LiDAR topographic surveying that excels in no-drone zones: Advantages for urban surveys

Table of contents

• Areas where drones cannot be flown

• What is LiDAR topographic surveying?

• Benefits of using LiDAR for urban topographic surveys

• Comparison with conventional surveying methods

• Use cases of LiDAR surveying in urban areas

• Simple surveying with LRTK

• Frequently Asked Questions (FAQ)

In recent years, new technologies such as drones and 3D scanning have been introduced one after another to surveying sites, improving efficiency and accuracy. However, in areas where drones cannot be flown, such as urban centers, there are still many situations where traditional methods must be relied upon. One solution attracting attention is topographic surveying using LiDAR. LiDAR is a technology that measures surrounding shapes with laser light to obtain detailed 3D data, and it performs powerfully even in environments where drones cannot be used. This article explains the mechanisms of LiDAR topographic surveying and the benefits it brings to urban surveys, taking into account drone-no-fly zones and the challenges of urban surveying. Finally, we introduce a simple surveying method using the latest solution, LRTK.

Areas where drones cannot be flown

First, let’s confirm why there are areas where drones cannot be used. Under Japan’s Civil Aviation Law, for safety reasons, designated no-fly airspaces are established, and permission from the Minister of Land, Infrastructure, Transport and Tourism is required for unmanned aircraft (drones) to fly in airspace around airports, at altitudes of 150 m (492.1 ft) or higher above ground level, and over densely inhabited districts (DID). Densely inhabited districts refer to areas where houses are concentrated, and most major urban areas fall into this category. In other words, much of the airspace over cities like Tokyo and Osaka is, in principle, not freely available for drone flights.

Even when flights are legally possible, strict safety measures are required to fly drones in urban areas. In the unlikely event of a crash, there is a high risk of damage to people or buildings on the ground, so thorough caution and prior notifications are necessary. In densely populated areas, residents also tend to have strong privacy concerns and resistance toward unmanned aircraft operations. For these reasons, the hurdles to conducting aerial surveys with drones in urban areas are extremely high.

As a result, urban topographic surveys still often have to rely on traditional methods such as ground surveying by personnel or the use of aerial photographs. However, those methods can be time-consuming and labor-intensive and may not provide sufficient accuracy or information. How can detailed topographic data be obtained efficiently in urban areas where drones cannot be used? One solution gaining attention is LiDAR-based topographic surveying.

What is LiDAR topographic surveying?

LiDAR stands for Light Detection and Ranging and is a sensor technology that measures the distance to targets by emitting laser light. A LiDAR unit emits laser pulses toward the ground or structures and calculates distance by measuring the time until the reflected light returns. Using the speed of light (approximately 300,000 km per second), distances can be determined to sub-centimeter precision from the round-trip time. Compared to traditional radar using radio waves, laser light has a much shorter wavelength, enabling detection of fine details and high-resolution range measurements.

LiDAR measurements repeatedly emit these laser pulses tens of thousands to hundreds of thousands of times per second and record the resulting myriad distance data as a collection of points. This vast collection of points is called point cloud data. Each point contains three-dimensional coordinates (X, Y, Z) and can precisely represent the surface shape of objects. For example, when laser pulses are directed at the ground or building walls, many points on those surfaces are captured, and plotting those points reproduces a 3D model of the site. The higher the point cloud density, the more detail is captured, making LiDAR a powerful means to digitize real-world spaces quickly.

LiDAR topographic surveying is a method of measuring ground surface elevations and terrain undulations from the acquired point cloud data. By using specialized software to extract ground points, creating contour lines and checking cross-sections of the terrain becomes straightforward. Topographic maps that were once created by manually measuring numerous points can now be obtained automatically as detailed digital terrain models (DTMs) with LiDAR. Another advantage is that laser light can be used day or night and does not depend on sunlight, so measurements can be taken in dark or dim environments.

Benefits of using LiDAR for urban topographic surveys

Introducing LiDAR technology to urban surveys brings many advantages not available with traditional methods. Here are the main benefits.

• Speed and efficiency: LiDAR scanners can perform hundreds of thousands of range measurements per second, allowing wide-area topographic data to be acquired in a short time. Compared with manually measuring one point at a time, on-site work time can be greatly reduced. Because large numbers of points are acquired automatically at once, measurements are comprehensive with no missed spots.

• Labor savings and safety: There is no need to carry heavy equipment and reposition it repeatedly, and surveying can be conducted smoothly by a small team. In some cases, surveying can be completed by a single person, simplifying operations considerably. Shorter work time not only reduces personnel costs but also lessens workers’ physical burden. Because measurements can be taken remotely without entering hazardous roads or high places, on-site safety is also improved.

