Dramatically Reduce 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 quantities actually used during construction. For example, many site managers have experienced cases such as “the ready-mixed concrete that was supposed to be sufficient according to the design ran short on site” or “the earthwork estimate was too optimistic and additional dump trucks had to be arranged.” Behind the frequent occurrence of such quantity discrepancies are the information shortages and reliance on heuristics in conventional preliminary quantity estimation methods. Pre-construction site surveys and drawings are limited to two-dimensional cross-sections and plan information, and cannot fully capture detailed terrain or structural undulations. As a result, quantity estimates at the design stage become inaccurate, leading to material shortages or surpluses on site.

In addition, preliminary quantity design often incorporates safety-side padding or gut feelings based on experience. If a designer decides “let’s estimate a bit more” or “let’s allow some margin just in case,” quantities may clearly be in excess at construction time, or conversely, the estimate might be barely sufficient and result in shortages. The attitude of “better to have a little extra than to run out” can lead to wasted costs, while shortages lead to schedule delays and increased costs from reorders.

In short, conventional analog preliminary quantity design inherently contains errors and uncertainties that are hard to avoid, and these become one of the causes of construction mistakes and rework on site. In this article, we will introduce in detail the methods and effects of high-accuracy preliminary quantity design, which is attracting attention as a key to solving the problem of quantity discrepancies.

Correlation between the accuracy of preliminary quantity design and on-site troubles

The accuracy of preliminary quantities calculated at the design stage is directly linked to the frequency and severity of troubles that occur on site. Preliminary quantity design is the process of estimating in advance the amount of soil, concrete, and materials required for the work. If these quantity estimates are inaccurate, various problems arise during construction.

For example, if the excavation volume estimate is too optimistic, arrangements for transport dump trucks or disposal sites may be insufficient, causing work to halt. Conversely, if the estimate is excessively large, unnecessary heavy machinery operation and material costs increase. Errors in calculating concrete placement volume can lead to emergency requests for additional ready-mix trucks or surplus waste, causing losses in both schedule and cost. In fact, this issue has become a common concern among construction managers to the extent that industry media publish articles on the topic.

Furthermore, poor quantity accuracy can necessitate design changes or redesigns. If it becomes clear only after entering the site that “the structure won’t fit with the design quantities” or “more ground improvement than expected is needed,” rushing to redo the design leads not only to schedule extensions and budget overruns but also affects trust with the client.

Conversely, if preliminary quantity design is highly accurate, troubles caused by quantity discrepancies are drastically reduced. When required materials and soil volumes are accurately understood, construction proceeds according to the plan, and additional orders or rework do not occur. As a result, the site progresses smoothly without waste, and the burden and stress on construction managers are reduced.

In other words, accuracy of quantities and the rarity of on-site troubles are proportional. So what kinds of discrepancies tend to occur particularly in slope works and water/sewer works, and to what extent do they impact projects? Let’s look at concrete examples in the next chapter.

Typical examples of “discrepancies” in slope and water/sewer works

Slope works (slope shaping and slope protection) and water/sewer works (burial of water and sewer pipes) are fields where quantity discrepancies due to differences in on-site terrain and conditions are especially likely. Below are typical examples of discrepancies seen in both types of work.

• Slope work discrepancy example: The design planned to cut and fill slopes at a uniform gradient, but the actual ground has pronounced irregularities and in places differs from the design cross-section. As a result, more soil may be cut than planned and fill material may be insufficient, or conversely some fill areas may become excessively thick and materials remain unused. Also, if the preliminary surface area calculation for slope protection measures such as shotcrete or slope-face cribbing is inadequate, it leads to material shortages. For example, the surface area estimated at design by simply multiplying length × height may be smaller than the actual area including irregularities, resulting in ready-mix concrete running out midwork. Moreover, anchor or pile installation positions may not be deployable at the intervals shown on drawings, causing the actual number of installations to increase or decrease due to on-site adjustments.

