High-precision Urban Mapping Pioneered by LiDAR Topographic Surveying That Also Handles Narrow Alleyways

Table of Contents

• The importance of urban topographic surveying and the challenges of narrow alleyways

• Limitations of conventional surveying methods in narrow alleyways

• What is LiDAR topographic surveying? Mechanism and characteristics

• Benefits of using LiDAR surveying in urban areas

• LiDAR surveying technologies effective in narrow alleyways

• Use cases enabled by high-precision mapping

• Efficiency gains realized by simplified surveying with LRTK

• FAQ

The importance of urban topographic surveying and the challenges of narrow alleyways

Urban topographic surveying involves precisely measuring the locations and heights of roads, lots, and buildings in urban areas and reflecting those measurements in maps and drawings. Such high-precision mapping is indispensable across a wide range of fields, from urban planning and infrastructure management to disaster prevention. However, Japanese cities are interlaced with traditional narrow alleyways, and surveying these spaces presents unique challenges.

Narrow alleyways generally refer to roads with extremely small widths, small paths where vehicle traffic is difficult or impossible. In densely built residential areas and older downtown districts, there are many roads with widths less than 4 m (13.1 ft) or winding lanes. While these spaces are important as local access roads, conventional surveying methods struggle to measure them accurately, and the creation of precise topographic maps tends to be postponed. As a result, gaps in detailed urban topographic data can occur, and a lack of information on narrow alleyways can become a bottleneck in disaster prevention planning and road maintenance planning.

In recent years, LiDAR technology has attracted attention as a means to solve surveying challenges in such confined spaces. Below we explain in detail the limitations of conventional methods in narrow alleyways and the new possibilities offered by LiDAR topographic surveying.

Limitations of conventional surveying methods in narrow alleyways

First, let’s organize the difficulties encountered when attempting conventional surveying in narrow alleyways. Each traditional surveying method has constraints particular to confined spaces.

• GNSS surveying (RTK-GNSS, etc.): In narrow alleyways, the sky is obstructed by buildings, creating an environment known as an “urban canyon.” Signals from satellites are reflected or attenuated by buildings, making high-precision positioning with GNSS difficult. Even with an RTK setup using a base and rover, positions cannot be solved without direct line-of-sight to satellites, and positioning is unstable inside narrow lanes.

• Total station surveying: Optical total stations require line-of-sight from the instrument to the prism, but in winding lanes and complex spaces it is difficult to ensure straight-line visibility. It often becomes necessary to relocate measurement points repeatedly and relay measurements, greatly increasing work time and effort.

• Mobile mapping systems (MMS): Vehicle-mounted mobile mapping systems with GPS and laser scanners are effective for 3D surveying of arterial roads, but vehicles cannot physically enter narrow alleyways. Even if a small vehicle can enter, parking or stopping is difficult and ensuring safety for the surroundings becomes an issue.

• Photogrammetry (drones, etc.): Creating 3D models from aerial photos taken by drones is also difficult for narrow urban alleyways. Legally, drone flights are restricted in densely populated areas, and alleyways shaded by tall buildings do not capture detail from above. Capturing the interior of narrow lanes in a camera’s field of view would require extremely low-altitude flight, which is hazardous.

• Manual surveying: Manual measurements using tape measures and simple tools require enormous effort as each length and corner of the alley must be measured one by one. In addition, subtle building irregularities and vertical information such as heights cannot be recorded, resulting only in simple planar sketches.

As shown, conventional techniques impose many constraints on surveying narrow alleyways, making it difficult to obtain data efficiently and with high accuracy. Consequently, narrow alleyways have tended to become “blank areas” in urban mapping. How, then, can LiDAR topographic surveying overcome these challenges?

What is LiDAR topographic surveying? Mechanism and characteristics

Let’s look at what topographic surveying with LiDAR involves. LiDAR (Light Detection and Ranging) is a technology that measures distances by timing how long it takes laser pulses emitted from the sensor to hit surrounding objects and return. By combining angle information, it can calculate the three-dimensional coordinates of points in space, producing large amounts of ranging points known as point cloud data. By analyzing this point cloud, high-precision topographic maps, 3D models, contour maps, and more can be created.

