Table of Contents

• What is an as-built heat map?

• Benefits of as-built heat maps

• How to create an as-built heat map

• Check construction status in real time with AR display

• Summary

• FAQ

On construction sites, measuring and recording whether the finished work matches the drawings—known as as-built management—is indispensable. Recently, a new method called the as-built heat map has emerged in this field, and when combined with AR (augmented reality) technology it has become possible to intuitively check construction status on site. This article explains what an as-built heat map is, its benefits, and how to create one in practice. It also introduces how to project the heat map into real space using AR to check construction accuracy in real time. Learn this latest as-built management approach using digital technology to help streamline quality control.

What is an as-built heat map?

An as-built heat map is a three-dimensional visualization that compares the shape of the completed structure or terrain (the as-built condition) with the design data, and color-codes the differences. Specifically, point cloud data or 3D survey data acquired after construction are overlaid with the 3D design model (design surface data), and the error at each location is shown by color. For example, areas that are built higher than the design are displayed in red or warm colors, while areas that are lower due to insufficient cutting are shown in blue tones, and areas that match the design are shown in green. At a glance, you can intuitively understand which locations are higher or lower than the specification and whether they are acceptable or not.

As-built heat maps can be considered a visualization tool for as-built management. Subtle bumps or trends that are hard to notice in flat drawings or numerical lists can be easily discovered with colored 3D visuals. In recent years, the Ministry of Land, Infrastructure, Transport and Tourism has been promoting 3D measurement and surface-based as-built evaluation through initiatives like *i-Construction*, and heat map–based as-built management has begun to be adopted in official guidelines. In other words, as-built heat maps are becoming a new standard in the era of on-site DX.

Benefits of as-built heat maps

Using as-built heat maps provides many benefits that traditional methods could not offer. Here are the main advantages.

• Intuitive quality assessment: Because the magnitude of errors is shown by color, anyone from site workers to clients can understand construction accuracy at a glance. It is easier to understand than reports consisting only of numbers or text, and it makes it easier for the whole team to share the points that need correction.

• Prevention of measurement omissions: Because the entire surface can be evaluated with high-density data such as point clouds, irregularities and localized defects that are often missed by spot checks can be detected. A heat map that covers a wide area can uncover quality variations without omissions.

• Rapid feedback: If you scan during construction and generate heat maps as needed, you can immediately check the as-built status at that time. Early detection and rework of problem areas minimizes rework, shortens schedules, and helps assure quality.

• Records and traceability: Heat maps and point cloud data can be stored in the cloud as digital records. They preserve detailed construction histories that could not be kept on paper drawings, making it easy to compare with past data for cause analysis during future maintenance. Integrating as-built data into BIM/CIM models for asset management can also make them valuable information resources after completion.

• Labor savings and improved safety: Wide-area point cloud measurement and automated analysis greatly reduce the manpower and time required for measurement tasks. High or hazardous locations can be scanned remotely, contributing to worker safety. As-built confirmation in places that used to be difficult becomes easy with heat maps, reducing human error.

Thus, as-built heat maps greatly contribute to improving the accuracy and efficiency of quality control. So, what steps should you take to actually create such a heat map? The next section walks through the concrete process.

How to create an as-built heat map

Below is a step-by-step outline of the general flow for creating an as-built heat map, from preparing the necessary data to generating the heat map.

• Prepare the design data: First, prepare the 3D design data that will serve as the basis for comparison. For earthworks, this means the design ground model (TIN data or design surface data); for structures, it means the BIM/CIM 3D design model. In short, clearly define in data form “which shape is considered ideal (target).” This design model becomes the pass/fail criterion in as-built management.

• 3D measurement of the as-built condition: Next, measure the actual shape after construction in three dimensions. Point cloud measurement has become mainstream, and methods that scan the entire site using terrestrial 3D laser scanners or drone photogrammetry are commonly used. Recently, there has also been an increase in cases where point clouds are acquired easily using LiDAR-equipped smartphones. By combining an iPhone Pro model’s built-in LiDAR sensor with an RTK-GNSS receiver, it is possible to perform point cloud surveys with centimeter-level accuracy (half-inch accuracy) even with a smartphone. Regardless of the method, the important point is to measure the as-built condition comprehensively and obtain data with as high positioning accuracy as possible. Point cloud scans that can cover large areas in a short time provide a digital as-built model that includes fine details of terrain and structures.