• Water/sewer work discrepancy example: For water and sewer pipeline burial, subtle differences in longitudinal slope or burial depth directly cause quantity discrepancies. For instance, to secure the slope of a sewer pipe, it may be necessary to excavate deeper than planned, increasing excavation volume and disposal costs. Also, encountering unknown buried objects or bedrock not identified in pre-investigation can force a design change to an alternate route, resulting in insufficient pipe length. Additionally, interference with surrounding structures at the installation positions of manholes or valve chambers may be discovered on site, forcing the addition of precast concrete items. Water/sewer works require consistent quality and slope over long lengths, but intermediate terrain changes and obstacles frequently cause discrepancies between estimated and actual earthwork volumes and material counts.

As described above, slope works typically exhibit discrepancies in surface area and soil volume due to three-dimensional terrain undulations, while water/sewer works typically show discrepancies in earthwork quantities and component counts due to uncertainties in subsurface conditions and alignment changes. Conventional preliminary estimates that rely solely on 2D drawings and experience inevitably miss information, which becomes the cause of “discrepancies.”

So how can these problems be overcome? The key is the concept of “capturing the site in 3D” and high-precision coordinate control using GPS positioning. From the next chapter, we explain concrete measures for high-accuracy preliminary quantity design using the latest technologies.

The idea of capturing shapes as “surfaces” with 3D point cloud analysis

The first point to reduce the aforementioned discrepancies is to capture the on-site shape not as a “line” but as a “surface.” A promising means 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). Conventional 2D measurement used to pick up elevation points only along a few survey lines when drawing cross-sections. But using point clouds, the entire slope can be measured as a dense point “surface.” This allows one to grasp the actual shape almost completely, including irregularities and subtle undulations.

The reason the concept of capturing shapes as surfaces directly improves quantity accuracy is that you can accurately calculate surface area and volume. For example, when estimating the sprayed surface area for slope protection, there can be a large difference between a rectangular approximation from a 2D drawing using height and width and the measured surface area computed from a 3D point cloud. Point cloud analysis can automatically measure the surface area including subtle irregularities of the slope, enabling accurate calculation of the required concrete amount without over- or underestimating. Likewise, for excavation and fill volume calculations, taking the 3D difference between the design model and the as-built point cloud yields precise earthwork volumes that include small depressions and protrusions missed by traditional methods.

Furthermore, 3D point cloud analysis excels in visualization. Point cloud data can be rendered three-dimensionally on a computer, allowing virtual bird’s-eye views of the site and arbitrary sectional cuts for inspection. This enables designers and contractors to share a common “real image of the site.” Areas that were previously judged by experience become obvious when viewing the point cloud: “this slope projects more than expected,” “this is where the ground slope changes,” etc. All stakeholders can share these insights, dramatically improving the accuracy of quantity assessments and construction planning.

Means to acquire 3D point cloud data have also diversified and become easier. Fixed or mobile laser scanners, drone-mounted LiDAR, and even LiDAR-equipped devices like the latest iPhone/iPad make it possible for anyone on site to acquire point clouds in a short time. For example, at some sites, high-accuracy point cloud surveys are completed by walking around with a smartphone for about 5 minutes. Such rapid 3D scanning enables preparing full as-built 3D data before construction and refining quantity design based on it.

Eliminate the “source of positioning errors” with RTK-GNSS

Another cause of quantity discrepancies that should not be overlooked is survey coordinate error and reference inconsistencies. 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—is effective.

Traditionally, to align the coordinate system on drawings with the field survey coordinates, a survey network was built using transit or level instruments based on several known points. However, manual traverse and leveling surveys are accompanied by small point errors and reading mistakes that can accumulate and produce position or elevation discrepancies. Especially in long water/sewer pipeline works, there can be baseline mismatches on the order of a few cm to about 10 cm that cause deviations in pipe slope.