A major characteristic of LiDAR is its ability to capture extensive shapes in an areal manner at once. Unlike traditional point-by-point measurement, it can instantly acquire high-density point clouds of tens of thousands to millions of points per second. Because it uses laser light, measurements can be made at night or in dim locations and are not affected by sunlight or subject contrast as photogrammetry is. Even in environments such as narrow alleyways, where laser reaches, LiDAR can comprehensively record everything from fine wall surface irregularities to slight road gradients.

There are various approaches to LiDAR topographic surveying. For example, airborne laser surveying mounts LiDAR sensors on aircraft or drones to survey from above; mobile mapping mounts sensors on vehicles or carts to measure while moving; and terrestrial laser scanners use tripod-mounted stationary scanning for fixed-point observation. Recently, miniaturization of sensors and computing devices has enabled backpack and handheld LiDAR units that can be carried on foot, promoting use in narrow sites where equipment could not previously be brought in.

To provide real-world coordinates to point clouds acquired by LiDAR, it is necessary to accurately know the sensor’s position and orientation. For vehicle-mounted LiDAR, high-precision GNSS and inertial measurement units (IMU) record the sensor trajectory, while for ground-based setups the instrument’s installation point is tied to known control points to assign absolute coordinates. Through such georeferencing, any point in the point cloud can be attributed with geodetic information such as latitude, longitude, and elevation.

In summary, LiDAR topographic surveying is a method that rapidly acquires detailed three-dimensional information about the environment using lasers and maps it into real-world coordinate systems. These characteristics make LiDAR especially effective for surveying narrow alleyways.

Benefits of using LiDAR surveying in urban areas

Incorporating LiDAR topographic surveying into urban mapping provides numerous advantages not available with conventional methods. Key benefits are listed below.

• Efficient data acquisition: Because it can survey wide areas in a short time, on-site work time in urban areas can be dramatically reduced. For example, where conventional methods might take several days to survey a complex network of lanes, a LiDAR scan can be completed in a few hours. Since surrounding point clouds can be collected in a single pass while walking or driving, omissions are minimal and costly re-surveys are avoided.

• Labor savings and improved safety: LiDAR surveying is non-contact, reducing the need to repeatedly set up equipment on narrow roads or for personnel to remain in the roadway. This lowers operator burden and reduces traffic accident risk. Scanning at night or during off-peak hours with less traffic can also minimize impacts on surroundings and enable safer work.

• Highly detailed and comprehensive data: LiDAR point clouds capture all urban objects such as roads, buildings, fences, utility poles, and overhead wires. This allows the complex arrangement of structures and road geometries within narrow alleyways to be captured without omission. Because cross-sections can be extracted and slopes and heights analyzed freely after surveying, the need to revisit sites due to missed measurements is reduced.

• Robust in dark or complex terrain: In back alleys where sunlight doesn’t reach or in low-light nighttime conditions, LiDAR can emit its own light for measurement and is unaffected. In intricate terrain, as long as the laser reaches, fine details can be captured. Monotonous concrete walls and subtle pavement irregularities that are hard to capture photographically can be understood through LiDAR point cloud intensity data.

• Immediate use and sharing: LiDAR measurements are digital data that can be used immediately. Point clouds can be reviewed on a tablet right after a field scan or shared via the cloud with the office for on-the-spot discussion of results. Data can be exported in formats directly importable to CAD drawings or GIS, shortening post-processing time.

By leveraging LiDAR, urban surveying becomes markedly more efficient and advanced. How is LiDAR technology applied specifically to the special environment of narrow alleyways?

LiDAR surveying technologies effective in narrow alleyways

Applying LiDAR technology to surveying narrow alleyways has made mapping these previously challenging spaces a reality. The key is the existence of portable devices known as small mobile LiDAR. Using backpack-mounted or handheld scanners, sensors can be carried into any alleyway that people can walk through for measurement. In practice, there are increasing cases where operators walk through narrow lanes, stairs, or overgrown back alleys—places that were previously inaccessible with equipment—and scan the surroundings to acquire point clouds.

In such mobile LiDAR, the self-localization technology called SLAM plays an important role. SLAM (Simultaneous Localization and Mapping) is a technique that simultaneously estimates the device’s position while building a map by matching surrounding shapes obtained by LiDAR in real time. In other words, even in environments where GPS cannot be used, the LiDAR sensor can autonomously estimate its current position by using observed features and sequentially stitch together point clouds. Thanks to SLAM, even winding lanes can be mapped continuously by progressing the sensor through them.

