transport dumps or arrangements for disposal sites will be insufficient, causing work to stall. Conversely, overestimation leads to unnecessary heavy equipment operation and material costs. Errors in calculating the amount of concrete to be placed can result in requests for additional concrete trucks or excess waste, causing losses in both schedule and cost. In fact, this issue has been covered in industry media and is a common concern among construction managers.

Furthermore, low quantity accuracy can force design changes or redesigns. If it is only after entering the site that it becomes apparent “the designed quantity won’t accommodate the structure” or “substantially more ground improvement is needed than expected,” having to redo the design on short notice affects not only the schedule and budget but also the client’s trust.

On the other hand, when preliminary quantity design is highly accurate, such quantity-related troubles are drastically reduced. If required materials and earth volumes are accurately known, construction proceeds according to plan without additional orders or rework. As a result, the site operates smoothly with less waste, reducing the burden and stress on construction managers.

In other words, higher quantity accuracy correlates with fewer on-site troubles. Next, let’s look at specific examples of the kinds of discrepancies that commonly occur in slope works and water/sewer works, and the extent of their impact.

Typical Examples of “Mismatch” in Slope and Water/Sewer Works

Slope works (slope shaping and protection) and water/sewer works (burial of water and sewer pipelines) are areas particularly prone to quantity discrepancies caused by differences in on-site terrain and conditions. Below are typical examples of such mismatches observed in both work types.

• Examples of mismatches in slope works: On design drawings, slopes may be planned to be cut and filled at a constant gradient, but the actual ground may be highly irregular, causing local discrepancies between the design cross-sections and the actual terrain. As a result, more soil than planned may be excavated, leading to a shortage of filling material, or conversely, some fill areas may become excessively thick, leaving surplus material. Additionally, quantities for shotcrete or slope protection frames can be insufficient if surface area estimates made in advance are too conservative. For example, if the slope area is roughly estimated during design by length × height, the actual area including irregularities may be larger, and ready-mixed concrete can run out mid-job. Moreover, the number of anchors or piles actually installed may increase or decrease because they cannot always be placed exactly at the intervals shown on drawings, causing on-site adaptation changes.

• Examples of mismatches in water/sewer works: For pipeline burial, subtle differences in longitudinal gradient and burial depth directly cause quantity discrepancies. For instance, if deeper excavation becomes necessary to secure the slope for a sewer pipe, the excavated soil volume increases, inflating disposal costs. Unexpected buried objects or bedrock discovered during initial surveys can force a detour and design change, resulting in insufficient pipe lengths. There are also cases where interference with nearby structures is discovered at manhole or valve chamber installation points, forcing the addition of precast concrete products. In water/sewer works, consistent quality and gradient must be maintained over long lengths, but changes in terrain or obstacles frequently cause mismatches between estimated and actual earthwork volumes and material quantities.

As shown above, slope works typically suffer mismatches due to three-dimensional terrain undulations affecting surface area and earth volume, whereas water/sewer works commonly face mismatches from subsurface uncertainties and alignment changes affecting earthwork volumes and component quantities. Conventional 2D-based drawings and reliance on experience miss information that cannot be captured, and this has been a root cause of those mismatches.

So how can we overcome these problems? The key is to “capture the site in 3D” and use high-accuracy coordinate control via GPS positioning. The following sections explain concrete measures for high-accuracy preliminary quantity design using the latest technologies.

The Concept of Capturing Shapes as “Surfaces” with 3D Point Cloud Analysis

The first point to reduce the mismatches described above is to capture the on-site shape not as a “line” but as a "surface". A practical way to achieve this is 3D point cloud analysis.

3D point cloud analysis is a technology that acquires three-dimensional site shape data using laser scanners or photogrammetry and digitally reproduces the terrain and structures as a collection of countless points (a point cloud). In traditional 2D measurement, for example, only elevation points along a few survey lines would be sampled when drawing cross-sections. But with point clouds, the entire slope can be measured as a high-density point “surface.” This enables near-complete capture of the actual shape, including irregularities and subtle undulations.

The reason why thinking in terms of surfaces directly improves quantity accuracy is that surface area and volume can be calculated accurately. For instance, when estimating the area requiring shotcrete on a slope, there can be a significant difference between a rectangle approximation derived from a 2D drawing and the actual measured area from a 3D point cloud. Point cloud analysis can automatically measure slope areas including small irregularities, enabling accurate calculation of required concrete. Likewise, for excavation and fill volumes, three-dimensionally computing the difference between the design model and the as-built point cloud yields precise earthwork volumes that include fine depressions and protrusions overlooked by conventional methods.