By using RTK-GNSS, you can fundamentally eliminate such sources of coordinate error. RTK-GNSS is a technology that obtains position coordinates with centimeter-level accuracy by applying real-time corrections to satellite positioning (GPS, etc.). Specifically, a base station (fixed station) and a rover (survey instrument) simultaneously observe GNSS satellites, and the rover applies correction information sent from the base station to calculate highly accurate coordinates. This allows any point on site to be fixed to public coordinate systems at centimeter accuracy.

There are two main advantages to applying RTK-GNSS to preliminary quantity design. First, because you can compare design drawings and the as-built condition on the same coordinate base, discrepancies in position or elevation can be detected in advance. For example, if the design ground elevation for a slope differs from the measured field elevation, RTK positioning reveals it immediately. In water/sewer works, verifying the elevations and coordinates of start and end points with RTK can reveal deviations from the design longitudinal profile early.

Second, on-site surveying becomes faster and simpler. Tasks that previously required surveying specialists using total stations and levels for extended periods can now be performed by a single person using an RTK-GNSS receiver to obtain point coordinates instantly. Moreover, with the recent launch of the domestic quasi-zenith satellite “Michibiki” and the advent of the centimeter-level positioning augmentation service (CLAS) (cm level accuracy (half-inch accuracy)), high-precision positioning is now possible even in mountainous areas outside mobile network coverage. This allows mountain sites with many slopes and regions with poor communications to benefit from RTK positioning.

In short, RTK-GNSS is a tool that dramatically improves the reliability of field coordinates. Because it lets you grasp even minute differences between design values and measured values, it removes the seeds of construction mistakes such as “the position is different than expected” or “the slope is off.”

Pre-construction verification by BIM model and point cloud matching

Once you have high-precision digital representations of on-site terrain and coordinates through 3D point cloud analysis and RTK-GNSS, the next step is to reconcile them with the design data. In other words, overlay the BIM/CIM design model and the as-built point cloud data to detect discrepancies before construction.

The concrete workflow is as follows:

• Current point cloud data acquisition: Using the methods described earlier, acquire point cloud data of the construction area’s existing terrain and surrounding structures. For slope works, scan the existing slope; for water/sewer works, scan the terrain of the planned excavation area or, if existing pipes are present, perform test excavations and point-cloud-measure their positions.

• Prepare the design 3D model: Obtain BIM/CIM models or 3D design data provided by the designer (even if only 2D drawings exist, creating a simple model for comparison makes the process easier). For water/sewer works, models include pipe diameters, lengths, and burial depths; for slopes, models include finished cross-sections and structural placements.

• Align the point cloud and model: Place both datasets in the same coordinate system. If the point cloud was obtained with RTK-GNSS, it is already in a public coordinate system, so if the design model is also created or transformed into the same coordinates, automatic overlay is possible. With proper coordinate integration, the design and actual conditions can be displayed to align precisely in 3D space.

• Visualize and analyze differences: Compare the overlaid datasets and extract areas with discrepancies. For example, when comparing the design slope surface and the point cloud terrain, gaps or overlaps indicate mismatches. Dedicated tools can visualize differences as a heat map—showing conforming areas in blue/green and discrepant areas in red—making nonconformities immediately apparent. Furthermore, by automatically calculating volume differences between the point cloud and the model, required additional fill or cut volumes can be computed instantly. This directly supports decisions such as “how many more cubic meters of earth must be cut to bring the as-built ground down to the design elevation.”

This pre-construction verification allows correction of insufficient assumptions or errors in the design stage before breaking ground. For example, if buried objects thought to be non-interfering on the design model are shown to interfere in the point-cloudified field data, route change options can be considered in advance. For slopes, if the as-built terrain projects beyond the design slope in places, you can pre-arrange additional rock removal or moderately revise the design section.

BIM model and point cloud matching functions as a digital rehearsal of construction. It prevents “it didn’t fit when we tried it” situations, avoiding redesign and rework. Processes that used to require on-site surveying, drafting, and lengthy review can now be completed quickly because, once datasets are overlaid, software automatically detects and calculates discrepancies. It effectively strengthens the Plan phase of the PDCA cycle by surfacing potential issues in advance.

