Color-Coded As-Built Checks: AR Heat Maps Intuitively Indicate Construction Quality

Table of Contents

• What is an AR heat map?

• How to create an as-built heat map

• On-site use of AR heat maps

• Benefits of as-built management using smartphones

• Recommendation for simple surveying with LRTK

• FAQ

What is an AR heat map?

An AR heat map is a drawing that intuitively visualizes, by color-coding, whether completed structures or terrain in civil and construction works have been finished according to design. Specifically, it compares the 3D model created at design time (or the design surface height data) with the as-built data (the terrain/structure shape after construction) measured on site, and expresses the slight differences as a color gradient. Areas with no deviation are shown in colors such as blue or green, while areas that are excessively high or low compared to the design are highlighted in red or orange, allowing you to judge construction accuracy at a glance. Traditional as-built management typically involved recording heights at key points as numbers and cross-checking with drawings, but using a heat map lets you understand the overall finish of the site in a surface-based manner and prevents overlooking height differences of a few centimeters.

The reason color-coded as-built checks are important is that if a discrepancy between the design drawings and the as-built condition is pointed out after construction, it can lead to rework and disputes. For reliable quality assurance, it is ideal to verify the as-built immediately after construction and correct any problem areas early. However, conventional as-built measurements required measuring each point one by one using tape measures, staffs, levels, total stations, etc., making it very time-consuming to cover large areas. On top of that, measurement results had to be taken back to the office to compare with the design drawings, identify defect locations, and plan corrective measures, making it difficult to respond immediately on site. The solution that has emerged to address these challenges is the as-built heat map. A heat map that indicates differences by color allows on-site verification of the finish in place, preventing missed quality defects. In recent years, it has also become possible to overlay these heat maps onto the real world using AR (augmented reality) technology, enabling intuitive quality checks directly at construction sites.

How to create an as-built heat map

So, how is an as-built heat map actually created? With the recent advancement of smartphones and ICT technologies, it is now possible to easily create heat maps using only a smartphone without special surveying equipment. The basic steps are as follows.

• Preparation of design data: First prepare the design data that will serve as the reference. For as-built checks of roads or land development, this corresponds to the 3D model data created during design or the reference design height information. Even if you only have paper drawings, you can read reference cross-sections and design heights from the drawings and input them into software to generate reference surface data for comparison. An important point is to prepare data that matches the design coordinate system for later comparison.

• Measure the as-built with a smartphone on site: Next, measure the shape of the completed terrain or structures on site using a smartphone. Modern smartphones are equipped with high-performance cameras and LiDAR sensors, and by using a dedicated app to scan the surroundings you can acquire the current 3D shape as countless points (point cloud data). Even on models without LiDAR, point cloud generation is possible by taking multiple photos with the smartphone camera and using cloud processing for photogrammetry. Furthermore, by combining the smartphone with a high-precision GNSS unit to perform RTK positioning, you can attach precise earth coordinates (latitude, longitude, elevation) in real time to the acquired point cloud or arbitrary measurement points. This makes it possible to measure as-built data in absolute coordinates on site without performing special surveying reference setup.

• Compare with design data and generate the heat map: Compare the point cloud data obtained with the smartphone to the design data and generate the heat map. For example, using cloud-based comparison functions, you can upload the design model and on-site point cloud and overlay them with a few clicks to calculate the differences. If the point cloud is RTK-enabled and coordinates match, tedious alignment is unnecessary and numerical comparison calculations are automated. The resulting heat map automatically displays height excesses and deficits by color. Each point is color-coded to show how many centimeters higher or lower it is than the design, giving a visual grasp of overall as-built accuracy. Generally, blue or green indicate acceptable ranges, areas higher than the reference are set to red, and low areas to yellow, so OK and NG areas are distinguishable at a glance.

• Adjust heat map display: Adjust the heat map display settings as needed. You can change the size of the grid (mesh) used to display height information to create a coarser or finer heat map, and modify the color-coding thresholds (allowable error ranges) to match site standards. For example, if the allowable error is set to ±3 cm (±1.2 in), set the thresholds so that colors change at that value to create a heat map that allows stricter pass/fail judgments. Conversely, if you want to see general trends, you can set wider thresholds. Flexibly configure the color scale (color steps) according to site needs.