• Align the data spatially: Overlay the design data and the acquired as-built data in the same coordinate system. If you measured in a surveying coordinate system (such as a global geodetic reference) from the start, the two will align automatically and little positional adjustment is required. For example, if you acquired point clouds with RTK-capable equipment, the acquired data already has high-accuracy coordinates, so you can simply overlay the design model. If you measured in a local coordinate system or there is some misalignment, perform a fitting adjustment using known control points to align the data. If the positioning is incorrect here, subsequent heat maps cannot be trusted, so check carefully.

• Generate the heat map: Compare the prepared design data with the point cloud data of the as-built condition to create the as-built heat map. Running the “create heat map” function in dedicated analysis software or cloud services will automatically calculate the height difference at each point and generate a color-coded heat map of those differences. In common color schemes, small errors are shown in green–blue, areas built higher than design are shown in warm colors (yellow–red), and areas lower than design are shown in cool colors (blue–purple). If you set the allowable error thresholds in advance, the range within tolerance is shown in green and out-of-tolerance areas are emphasized in red or blue, making it easy to identify locations that deviate from specifications. Some tools let you adjust the heat map mesh (grid) size and color ranges. Because the comparison is computed quickly by computers, results can be obtained in a short time even for hundreds of thousands of points.

• Review and analyze the results: Inspect the generated heat map on screen and analyze the construction quality. The color distribution lets you intuitively read “where and how much higher or lower” areas are. For example, you might identify that “the center of area ○○ is overbuilt by design +5 cm (+2.0 in)” or “area △△ has -3 cm (-1.2 in) of undercut.” You can also check numeric errors at each point on the heat map and analyze overall trends (e.g., generally slightly high, or only specific locations low). Because heat maps are visual, they are easy for site craftsmen and heavy equipment operators to understand, making them effective communication tools to share what needs correction. Uploading data to the cloud also allows stakeholders in remote offices to view the same 3D heat map in a web browser. You can share information in real time with remote supervisors or clients and obtain accurate instructions or approvals.

• Correction work and records: If defective areas are revealed by the heat map, perform the required rework on site (such as re-cutting or adding fill). After correction, perform 3D measurement again and confirm the finish with another heat map. Once you confirm the problem is resolved, output the final as-built heat map and inspection results as as-built management charts (forms). Modern systems can automatically generate reports with heat maps attached, enabling one-click submission materials that combine photos and drawings. Because the workflow can be completed with digital data, the effort of report creation is greatly reduced. Store the resulting heat maps and point cloud data within the company to aid future projects and share knowledge among engineers.

That is the basic flow for creating an as-built heat map. The key points are obtaining high-accuracy as-built data, proper spatial alignment, and leveraging automation tools. Next, we explain AR display as a method to use these heat maps to check construction status in real time on site.

Check construction status in real time with AR display

Once you have created an as-built heat map, you can display it on site with AR (augmented reality) to overlay digital information on the real world and check construction status. The heat map data is loaded into a mobile device via a dedicated AR-capable app or system, and a virtual heat map is overlaid on the camera view. This lets you view color-coded as-built acceptability composited with the actual site scenery, enabling you to intuitively understand “which locations and by how much they need to be corrected” on the spot.

The AR display procedure first transfers the heat map data (a colorized 3D model or point cloud) to the mobile device. On site, point your smartphone or tablet camera and overlay the heat map on the actual structure or terrain visible on screen. For accurate alignment, it is effective to use not only the device’s GPS and gyroscope sensors but also high-precision positioning and reference markers. For example, you can correct the device position to centimeter-level (half-inch accuracy) with RTK-GNSS or fix virtual objects to known site control points to minimize misalignment between the heat map and the site. Supported systems achieve high-precision AR through such corrections so that the virtual heat map does not drift even when you walk around with the device, and it is always displayed in the correct position.