• High-density, comprehensive data acquisition: LiDAR point cloud data are extremely dense and capture details of the ground surface and structures. Reviewing point clouds in the office makes it easy to identify any missed or overlooked areas. Heights and distances at any desired point can be measured in the data, reducing the need to return to the site for re-measurement. As a result, detailed and comprehensive 3D surveying is possible.

• Non-contact, remote measurement: Laser scanners can measure from a distance without contacting targets, making it easier to capture terrain in narrow or restricted areas where people cannot enter. For example, you can scan adjacent terrain across a restricted site or understand the shapes of high structures out of reach from the ground. Even in the complex environments typical of urban areas, LiDAR enables data collection from various angles thanks to its mobility.

• Independent of time of day and lighting: As mentioned earlier, LiDAR emits its own light to measure distance, so accuracy does not degrade in the dark or under low light. The ability to survey at night when traffic is light, for example, greatly widens scheduling options for urban surveys. Lasers also generally operate in light rain (※ heavy rain or dense fog can reduce measurement accuracy, but light rain is usually not a problem).

Comparison with conventional surveying methods

To understand LiDAR surveying, here is a brief comparison with other methods traditionally used.

• Conventional ground surveying: Surveyors use total stations or levels to measure angles and distances point by point on site. While accurate, surveying large areas requires substantial manpower and time. In urban areas, space for tripods and traffic control may be needed at each survey point, posing efficiency challenges. Also, only the coordinates of measured points are obtained, and the terrain between points must be interpolated. Capturing complex shapes of structures or terrain realistically would require measuring an impractically large number of points.

• Aerial photogrammetry: This method creates 3D models and contour lines from photographs taken from aircraft or drones using image analysis. It can cover wide areas quickly, but drone flights are restricted in urban areas as mentioned above. Manned aircraft aerial photography is costly and has limited resolution for fine details. Photogrammetry is also susceptible to sunlight and weather conditions, and it is difficult to capture terrain in areas hidden from the camera such as building shadows or beneath trees. Ensuring accuracy often requires placing known ground control points, which is additional work.

• LiDAR surveying: This method directly acquires 3D point clouds via laser scanners. Huge volumes of high-density data can be captured in a single measurement, addressing the issues above. Because there is no need to pick points manually like in ground surveying and it is less affected by lighting or visibility than photogrammetry, LiDAR provides stable accuracy and efficiency. LiDAR equipment used to be expensive, but recently the proliferation of small drone-mounted and handheld models has lowered the entry barrier. Using portable LiDAR devices in urban areas makes it possible to obtain detailed terrain information on site without lengthy permit procedures or extensive preparations.

Use cases of LiDAR surveying in urban areas

LiDAR topographic surveying is useful in a variety of urban scenarios. Below are typical use cases in urban areas.

• Surveying narrow roads and alleys: On narrow streets or intricate alleys, it is difficult to set up tripods or fly drones. With LiDAR, you can walk with a sensor or mount it on a vehicle and drive along roads to efficiently scan roadside terrain and building façades. Conducting short surveys at night when traffic is low minimizes the need for road closures and reduces disruption to the surroundings.

• Grasping conditions in redevelopment areas: In urban redevelopment sites with many aging buildings, LiDAR is effective for understanding narrow plots and varied elevation terrain. Recording building layouts and ground elevations before demolition helps in design review and construction planning. Creating a current 3D model from LiDAR point clouds enables stakeholders to share and streamline consensus-building.

• Damage assessment after disasters: After earthquakes, fires, or other disasters in urban areas, quickly grasping the state of collapsed buildings and debris is crucial. Even in areas where drones cannot be flown, carrying LiDAR equipment and walking a damaged site allows for rapid acquisition of precise point cloud data of the affected area. Because measurements can be made at night, current-condition recording can be done without disrupting daytime rescue operations. The obtained 3D data can be used for restoration planning and cause analysis.

Simple surveying with LRTK

Finally, we introduce LRTK as a cutting-edge tool that makes LiDAR surveying in urban areas dramatically easier. LRTK (L-R-T-K) is a solution that combines GNSS (Global Navigation Satellite System) real-time correction technology (RTK) with LiDAR measurements, designed so that even non-experts can perform high-accuracy 3D surveys.

