How SfM Processing Is Changing As-Built Management: Improving Accuracy and Efficiency on Site

1. What As-Built Management Is and Traditional Challenges (Limits of Survey Points, Labor, Work Time)

As-built management is the construction management process of verifying that completed structures or terrain in civil engineering works conform to the design in shape and dimensions, and of recording and reporting the results. It is key to ensuring quality and is a critical step required for passing inspections and handing over the work. Traditionally, site managers and surveyors measured designated survey points using tape measures, levels, total stations, and so on, compared measured values against design values, recorded the measurements in field notebooks, and later compiled charts and reports in the office.

However, this traditional approach to as-built management has several noted problems. The main issues are as follows:

• Limit on the number of survey points: There is a limit to how many points can be measured manually, making it difficult to comprehensively capture the as-built condition over a wide area. Inspections tend to be limited to a few selected points, risking oversight of subtle irregularities or out-of-spec areas. For example, when checking the as-built condition of a road slope, measuring a few representative cross-sections cannot capture the finish of the entire slope in three dimensions, and it is common in later inspections to be told “the main survey points were within tolerance, but there were errors in the intermediate areas.”

• Labor and time burden: Traditional surveying often requires a team of several people working a full day. Manual measurements with tapes and levels, or setup and observation with a total station, demand considerable time and effort; the wider the site or the larger the structure, the more measurement and record-keeping hours are required. With a growing workforce shortage, this labor and staffing burden has become a significant strain on site operations.

• Accuracy, reliability, and record errors: Manual recording carries the risk of human error. If notes are taken incorrectly on site, transcribed wrongly later, or photos are forgotten, the reliability of measurement results can be compromised and may lead to quality issues. As long as as-built management relies on partial point measurements, it is hard to grasp the whole picture and prone to mistakes, causing significant stress for site engineers.

As described above, traditional as-built management methods required substantial time and labor yet offered only limited coverage, presenting problems in both efficiency and accuracy.

2. How Photogrammetry with SfM Makes Area-Based As-Built Management Possible

Recently, advances in digital technology have begun to significantly change how as-built management is conducted. The Ministry of Land, Infrastructure, Transport and Tourism promotes *i-Construction* and *ICT construction*, encouraging the use of 3D measurement technologies on site such as drone photogrammetry and 3D laser scanners. Among these, photogrammetry technology—generating high-density 3D point cloud data from photos taken by drones or cameras—is attracting particular attention. Using the representative method *SfM (Structure from Motion) processing*, features are extracted and matched across many overlapping images to simultaneously estimate camera positions and orientations and the 3D shape, producing a point cloud model that realistically represents the entire site.

Point cloud data obtained from photogrammetry is a collection of countless points that make up the space, effectively a 3D model that copies the site. Each point contains XYZ coordinates (position) and RGB color information, recording the surfaces of terrain and structures with high accuracy. This 3D measurement makes it possible to capture as-built conditions in area and three dimensions, rather than only partially as before. In other words, because the construction area can be measured densely down to the smallest details, as-built conditions can be assessed “by surface rather than by point.”

For example, in paving work, the traditional approach checked surface thickness at a few locations per section, but with point cloud data you can evaluate the flatness of the entire finished surface using a heat map display. The surface can be visualized as color distributions to show whether it matches the design, so you can immediately see which areas are overfilled or underfilled. The Ministry of Land, Infrastructure, Transport and Tourism’s as-built management guidelines (revised 2022) also include methods for “surface management” using 3D measurement technologies, and this is expected to prevent omissions in inspection points and enhance quality control.

By utilizing photogrammetry with SfM processing, wide-area as-built conditions can be digitally measured in a short time, capturing details previously overlooked. As a result, as-built management is evolving from the “manual measurement of scattered points over a small area” to “measuring the entire site and processing the data.”

3. Operational Example: Generating Point Clouds via Drone Aerial Photography + SfM and Comparing/Differencing with Design Models

Let’s look at an actual workflow combining drone aerial photography and SfM processing for as-built management. For example, at an embankment construction site, after completion the site was photographed from the air with a drone to obtain numerous images. By analyzing these images with an SfM algorithm, a high-density 3D point cloud model representing the current embankment condition was generated.