Moreover, recent high-performance LiDAR surveying devices are designed to maintain high positioning accuracy in narrow alleyways by fusing SLAM with GNSS and IMU. For example, if GNSS can be received at the entrance or nearby points of a lane, the reference positions obtained there can be used to correct SLAM coordinates and give absolute coordinates to the overall point cloud. Alternatively, by tightly integrating multiple sensor inputs, even if satellite positioning is temporarily lost inside a lane, the IMU can capture attitude changes while SLAM supplements self-positioning, ensuring continuity and accuracy of positioning.

Traditionally, terrestrial laser scanning of confined spaces required setting targets (control points) at various locations and later merging separately measured point clouds, which was labor-intensive. Mobile LiDAR, however, produces spatially consistent point clouds from a single traversal, and once reference positions are obtained the map is immediately practical. The advantage of “no target placement required” greatly simplifies on-site work.

In narrow alleyway surveys, hybrid approaches with conventional methods are also used when necessary. For instance, setting control points at the lane entrance with a total station or GNSS and then scanning the interior with LiDAR allows detailed LiDAR geometry to be tied to highly accurate geodetic reference information. These combinations make topographic surveying at practical accuracy possible even in narrow alleyways.

Today, simplified LiDAR sensors are being incorporated into some smartphones and tablets, making 3D scanning more accessible. However, accurate topographic surveying still requires dedicated surveying instruments and positioning systems. Professional LiDAR surveying systems realize reliable data acquisition in narrow alleyways through advanced sensor fusion and correction techniques, which is a major differentiator.

Use cases enabled by high-precision mapping

When LiDAR makes detailed topographic data of urban areas including narrow alleyways available, the range of applications expands considerably. Below are major application areas enabled by high-precision mapping.

• Urban planning and road maintenance: Accurate current-condition data is essential for plans to widen or redevelop narrow roads. Using 3D models of lanes obtained by LiDAR, widening plans can account for the position of buildings and fences. They are useful for simulating setbacks and optimizing road geometry, and for materials to build consensus with residents.

• Disaster prevention and response: In densely built urban areas there are disaster challenges such as fire trucks being unable to enter, but detailed maps of narrow alleyways aid firefighting planning and evacuation route design. High-precision lane data also provides realistic initial conditions for simulations of building collapse or fire spread during earthquakes. After a disaster, LiDAR surveying is powerful for assessing damage and planning recovery in compact affected areas.

• Infrastructure maintenance and management: Lifeline facilities such as power lines and water/sewer pipes can be concentrated in narrow roads. By creating precise drawings from LiDAR point clouds, the positional relationships between aboveground objects and buried utilities can be accurately understood, reducing excavation risk and aiding pipeline routing during construction planning. Furthermore, periodic LiDAR surveys for time-series comparison are effective for monitoring road deformation and aging.

• 3D city models and smart cities: Point clouds of narrow alleyways contribute to building full 3D models of cities. If back alleys shaded by buildings are included, a complete 3D urban model allows diverse analyses and services as a digital twin. High-precision maps accelerate smart city initiatives based on them, such as simulation for autonomous vehicles and delivery robots, radio propagation analysis, and urban landscape visualization.

• Preservation of historic districts and tourism: In historic districts with traditional narrow lanes, detailed streetscape archives from LiDAR measurements help preserve cultural assets. Full 3D datasets of buildings and alley spaces provide valuable records for future restoration or reconstruction, and can be used as virtual tours for tourism.

Thus, high-precision urban mapping yields benefits across a wide spectrum from solving urban issues to creating new services. Detailed topographic information including narrow alleyways reveals previously unseen aspects of the city and becomes an indispensable foundation for safer and more comfortable urban development.

Efficiency gains realized by simplified surveying with LRTK

One solution gaining attention for improving on-site surveying efficiency, including high-precision mapping of narrow alleyways, is LRTK. LRTK is a compact positioning device used in combination with a smartphone that enables centimeter-level surveying without relying on specialized equipment (cm level accuracy (half-inch accuracy)). Tasks that previously required skilled technicians can be performed intuitively by anyone on site using LRTK. With the concept of a “one-device-per-person universal surveying instrument,” this system’s ease of use and affordable price point have driven rapid adoption, particularly in the construction and civil engineering industries.