LRTK consists of a dedicated GNSS receiver device that attaches to a smartphone or small tablet and an app, and it allows easy use of satellite position correction information (such as network RTK or augmentation signals). This can reduce typical smartphone GPS position errors of several meters (several ft) to the order of a few centimeters (a few inches), enabling the captured point cloud data to be tagged with high-accuracy coordinates. For example, LRTK-compatible devices contain high-sensitivity antennas supporting multiple satellite constellations, achieving stable centimeter-level positioning.

Operation is simple: attach the device to a smartphone, press a button in the app, and positioning begins. No complicated settings or specialist knowledge are required, and the user interface is intuitive even for those with limited surveying experience. The acquired point cloud data can be automatically saved and shared in the cloud, allowing colleagues in the office to check 3D data immediately after on-site measurement.

With the advent of LRTK, urban topographic surveying is changing significantly. Because detailed terrain information can be obtained by a single person walking the site without expensive equipment or large personnel, the barrier to surveying has been greatly lowered. Even in no-drone zones, LRTK enables faster and safer 3D surveying than before. Simple surveying with LRTK, incorporating the latest technologies, is expected to play an increasingly active role in urban surveying sites.

Frequently Asked Questions (FAQ)

Q: What kinds of places are considered areas where drones cannot be flown? A: Representative examples are densely inhabited districts (DID), which are urban airspaces. For example, residential and commercial areas in city centers are almost all subject to this, and legally require permission from the Minister of Land, Infrastructure, Transport and Tourism to fly drones. Other no-fly zones include airspace around airports, areas above 150 m (492.1 ft) above ground, and certain important facilities (such as power plants and areas near government residences).

Q: Is permission required to conduct LiDAR surveying in urban areas? A: Ground-based LiDAR surveying generally does not require special permission. If drones are not used, the Civil Aviation Law does not apply, so anyone can perform measurements in principle. However, when working on roads, there is a duty to ensure safety, and you may need to confirm road usage permits in some cases. Also, avoid scanning the interiors of private land without permission; obtain the landowner’s consent as needed.

Q: What is the accuracy and range of LiDAR surveying? A: Accuracy depends on the equipment used, but modern LiDAR surveying systems can measure terrain with accuracy on the order of a few centimeters (a few inches). Especially when combined with GNSS RTK corrections, planar and vertical errors often fall within a few centimeters. The measurable range (effective distance) depends on sensor output, but ground-based laser scanners can acquire point clouds at radii of tens to hundreds of meters (tens to hundreds of ft). For urban surveys, scanning visible ranges sequentially can cover the required area.

Q: Can surveying be done at night or in rainy weather? A: Yes. Because LiDAR measures with laser light, it can be used at night without issue. In fact, surveying at night, when traffic is lighter, is advantageous for urban work. Light rain is usually acceptable, but heavy rain or dense fog can scatter laser light and reduce accuracy, so avoid measuring in such severe conditions. Don’t forget to take waterproofing measures for equipment and avoid exposing devices to water.

Q: Is processing and using the acquired point cloud data difficult? A: Processing point cloud data requires dedicated software, but user-friendly tools have become more common and basic operations are not very difficult. For example, extracting ground points to create contour lines or automatically removing noise can be automated. Loading point clouds into CAD or 3D modeling software allows you to display cross-sections, take measurements, and produce drawings. Using LRTK cloud services, on-site data can be shared within the company immediately, making point cloud utilization even smoother.

Q: What equipment is needed to start LiDAR surveying? A: At minimum, a LiDAR sensor (laser scanner), a GNSS positioning device, and PC software to process acquired data are needed. LiDAR sensors come in types such as ground-based, vehicle-mounted, drone-mounted, and handheld, and should be chosen according to the application. For GNSS, an RTK-capable receiver is desirable for high-accuracy positioning, but small RTK receivers that pair with smartphones (such as LRTK) have become available for easy adoption. Some modern smartphones even include simple LiDAR sensors, so small-scale scans can be started with familiar devices. Select equipment incrementally according to your objectives and accuracy requirements.

Q: What are the weaknesses and precautions for LiDAR surveying? A: LiDAR is extremely useful but not omnipotent. For example, areas hidden from direct laser paths, such as behind buildings or in shadows, will not produce point clouds, so measurements from multiple positions are needed to fill blind spots. Materials that transmit or mirror light, like glass or water surfaces, can be difficult to measure accurately. Note also that data volumes can be enormous, so high-capacity storage and powerful computers are required to handle high-resolution point clouds. High-precision LiDAR equipment used to be quite expensive. However, more compact and lower-cost products are now available, and processing software has improved significantly, so these issues are gradually being resolved.