Next, the design-stage 3D model or final design data is prepared and overlaid on the point cloud for as-built comparison. Using dedicated software or cloud services, when the design model and the as-built point cloud are overlaid, you can check across the entire site whether the finish matches the design. With a difference display function, areas that match the design can be shown in green, parts exceeding the specification in red, and low areas in blue, automatically color-coded. This makes it possible to instantly identify construction defects or areas of over- or under-fill and decide where rework is needed.

Specifically, height differences between the point cloud and the design surface are computed and color-mapped according to the difference. For example, setting parts +5 cm or more high to red, parts −5 cm or more low to blue, and parts within the tolerance to green makes a numerical-based as-built judgment immediately apparent. Because this enables quantitative verification instead of relying on an expert’s intuition, pre-inspection self-checks become more reliable. Moreover, by calculating the volumetric difference between the design and the as-built, you can instantly determine “how many cubic meters of soil need to be removed to meet the design.” With this kind of drone + SfM point cloud vs. design data comparison, as-built management advances into a digital inspection that takes an overall view of the site.

4. Efficiency Methods: Automatic Cross-Section Generation, Heat Maps, Volume Calculations from Point Clouds

Using the point cloud data obtained from photogrammetry dramatically streamlines many as-built management tasks. Representative uses include:

• Automatic cross-section generation: You can extract longitudinal and cross sections at any location on the point cloud and automatically generate section drawings. Cross-sections that were previously measured on site can be obtained freely from digital data. For example, you can generate continuous cross-sections every 10 m for a road or embankment or later add checks of complex terrain cross-sections at the push of a button. The time to create as-built drawings is drastically reduced, and comparison with the design drawings becomes easier.

• As-built evaluation via heat maps: As mentioned, displaying the difference between the point cloud and the design model as a full-area heat map enables quality control through “surface visualization.” Local errors are not missed, and spatial unevenness in finishing can be quantitatively evaluated. For tasks like pavement flatness checks or slope gradient confirmation, intuitive color displays allow easy judgment of quality, increasing the persuasiveness of inspection documents.

• Volume and quantity calculations: Volumes of fill and cut can be quickly calculated from point cloud data. By comparing pre- and post-work terrain point clouds to compute earthwork volumes, this also supports progress-based quantity management. Where previously quantities were approximated from limited sections, point clouds provide accurate quantities based on the entire current condition in a short time. Volume calculations on the order of thousands of cubic meters can be completed in seconds to minutes on software, speeding up quantity aggregation and progress reporting.

Beyond these, you can directly measure dimensions, gradients, and areas on the point cloud, generate high-precision orthophotos (composite aerial photos viewed from directly overhead) for use as drawing backgrounds, and take advantage of diverse capabilities unique to digital measurement. With cloud services that support point cloud processing, data collected on site can be uploaded immediately and the office can begin analysis and drawing creation right away. The DX of as-built management (digital transformation) dramatically reduces record-keeping workload and makes information sharing among stakeholders much smoother.

5. How LRTK Contributes to Eliminating or Simplifying GCPs (Unifying Coordinate References, Smartphone GNSS Auxiliary Observations, Ground Photography for Occluded Areas)

A major challenge in improving the accuracy of photogrammetric measurements is providing accurate position coordinates to the captured images. Traditionally, aerial photogrammetry required placing multiple known-coordinate markers called GCPs (Ground Control Points) on site and using them to correct the model’s coordinates. However, installing and surveying many GCPs takes time and effort and becomes a site burden.

To solve this, the use of RTK-GNSS for direct georeferencing (positioning-based alignment) has become widespread in recent years. Particularly innovative is LRTK technology, which enables high-precision positioning with a smartphone. LRTK is a system composed of a compact RTK-GNSS receiver developed by Reflexia and a dedicated app, used by attaching it to a handheld smartphone such as an iPhone or iPad. Combining the convenience of a smartphone with RTK positioning accuracy, it is gaining attention as a pocket-sized surveying instrument that allows centimeter-level positioning anywhere by anyone.