Following the above process, you can create an as-built heat map from point cloud data measured with a smartphone. Even without expensive specialized software, there are increasingly many cloud services and apps that can handle everything from surveying to automatic calculation and heat map display. The important thing is to compare the design data and the current data in a common coordinate system. Using RTK-enabled smartphone surveying eliminates this concern. The era in which as-built inspection and heat map creation can be completed with just a smartphone is already becoming a reality.

On-site use of AR heat maps

Heat maps that have been created demonstrate their true value when they are used directly on site. Incorporating heat map–based as-built checks into on-site operations dramatically improves the speed and accuracy of quality control. Here are the main benefits of performing on-site as-built checks with AR heat maps.

• Immediate pass/fail judgment and rework: With a heat map, you can judge whether the finish is acceptable on the spot immediately after construction. For example, in paving work you can measure the as-built with a smartphone immediately after finishing, generate a heat map, and instantly check for areas that are short of the design height (shown in red). If nonconforming areas are found on the heat map, you can immediately identify the location and perform rework—such as adding additional paving thickness or cutting down—right away. Traditionally, you would have to return to the office to locate defects on the drawing and then mark and redo the area at a later date, but with heat maps you can complete the “measure → compare → fix” cycle the same day. This minimizes rework and enables early correction of quality defects.

• AR-based on-site visualization: The generated heat map can not only be viewed on a smartphone or tablet screen but can also be overlaid onto the real-world scene through the camera (AR display). Recent smartphones have robust AR features, and when high-accuracy heat map data is loaded into the device and composited with the camera feed, the colored display can be superimposed directly over the site structures. For example, holding the smartphone over the ground with the heat map displayed makes it possible to visually see how much higher or lower that spot is compared to the design as a color distribution (essentially a heat map overlaid on the site view). This allows pinpoint identification of defects without referring to drawings or numbers or relying on intuition and experience. You can give intuitive on-site instructions such as “let’s shave down this red area a bit” while viewing the smartphone screen over a hard hat. Thanks to centimeter-level accurate display using smartphones and AR, laborious layout work is reduced and construction management speeds up.

• Measurement and safety management in difficult areas: Smartphone-based as-built heat maps also support site safety. Previously, surveying steep slopes required surveyors to climb the slope to measure heights, which involved risk. But with smartphones and heat maps you can scan the entire slope from a distance, digitize it, and evaluate the finish from a safe remote location. This enables non-contact as-built inspection without personnel entering areas with poor footing or where heavy machinery is operating, thereby improving labor safety. You can also use heat maps to check whether materials are protruding outside the construction area or whether fill has been placed beyond designated boundaries, providing both as-built management and a safety-patrol–like effect.

• Data sharing and report generation: With cloud-enabled heat map tools, it is easy to share the day’s as-built heat map with the office and stakeholders immediately. For example, upload the point cloud and heat map from work done in the morning to the cloud so that managers in the head office can perform a quality check in the afternoon and give feedback to the site if corrective action is needed. Previously, compiling inspection results took days and corrective meetings tended to lag, but cloud use enables near-real-time construction PDCA cycles. Also, heat maps and point cloud data are stored on the cloud in time series, serving as daily as-built records. This eliminates the time-consuming process of recording numbers on paper and later transcribing into Excel, and prevents human error since digital data are stored automatically. Functions that automatically output as-built management reports (heat map images, cross-sections, lists of measurement point coordinates, etc.) at the push of a button are now available, greatly reducing the time required to prepare inspection documents. In the future, it may become possible to automatically prepare the full set of deliverables required by supervisors from on-site data. In this way, as-built heat maps are a revolutionary method that visualizes construction quality on site while enabling integrated use of data recording, sharing, and reporting.

Benefits of as-built management using smartphones

As described above, as-built heat maps make it possible to confirm construction quality with unprecedented speed and accuracy, and the factor that brings out their full potential is the use of smartphones. Finally, here are the main benefits that smartphone + latest technologies bring to on-site as-built management.

• Significant labor and time savings: With smartphone surveying and point cloud scanning, one person can measure as-built conditions over a wide area in a short time. Measurements that used to take multiple people several hours can be completed in a matter of minutes, minimizing work interruptions. Small daily as-built checks can be performed easily, which directly contributes to shortened construction periods.