Moreover, 3D point cloud analysis excels at visualization. Point cloud data can be displayed in 3D on a computer, allowing virtual overview of the site or inspection by arbitrary cross-sections. This enables designers and contractors to share a common “real image of the site.” Locations that were previously left to intuition become immediately clear when viewing the point cloud: “this slope protrudes more than expected” or “this is where the surface gradient changes.” Such shared understanding dramatically increases the accuracy of quantity reviews and construction planning.

Methods for acquiring 3D point cloud data have also diversified and become easier. Fixed or mobile laser scanners, drone-mounted LiDAR, or devices like the latest iPhones/iPads equipped with LiDAR sensors mean that anyone can obtain point clouds at the site in a short time. For example, some sites complete high-accuracy point cloud surveys by simply walking around with a smartphone for about five minutes. With such rapid 3D scanning, a full 3D as-built dataset can be prepared before construction and used to refine quantity design.

Eliminating Sources of Coordinate Error with RTK-GNSS

Another overlooked cause of quantity discrepancies is survey coordinate error or inconsistencies in reference systems. Here, the use of RTK-GNSS ([Real-Time Kinematic GNSS](https://ja.wikipedia.org/wiki/%E3%83%AA%E3%82%A2%E3%83%AB%E3%82%BF%E3%82%A4%E3%83%A0%E3%82%AD%E3%83%8D%E3%83%9E%E3%83%86%E3%82%A3%E3%83%83%E3%82%AF))—a high-precision positioning technology—proves useful.

Traditionally, to align the coordinate system on drawings with on-site survey coordinates, a few known points were used to build a survey network using total stations or levels. However, manual traverse or leveling surveys involve small measurement errors and reading mistakes that can accumulate, producing positional or elevation discrepancies. In long pipeline projects for water and sewer systems, for example, reference mismatches of several centimeters to ten centimeters can occur between start and end points, potentially disrupting pipe gradients.

Using RTK-GNSS can fundamentally eliminate such sources of coordinate error. RTK-GNSS is a technique that achieves centimeter-level position accuracy by applying real-time corrections to satellite positioning (GPS, etc.). Specifically, a fixed station (base station) and a rover (mobile receiver) simultaneously observe GNSS satellites; the correction information sent from the base is applied at the rover to compute highly accurate coordinates. This allows any point on site to be fixed in a public coordinate system to centimeter precision.

There are two main advantages to applying RTK-GNSS in preliminary quantity design. First, because design drawings and as-built conditions can be compared on the same coordinate foundation, discrepancies in position or elevation can be detected in advance. For example, if there is a difference between the design ground elevation and an RTK-measured on-site elevation in a slope project, it can be discovered immediately. In water/sewer works, confirming start and end elevations and coordinates with RTK allows early detection of deviations from the designed longitudinal profile.

Second, on-site surveying becomes faster and simpler. Tasks that previously required specialized surveyors using total stations and levels over extended time can be performed by a single person with an RTK-GNSS receiver, obtaining coordinates instantly. Recently, with the emergence of the Japanese Quasi-Zenith Satellite System “Michibiki” and its centimeter-level positioning augmentation service (CLAS), high-precision positioning is possible even in mountainous areas without internet coverage. This makes RTK positioning beneficial for mountain slopes and regions with poor communication.

In short, RTK-GNSS is a tool that greatly enhances the reliability of on-site coordinates. With the ability to capture minute differences between design and measured values, it helps eliminate causes of construction errors such as “the position was different than expected” or “the water slope is off.”

Workflow for Pre-Construction Verification by Matching BIM Models and Point Clouds

Once the actual site terrain and coordinates have been digitized with high accuracy using 3D point clouds and RTK-GNSS, the next step is verification against the design data. In other words, overlay the BIM/CIM design model with the as-built point cloud data and detect discrepancies before construction begins.

A typical workflow is as follows:

• Acquiring as-built point cloud data: Using the methods described above, capture the as-built terrain and surrounding structures of the construction area as point cloud data. For slope works, scan the existing slope; for water/sewer works, scan the area to be excavated, or if existing pipes are present, perform trial excavation and measure their positions as a point cloud.

• Preparing the design 3D model: Obtain BIM/CIM models or 3D design data from the designer (if only 2D drawings exist, it’s helpful to create a simple model for comparison). For water/sewer works this includes pipe diameters, lengths, burial depths, etc.; for slope works this includes as-built cross-sections and structural element placements.

• Aligning the point cloud and the model: Place both datasets in the same coordinate system. If the point cloud was acquired with RTK-GNSS, it is already in a public coordinate frame, so if the design model is created or converted into the same coordinates, the datasets can be automatically overlaid. With proper coordinate integration, the design and as-built can be displayed perfectly aligned in 3D space.