Using LRTK on site makes it possible to unify the coordinate reference for drone photogrammetry and greatly reduce the need to install GCPs. For example, using RTK-equipped drones or drone images corrected by LRTK gives the point cloud derived from aerial photos accurate absolute coordinates from the outset. As a result, you can align the as-built point cloud with the existing survey coordinate system without placing many ground markers. Time-consuming coordinate alignment tasks become unnecessary, and the speed from capture to point cloud generation improves dramatically.

Additionally, an LRTK-equipped smartphone is effective as a supplemental observation tool on site. Areas where drones have occlusions—under bridge girders or beneath tree canopies, for instance—can be photographed or scanned from the ground with a smartphone. Since photos and point clouds obtained with the smartphone include RTK position information, they can be automatically overlaid with the drone point cloud and treated as a single unified model. This ensures detailed measurement of areas the drone cannot cover, enabling comprehensive 3D data capture of the entire site. Parts that previously required separate ground laser scanning or manual supplementation can now be easily covered with an LRTK smartphone.

Furthermore, if you use LRTK on a smartphone to perform control point surveying and obtain coordinates of known points, it becomes straightforward to fit the point cloud model to those coordinates in post-processing. Without expensive dedicated equipment, you can carry out control point measurement and as-built measurement with just a smartphone and a small device, making the process accessible even to technicians without specialized surveying expertise. LRTK is thus a powerful solution that can make GCPs unnecessary or minimal and simplify coordinate correction.

6. Combining LRTK Drone and Smartphone Workflows to Balance Responsiveness and Speed on Site

The true value of LRTK lies in its ability to dramatically increase on-site responsiveness and speed by flexibly combining drone surveying and smartphone surveying. The drone can quickly capture wide areas from above, while the smartphone can easily capture detailed information from the ground—using both approaches according to site conditions and centralizing the data is the strength of the LRTK system.

For example, at a large-scale development site you might first capture an overall point cloud by drone to grasp the broad as-built condition. For fine structures or locations that are hard to see from above, workers can walk the site with a smartphone to perform supplementary measurements, ensuring nothing is missed. Because the data from both sources is integrated using a common RTK coordinate system, a complete 3D model of the site can be finished the same day. What would have previously taken a surveying team several days to measure can, with LRTK drones and smartphones, in some cases be completed by one person in half a day.

Moreover, cloud integration enables real-time information sharing between the site and the office. Positioning data and point clouds uploaded from the LRTK app can be immediately viewed on cloud-based viewers and maps, allowing remote offices to begin as-built checks and drawing creation almost in real time. There is no need to bring data back on a USB memory stick—measure on site and share on site becomes a speedy workflow.

Thus, LRTK-based drone and smartphone integrated surveying is a survey style that balances accuracy and efficiency while responding quickly to changing conditions. It allows short, opportunistic measurements around weather windows or between tasks and immediate supplementing of measurements by smartphone if additional data are needed. Because it does not require heavy equipment or large teams, it can respond nimbly to sudden measurement requests and accelerate the PDCA cycle of construction management.

7. Guiding the Natural Adoption of Simple Surveying with LRTK as a Technology Directly Improving Site Quality and Productivity

As shown so far, digitalizing as-built management with SfM photogrammetry and LRTK is a groundbreaking technology that directly improves site quality and productivity. Area-based understanding of as-built conditions from point clouds prevents construction errors from being overlooked and ensures quality, while streamlining and reducing manpower for surveying achieves substantial time savings and staff reductions. 3D as-built management, previously limited to some large-scale projects or specialist contractors, can now be easily introduced on small- and mid-sized sites by leveraging smartphones and the cloud.

In fact, the 2022 revision of the Ministry’s standards formally positioned simple 3D measurement with smartphones and similar devices within as-built management, supporting this trend. Today, as-built measurement meeting accuracy requirements is possible with just a smartphone and a small device, and both public and private sectors are increasingly adopting such practices. This goes beyond mere adoption of the latest gadgets; it is also an effective measure against the worsening labor shortage and a support measure for young engineers, contributing to the construction industry’s overall DX push.

Finally, LRTK-enabled simple surveying has made “affordable, user-friendly one-person as-built management” a reality. This solution, which reduces site burden while strengthening quality control, has the potential to become a new standard. If your company is considering improving as-built management efficiency or introducing 3D technology on site, consider adopting smart surveying with LRTK. Embracing digital technology is a chance to dramatically uplift both productivity and quality on site.