• Prevention of oversights through surface-based measurement: The conventional point-measurement–centric method risked overlooking unevenness in areas that were not measured. But smartphone point cloud scanning can capture the entire ground surface, so every area is digitized and even slight mid-area irregularities are revealed on the heat map. This greatly reduces omissions where “only this spot differed from the design…” is discovered after inspection.

• Real-time feedback: Smartphone-based as-built management enables measurement to judgment on the spot, delivering overwhelmingly fast feedback. Finding and correcting defects with a heat map on the day of construction reduces the risk of failing later inspections. Cloud linkage enables real-time information sharing with stakeholders, allowing the construction quality PDCA cycle to run at high speed.

• No specialist skills required, reducing reliance on individuals: Because measurement can be performed with a familiar smartphone and intuitive app operation, site staff can handle it without advanced surveying qualifications. Tasks that used to rely entirely on veteran surveyors can be shared across the team, helping solve labor shortages and reduce dependence on specific individuals. With short training, young staff and heavy-equipment operators can become proficient, contributing to site-wide digital transformation.

Recommendation for simple surveying with LRTK

So far we have seen how to create and use as-built heat maps with smartphones and their advantages. Finally, we introduce one concrete solution that supports this: LRTK. LRTK is an innovative system that transforms a smartphone into an “easy-to-use universal surveying instrument,” consisting of a high-precision GNSS receiver device called the LRTK Phone and dedicated apps and cloud services.

By attaching an LRTK device to your smartphone and performing a simple setup, you can make smartphone GPS—which normally has errors of several meters—measure positions with an accuracy of ± a few centimeters. This is achieved by using RTK-GNSS correction technology, enabling handheld devices to achieve positioning accuracy comparable to stationary, expensive surveying equipment. The device connects to the smartphone via Bluetooth or USB and uses correction information received over the internet (from national or commercial VRS services, for example) to perform high-precision, real-time positioning. As long as there is a clear view outdoors, smartphone surveying using LRTK allows anyone to perform stable centimeter-level measurements.

A major feature of LRTK is its low cost and ease of use. Device costs are suppressed compared to dedicated equipment, making it possible for each worker to have high-precision GNSS equipment that previously cost millions of yen. Because it can be attached to the smartphones or tablets you already use, there is no need to purchase many special surveying machines. For those who want to start with low initial investment, subscription plans that include cloud services are available so you can introduce the system with monthly operating expenses for only the period needed. LRTK is also small and lightweight for easy transport to the site, and the app operation is intuitive and simple. Even those without surveying expertise can start using it after a short training session. It’s like every field worker carrying a personal surveying device in their pocket—“measure when you want to”—which is its strength. For example, where one device used to be shared by a team, with LRTK each person can measure at their preferred timing, dramatically improving overall site productivity. Its compact size makes surveying at height or in confined spaces easy, enabling access to points that were previously impractical. Acquired data are automatically synchronized to the cloud, eliminating concerns about backups and internal sharing for each measurement.

Thus, LRTK is a low-cost, easy-to-introduce, and simple-to-operate smartphone surveying solution. The benefits from modest investment—such as reduced survey time, prevention of rework due to human error, and fewer reworks due to improved quality—are substantial, and payback periods are often short. Moreover, LRTK aligns with construction DX initiatives promoted by the Ministry of Land, Infrastructure, Transport and Tourism, such as i-Construction, and its concept of “anyone can perform high-precision surveying with a smartphone” is well received on site. By enabling not only experienced surveyors but also young workers and heavy-equipment operators to operate the system, organizational digitalization advances, and one-person surveying contributes to labor savings and speed improvements. Smartphone + LRTK can truly be called an innovative partner for future on-site construction management.

If voices arise at your site saying “we want to streamline as-built management” or “we want to try easy surveying with a smartphone,” consider introducing simple surveying with LRTK. Simply pick up a smartphone and a small device, and the surveying and as-built verification work that used to require manpower and time will be dramatically streamlined. By raising on-site quality management levels through digital visualization such as heat maps, you can also promote workstyle reform and improve safety management. LRTK is a tool that strongly supports on-site DX and will surely be useful at your construction sites.