AR-based on-site checks offer many advantages. First, identifying problem areas becomes rapid. Because red or blue spots displayed on the screen clearly indicate where they correspond on the ground, you can mark the ground there or directly instruct a machine operator, “Please cut here by ◯ cm.” Previously, teams used heat map reports and surveying equipment to locate the corresponding on-site positions, but that effort is no longer necessary. Second, AR helps streamline acceptance inspections. Showing the heat map AR on a tablet during an on-site inspection with clients or supervisors allows immediate sharing of pass/fail status. Because you can visualize “which areas were corrected and by how much,” explanations and consensus-building become smoother. Third, AR can reduce re-measurement work. If high-accuracy point cloud measurement was performed when creating the heat map, AR can identify positions on site, reducing the need to re-measure many points for the inspection. From a safety perspective, checking hazardous locations from a distance using AR instead of entering them reduces risk.

In this way, displaying as-built heat maps with AR brings digital pass/fail judgment into the real world and enables real-time construction management. The combination of heat maps and AR is not just a record of inspection but a quality-improvement tool that can be used immediately on site.

Summary

This article introduced how to create as-built heat maps and how to use them with AR. Compared with traditional survey-centered as-built management, the heat map + AR approach allows wide-area, high-accuracy checks in a short time and presents results visually and clearly, dramatically improving the efficiency and quality of construction management. Digital technology makes it possible to perform comprehensive inspections with small teams and smooth information sharing between site and office. As part of on-site DX (digital transformation), this approach will continue to spread.

That said, some may feel that advanced 3D scanning and analysis are difficult for their company. However, recently introduced easy-to-use surveying systems allow even non-specialists to perform point cloud measurement and heat map creation easily. For example, LRTK, which equips a smartphone with a small RTK-GNSS receiver, turns a phone into a high-precision 3D scanner and automatically generates as-built heat maps in the cloud from on-site scans. Furthermore, those heat maps can be displayed on the smartphone in AR for one-stop on-site verification. Using all-in-one solutions that minimize dedicated equipment and complex manual work, even first-time users can easily implement modern as-built management. Take this opportunity to adopt digital technologies on site to improve quality control and streamline operations.

FAQ

Q: What is an as-built heat map? A: An as-built heat map visualizes the difference between the actual shape after construction and the design shape by color. Acquired point cloud data are compared with the design model; areas with small errors are green, significantly raised areas are red, and excavated areas are blue, for example, allowing intuitive assessment of quality at a glance. It is an as-built management tool for quickly judging construction accuracy.

Q: What equipment and software are needed to create a heat map? A: Basically, you need equipment for on-site 3D measurement and software (or a cloud service) for data processing. Acquire point cloud data with a 3D laser scanner, drone, or LiDAR-equipped smartphone, then use dedicated desktop software or a cloud system to compare it with the design data and generate the heat map. Recently, platforms have appeared that automatically create heat maps by matching uploaded point clouds with design models in the cloud.

Q: Can I create an as-built heat map with a smartphone? A: Yes. Modern smartphones (e.g., iPhone Pro series) include LiDAR sensors, and combining them with a dedicated RTK-GNSS receiver allows a smartphone to function as a high-precision 3D scanner. Use a dedicated app to capture point clouds with the phone and upload them to the cloud, and services can automatically generate heat maps. Systems like LRTK enable heat map creation on a smartphone without specialized surveying knowledge.

Q: What is needed to overlay a heat map on site with AR? A: For AR display, you need an AR-capable smartphone or tablet and a dedicated app that loads heat map data. The device’s camera and sensors are used to overlay the virtual model on real space, but accurate overlay requires precise knowledge of the device’s position and orientation. For higher precision, correct the device position with RTK-GNSS or place reference markers on site for alignment. Compatible systems can achieve centimeter-level (half-inch) alignment without relying solely on the phone’s GPS, allowing accurate on-site display of heat maps.

Q: Are as-built heat maps accepted as official as-built management documents? A: As-built heat maps are increasingly recognized as an official as-built management method. The Ministry of Land, Infrastructure, Transport and Tourism’s guidelines include surface-based as-built management using 3D measurement, and heat map evaluation is being trialed and implemented in earnest. In some earthwork cases, comprehensive as-built measurement and heat map evaluation may be required. Therefore, it is possible to submit 3D as-built data including heat maps as inspection documents, and they are actively used in modern ICT-enabled construction sites. However, follow the contracting agency’s guidelines and submit printed heat map charts or electronic data as required.