LRTK consists of a small GNSS receiver attachable to a smartphone (LRTK Phone) and a dedicated app, acquiring high-precision coordinates using real-time kinematic (RTK) technology. By simply tapping a button on the smartphone screen, latitude, longitude, and elevation of the point to be measured can be recorded, and the system’s simplicity allows even those with limited surveying experience to operate it. Acquired data is saved to the cloud on site and can be checked immediately from an office PC. Photos taken are saved with geotags, enabling simultaneous documentation of conditions inside lanes.

The LRTK lineup also includes LRTK LiDAR, which combines a high-resolution laser scanner and GNSS. With this, broad-area point clouds can be obtained without placing markers. In one field case, a narrow-area topographic survey that previously required two people and more than half a day was completed by one person in a few hours using LRTK LiDAR. Obtained point clouds are automatically organized to public coordinate systems, allowing immediate use in drawing creation and analysis without additional steps. By utilizing LRTK in this way, surveying of narrow alleyways has become dramatically more efficient, directly improving on-site productivity and reducing labor.

Furthermore, centralized data management and immediate sharing afforded by LRTK contribute to on-site digital transformation (DX). Point cloud and survey point data are organized and shared in the cloud, making it easy to integrate measurements even when multiple people divide work in narrow areas. Local governments are also beginning to use LRTK for disaster prevention and infrastructure management, reporting that having detailed topographic data in-house enables faster decision-making and cost reductions.

Thus, simplified surveying with LRTK is bringing high-precision urban mapping within reach. Even in narrow alleyways, costly specialized equipment and large personnel are no longer necessary to obtain accurate data in short timeframes. As surveying technology continues to evolve, LRTK will further transform on-site surveying practices and strongly support safe and smart urban development.

FAQ

Q: What kind of method is LiDAR topographic surveying? A: It is a surveying method that measures surrounding distances using laser light and captures terrain and structures as a large collection of points (point cloud). Instead of measuring one point at a time as traditionally done, a LiDAR sensor scans the entire space to obtain a detailed 3D model. Q: What are the advantages compared to conventional surveying methods? A: The time required for surveying is drastically reduced, and data collection is efficient even in complex urban spaces. Also, because terrain is captured areally, omissions are minimized and comprehensive records including road widths and building shapes can be preserved. Additionally, it enables non-contact safe work and measurement in dark or low-light conditions. Q: Can you really survey places like narrow alleyways? A: Yes, it is possible. Where a person can enter, backpack or handheld LiDAR can be brought in to scan and comprehensively record the interior shapes of the lanes. Even when GNSS is unavailable, SLAM technology estimates self-position while mapping, enabling surveying in winding lanes and indoor spaces. Q: How accurate is the measurement data? A: It depends on the equipment and methods used, but with proper operation highly accurate results within several centimeters can be obtained. By combining high-performance LiDAR equipment with RTK-GNSS, horizontal positioning accuracy of 1–2 cm (0.4–0.8 in) and vertical accuracy within a few centimeters are achievable. However, accuracy may decline somewhat in heavily built-up environments, and corrections with control points can be applied as needed. Q: What equipment and preparations are required for LiDAR surveying? A: Basically, a laser scanner itself and a system to measure position (GNSS, IMU, or known control points) are necessary. Recently, mobile LiDAR systems integrate these components so scanning can start on site without special installation. For example, devices like LRTK LiDAR allow wide-area point cloud measurements by completing initial setup and powering on. Q: Is operation difficult? Can people without expertise use it? A: Modern LiDAR surveying systems are increasingly user-friendly and intuitive. Dedicated software and apps guide users, so people without surveying expertise can perform basic data acquisition. Systems like LRTK designed for smartphone control are built to operate with a map-app-like user experience. Q: Are there cost barriers? A: Traditionally, LiDAR devices and high-precision GNSS were very expensive, but miniaturization and mass production have produced more affordable products. Solutions like LRTK are lower cost than conventional systems and are available via subscription plans, making one-device-per-person deployment feasible within budgets. Overall, the cost barrier has significantly decreased compared to the past.