FAQ

Q1: Is the accuracy of as-built measurements taken with a smartphone really reliable? A: Yes. With appropriate corrections, smartphones can achieve high measurement accuracy. By using high-precision GNSS with RTK positioning, smartphones can achieve planar positioning accuracy of ± a few centimeters and vertical accuracy of a few centimeters. Considering that conventional smartphone-built GPS had errors of several meters, this is orders of magnitude better. However, achieving this accuracy requires good satellite signal reception conditions. Use outdoors with a clear view and ensure a stable communication environment to receive correction information (such as VRS).

Q2: What preparations and equipment are needed to create an as-built heat map? A: Basically you need the design data, a smartphone + surveying app, and, if possible, a high-precision GNSS unit (RTK-capable device). First prepare the design model or reference height data for comparison. On the smartphone side, use an app capable of point cloud scanning and, if necessary, attach an external RTK-GNSS receiver (for example, an LRTK device). RTK positioning also requires correction data from a reference station, so connect to a service that provides corrections over the internet (e.g., VRS services using Ntrip). With these preparations, you can perform high-precision on-site measurements with just a smartphone and generate heat maps from the data.

Q3: Can any smartphone be used? A: LRTK-style smartphone surveying devices support major iOS (iPhone/iPad) and Android models. Most modern smartphones meet the specifications. If the device has a built-in LiDAR scanner, direct point cloud scanning is possible within the app, but even LiDAR-free smartphones can generate point cloud models using photogrammetry by taking photos, so you are covered. Requirements include the ability for the smartphone to connect to an external GNSS receiver via Bluetooth or USB and that the dedicated app supports the device’s OS version. In general, most commercially available smartphones and tablets can be used.

Q4: How large an area can be measured with a smartphone LiDAR scan? A: The effective range of built-in smartphone LiDAR is roughly a radius of several meters to up to about 10 m (32.8 ft). Therefore, when scanning a wide site you divide the area into several blocks and walk around to scan them sequentially. On the other hand, photogrammetry mode can generate point cloud models over a wider area by processing many taken photos in the cloud. However, photogrammetry takes some time to produce results, so if immediacy is required, use real-time LiDAR scanning to get an overview and supplement detailed areas with photo mode. In this way, even a single smartphone can flexibly handle small to large-scale tasks with appropriate methods.

Q5: Compared to drone surveying or terrestrial laser scanners, what are the advantages of smartphone surveying? A: The biggest advantages are ease of use and immediacy. Drones and high-performance 3D laser scanners can survey large areas at once, but the equipment is expensive and requires specialist skills, as well as pre-permit applications, placement of control points, and time-consuming data processing. In contrast, smartphone + LRTK can be taken out and used by anyone on site for immediate measurement and verification. Smartphone surveying is especially suitable for small-scale as-built checks and daily inspections during construction. For very large-area overviews or aerial perspectives, drones are useful; for ultra-high accuracy to the millimeter, fixed laser scanners are appropriate. The ideal approach is to use smartphone surveying and other surveying methods in combination according to site scale and purpose.

Q6: Can as-built data obtained with a smartphone be submitted as official inspection documents? A: Yes. As-built data obtained with smartphone RTK + point clouds can be compiled in formats that conform to the Ministry of Land, Infrastructure, Transport and Tourism’s as-built management guidelines (draft). For example, LRTK stores measurement point coordinates and point cloud data in coordinate systems and accuracies that meet electronic delivery standards, allowing these data to be used as deliverables for inspection. It is possible to output records that satisfy accuracy requirements specified in guidelines for as-built management using RTK-GNSS (earthworks), and to export 3D data in standard formats such as LandXML or heat map figures as PDFs for submission to supervisors. As-built reports that were once submitted on paper or Excel are increasingly being accepted as digital data. Smartphone + ICT–based as-built management has reached a level that can sufficiently support official inspections.

Q7: I’m worried about the cost of introducing and operating a new system—will it be cost-effective? A: Introducing smartphone RTK and as-built management apps is overwhelmingly lower cost compared to conventional surveying equipment. Even including dedicated devices, the initial cost is modest, and subscription plans let you further reduce initial outlay by operating on monthly fees for only the needed period. Management of equipment is easy with just a smartphone and a small device, and specialist operators are not required. Considering labor cost reductions, shortened schedules, prevention of rework due to human error, and reduced rework from quality improvements, payback is often achieved in a relatively short time. Actual site reports show productivity gains from “one person completing as-built measurements” and quality improvements from “correcting defects on the spot,” leading to large overall productivity improvements. It is an investment that generally delivers good cost-effectiveness.