• Visualizing and analyzing differences: Compare the overlaid data to identify mismatched areas. For example, compare the design slope surface with the point cloud; any gaps or overlaps indicate inconsistencies. Specialized tools can display differences as heat maps (color distributions), showing compliant areas in blue/green and mismatched areas in red for quick identification. Moreover, automatically calculating volume differences between the point cloud and the model instantly provides required additional fill or excavation volumes. This directly informs decisions like “how many cubic meters more must be cut to bring the current ground to the design elevation?”

This pre-construction verification allows design assumptions or mistakes to be corrected before breaking ground. For instance, if an embedded object assumed clear in the design turns out to interfere based on the point-clouded site information along a proposed water/sewer route, alternative routing can be planned in advance. In slope works, if the actual ground protrudes beyond the designed slope in some areas, measures such as ordering additional rock removal in advance or relaxing the design cross-section can be taken.

Matching BIM models with point clouds is essentially a digital rehearsal of construction. It prevents situations where “it didn’t fit when we tried it” and helps avoid redesigns and rework. Processes that used to require field surveys, drawing creation, and lengthy reviews are shortened because once the data are overlaid, detection and calculation are automated, enabling rapid validation. This strengthens the Plan phase of PDCA by enabling early problem detection.

Changes in Design Collaboration from the Construction Manager’s Perspective

The adoption of 3D point clouds, RTK-GNSS, and BIM integration brings significant changes to the work of construction managers and their collaboration with designers.

Traditionally, after receiving design drawings, construction managers would survey and verify the site based on those drawings, then contact designers to clarify unclear points or obstructions. However, 2D drawings made it difficult to grasp the full picture of the site and limited the ability to accurately convey design intent to the field. Consequently, communication mismatches between site and design often led to attitudes like “it won’t fit on paper, but let’s try it” or “we’ll decide after installing it.”

High-accuracy digital integration changes this situation. Construction managers can acquire on-site point clouds themselves and prepare review materials by comparing them with the design model. For example, sending images with mismatched areas marked on overlaid point cloud and design helps visually share problems with designers. Designers can inspect the concrete site conditions in the data and respond or propose corrections quickly and accurately. This creates a "digital-mediated dialogue" that drastically reduces misunderstandings.

Also, construction managers can become more proactively involved in design. Backed by high-accuracy data obtained on site, they can make data-supported proposals like “this area will have a surplus/shortage—consider a design change,” transitioning from simply executing drawings to acting as partners in improving the project. This contributes to overall project quality from the viewpoints of both clients and designers.

Additionally, these digital tools promote information sharing among on-site staff. Showing the BIM model on a tablet to a foreman or operator and saying “we need to cut this red-highlighted area by an additional 5 cm” helps intuitive understanding. Using AR to project the design model on site allows everyone to check together. When the whole site team shares the same completion image and quantity targets, mistakes are prevented and motivation increases.

In summary, the introduction of high-accuracy estimates and digital integration is shifting the construction manager’s role from coordinator to collaborator, enabling data-driven communication and making on-site improvement proposals more readily accepted, thus facilitating smoother project progress.

If Quantity Accuracy Changes, Schedule, Cost, and Reliability Change Too

What positive changes occur in schedule control, cost management, and stakeholder reliability when high-accuracy preliminary quantity design is practiced? The main points are summarized below.

1. Shorter and more stable schedules: Reducing quantity errors means fewer unnecessary rework and waiting times. For example, losses due to “materials being insufficient and requiring urgent additional orders, during which construction is halted” are eliminated because the appropriate amounts are arranged from the start. Since issues are resolved through pre-verification, irregular on-site adjustments and rearrangements also decline. Work can progress on schedule or faster, making total schedule reduction achievable. In one actual case, improved efficiency in earthwork calculations enabled faster daily progress checks and resulted in an approximately 10% total schedule reduction. Better predictability also facilitates coordination with follow-on works, keeping the overall schedule stable.

2. Cost reduction and optimization: Accurate quantity control directly improves budget management. By reliably capturing necessary quantities, excess ordering and material waste or unnecessary machine idle time are minimized. Emergency additional orders prompted by shortages, which often cost above-market rates, are avoided. Reduced rework eliminates duplicate labor and equipment costs. As a result, the likelihood of completing the work within the execution budget significantly increases. Clients also benefit: contracts based on high-accuracy estimates have reduced risk of extra claims and increased cost transparency. Overall, unnecessary spending is curtailed while appropriate investments (such as ground improvement or reinforcement) are properly allocated, enabling cost-efficient construction.

3. Improved reliability and safety: Higher quantity accuracy and digital utilization improve project reliability in less visible ways. Between client and contractor, disputes like “the quantities weren’t as stated” or “who bears additional costs?” are diminished. With high-accuracy shared data, construction proceeds with mutual agreement, strengthening trust. On-site teams also experience increased morale as wasted tasks and mistakes decline, fostering a sense of "this project is going well." There are safety benefits too: when work proceeds according to plan without last-minute rushes, stress and confusion are reduced, lowering accident risk. For example, avoiding panic-driven additional concrete placements at night prevents excessive labor intensity and helps maintain safety.