Changes in design coordination from the construction manager’s perspective

Adoption of 3D point clouds, RTK-GNSS, and BIM integration brings major changes to how construction managers work and coordinate with designers.

Traditionally, construction managers surveyed and verified the site after receiving design drawings, then queried designers about unclear or obstructive points. However, 2D drawings made it difficult to understand the entire site and limited how accurately design intent could be conveyed to the field. This often caused miscommunication between site and design and led to responses like “it won’t fit as drawn, but let’s just try” or “let’s construct first and deal with it later.”

High-precision digital integration changes this situation dramatically. Construction managers can acquire point cloud data themselves on site and create review materials by comparing them with the design model. For instance, marking discrepancies on images where the point cloud and design are overlaid and sending them to the designer visually shares the issues. Designers can review the concrete site conditions in the data and provide quick, accurate responses or revision proposals. A “digital-mediated dialogue” emerges between the two, greatly reducing misunderstandings.

Moreover, construction managers can take a more proactive role in design. Using high-precision field data, they can support proposals like “this area will be over/under-supplied, so we should consider design changes” with backing data, shifting from simply executing given drawings to acting as partners who improve the project. This benefits owners and designers as well, raising overall project quality.

In addition, digital tools facilitate information sharing among other site staff. Showing the BIM model on a tablet to foremen or operators and saying “cut the red-shaded point cloud area by another 5 cm” helps them understand intuitively. Using AR to project the design model on site for collective verification is also possible. When everyone on site shares the same image of the finished state and quantity targets, mistakes are prevented and motivation improves.

In summary, with high-accuracy preliminary estimates and digital integration, construction managers become closer to designers—shifting from coordinators to collaborators. Data-driven communication increases the acceptance of site-originated improvement proposals, and the entire project proceeds more smoothly.

If quantity accuracy changes, schedule, cost, and reliability also change

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

1\. Shorter, more stable schedules: Reducing quantity errors eliminates unnecessary rework and waiting time. For example, losses such as “work stopped while urgently ordering additional materials due to shortage” disappear because the right amounts are procured from the start. Pre-construction reconciliation also eliminates many irregularities requiring immediate design changes on site, so progress can often proceed on or ahead of the schedule, making final schedule reduction achievable. In one case, improved efficiency in earthwork calculations sped up daily progress checks and shortened the total construction period by about 10%. Improved forecast visibility also helps coordinate subsequent works, stabilizing the overall schedule.

2\. Cost reduction and optimization: High-accuracy quantity capture directly improves budget control. Accurately capturing necessary and sufficient quantities minimizes material waste from over-ordering and machine idle time. It also prevents premium prices associated with emergency reorders. Reduced rework and redo work eliminate duplicated labor and machinery costs. Consequently, the probability of completing work within the execution budget increases significantly. For owners, contracts based on precise estimates reduce the risk of additional claims and increase cost transparency. Overall, wasteful spending is curtailed while necessary investments (e.g., ground improvement or reinforcement) are allocated appropriately, enabling cost-effective construction.

3\. Improved reliability and safety: Improved quantity accuracy and digital utilization enhance project reliability in ways that are not always visible. Between owners and contractors, issues like “the quantities you stated are different” or “who bears the extra cost?” are reduced. Construction proceeds based on shared, high-accuracy data, strengthening mutual trust. Within site teams, fewer unnecessary tasks and fewer mistakes improve morale, creating assurance that “this project is going well.” There are also safety benefits: when work proceeds according to plan, stress and confusion decrease and accident risk falls. For example, avoiding rushed additional concrete pours at night due to shortages prevents excessive workloads and helps maintain safety.

Thus, improving quantity accuracy is not merely a matter of numerical adjustment; it has ripple effects on schedule management, cost control, and stakeholder relationships on site. Beyond fewer construction mistakes, it yields multi-faceted benefits such as schedule reductions, cost savings, and higher stakeholder satisfaction. High-accuracy preliminary quantity design can be regarded as a form of quality assurance on site.

Conclusion: The future of sites transformed by high-accuracy estimates

This article has described how high-accuracy preliminary quantity design drastically reduces construction mistakes and brings various positive impacts to sites. Quantity discrepancies that were unavoidable with conventional 2D drawings and experience-based estimation can be dramatically reduced by a process of capturing the as-built condition as surfaces with 3D point clouds, fixing coordinates accurately with RTK-GNSS, and reconciling these with BIM models for pre-construction verification.

In slope works, you can conduct detailed earthwork and surface area calculations that reflect the true terrain, eliminating shortages or excesses of sprayed materials and cut/fill volumes. In water/sewer works, checking whether the piping plan aligns with field conditions in advance prevents mid-construction design changes and reexcavation. As a result, sites will have fewer uncertainties that “you don’t know until you try,” allowing construction to start in a state where the goal is nearly visible at the planning stage.

If high-accuracy preliminary quantity design becomes widespread, the future may hold sites where there are no surprises after construction and contracts without additional works—an ideal outcome for both owners and contractors. Owners will worry less about budget overruns; contractors can safely reduce risk margins and improve competitiveness in bidding. With the development of ICT and digital twin technologies, it may become 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 the future of sites that adopt it is smart construction sites without waste, variability, or mistakes.

Of course, conditions differ by site and perfect foresight is difficult, but by combining current available technologies, preventable discrepancies can be reduced to near zero. For construction managers and designers who have not yet implemented these approaches, try them first on small sites or partial tasks and experience the effects.

When high-accuracy estimates become the norm, the phrase “construction mistake” itself may become a thing of the past. The time has come to take the first step: improving quantity accuracy to change the future of sites.

Appendix: Getting started with smartphone × simple RTK surveying × 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. A small lightweight receiver weighing about 150 g and approximately 13 mm (0.51 in) thick simply attaches to the smartphone—no troublesome wiring or mounting is required. Launch the dedicated app to obtain RTK high-precision position information while using the phone’s built-in camera or LiDAR to perform 3D point cloud scanning. Since each point in the acquired point cloud is assigned absolute coordinates, scanning on site immediately produces a coordinate-stamped point cloud model.

LRTK also integrates with cloud services, enabling one-tap upload of site-measured data. From an office PC, point clouds and survey point information can be displayed in a web browser without dedicated software, and distance/area/volume measurements and overlays with design 3D models can be done in the cloud. In other words, LRTK’s strength is a one-stop workflow from point cloud acquisition to earthwork calculation and BIM reconciliation.

The on-site workflow is simple. For example, at a slope work site, walking along the slope while scanning with a smartphone equipped with LRTK can obtain a high-precision point cloud in about 5 minutes. Upload the data to the cloud, overlay the design slope model, and you can inspect required cut volumes and surplus areas as a color-coded heat map before construction. For water/sewer works, scan the roadway before excavation to create an existing terrain model, and compare it with the design pipe model to accurately calculate excavation and backfill volumes and to check clearances with underground utilities.

LRTK is also very affordable compared with conventional surveying equipment and aims for a “one-per-person” approach so field technicians can carry it. Pocketable and ready to use anytime, it is suitable for everyday as-built management and progress checks. The app is intuitive for first-time users: just press a button at the point you want to measure to record coordinates and acquire point clouds. In CLAS-enabled models using Japan’s Quasi-Zenith Satellite System “Michibiki,” centimeter-class positioning (cm level accuracy (half-inch accuracy)) is possible even in areas without cellular reception, making it highly suitable for civil engineering and construction fields.

As a first step in high-accuracy preliminary quantity design, making site surveying and point cloud capture familiar is important. Smartphone × RTK solutions like LRTK allow you to digitize site reality without large-scale equipment or specialized skills. If interested, try LRTK on a site—smartphone-based surveying and point cloud integration could elevate your construction management to the next stage.