Thus, improving quantity accuracy is not merely a matter of numerical alignment; it has ripple effects across schedule control, cost management, and stakeholder trust on construction sites. Beyond reducing construction errors, it enables schedule shortening, cost reduction, and higher stakeholder satisfaction. High-accuracy preliminary quantity design can be regarded as a form of "quality assurance" for on-site work.

Conclusion: The Future of Construction Sites Changed by High-Accuracy Estimates

This article described how high-accuracy preliminary quantity design can dramatically reduce construction errors and bring various positive effects to construction sites. Quantity discrepancies that were unavoidable with conventional 2D drawings and experience-based estimates can be dramatically reduced by a process of capturing as-built conditions as surfaces with 3D point cloud analysis, fixing coordinates precisely with RTK-GNSS, and verifying them against BIM models.

In slope works, it becomes possible to tightly estimate earth volumes and surface areas based on actual terrain, eliminating shortages and surpluses of shotcrete and cut/fill materials. In water/sewer works, prechecking whether pipe layouts align with actual conditions prevents design changes and re-excavation during construction. As a result, uncertainties that previously required "trying it to know" are reduced, and construction can begin with a state where the goal is largely visible during planning.

If high-accuracy preliminary quantity design becomes widespread, a future of "no surprises after construction" and "contracts without additional works" may become the norm. This is desirable for both clients and contractors: clients worry less about budget overruns, contractors can reduce risk margins while working safely, and competitiveness for bids may improve. With the advancement of ICT and digital twin technologies, it will even be possible to provide real-time as-built feedback during construction. In that sense, high-accuracy preliminary quantity design is an important piece of construction DX, and sites that adopt it will become smart workplaces free of waste, inconsistency, and mistakes.

Of course, each site has different conditions and perfect foresight is difficult, but combining currently available advanced technologies can reduce "predictable mismatches" to near zero. For those construction managers and designers who have not yet adopted these methods, start by trialing them on small sites or portions of a project and experience the benefits.

When high-accuracy estimates become the norm, the very term "construction error" may someday belong to the past. Improving quantity accuracy will change the future of construction sites—now is the time to take the first step.

Appendix: Getting Started with Smartphone × Simple RTK Survey × Point Cloud Integration Using LRTK

Finally, as a recommended tool to easily practice "high-accuracy preliminary quantity design," we introduce LRTK. LRTK is a solution consisting of a smartphone-integrated RTK-GNSS receiver device and a companion app developed by the startup Reflexia.

With LRTK, anyone can turn their iPhone or iPad into a versatile surveying instrument capable of centimeter-level positioning. By attaching a compact receiver weighing about 150 g and 13 mm thick to a smartphone, you eliminate complex wiring and setup. Launching the dedicated app lets you acquire RTK-level high-precision position information while performing 3D point cloud scans using the phone’s built-in camera and LiDAR. Each point in the acquired point cloud contains absolute coordinates, so simply scanning the site yields an immediately usable coordinate-tagged point cloud model.

LRTK also integrates with cloud services; on-site data can be uploaded to the cloud with one tap. From an office PC, point clouds and survey points can be displayed in a web browser with no specialized software required, and distance, area, and volume measurements as well as overlays with 3D design models can be performed in the cloud. In short, LRTK enables a one-stop workflow from point cloud acquisition to earthwork calculation and BIM verification.

Practical on-site use is simple. For a slope project, walking the slope while scanning with a smartphone fitted with LRTK can obtain a high-accuracy point cloud in about five minutes. Uploading that data to the cloud and overlaying the design slope model lets you pre-check required cut or fill quantities with a color-coded heat map. For water/sewer works, scanning the road surface before excavation creates an as-built terrain model that, when compared with the pipe design model, facilitates accurate calculation of excavation and backfill volumes and easy confirmation of clearances with buried objects.

LRTK is much more affordable than traditional surveying instruments, aiming for each site technician to carry a device like a personal tool. Pocketable and ready to use when needed, it is suitable for everyday quality management and progress checks. The app is intuitive for first-time users; simply press a button at the point you want to measure to record coordinates or acquire a point cloud. Also, CLAS-enabled models that utilize Japan’s Michibiki quasi-zenith satellites can achieve centimeter-level positioning even in areas without cellular signal, making LRTK highly suitable for civil engineering and construction fields.

As a first step toward high-accuracy preliminary quantity design, making on-site surveying and point cloud capture more accessible is crucial. Smartphone × RTK solutions like LRTK let you digitize on-site reality and use it without large-scale equipment or specialized skills. If interested, try LRTK on a jobsite—the world of surveying and point cloud integration enabled by a single smartphone can elevate your construction management to the next stage.