9.5. Comparison between ALS and TLS

Airborne Laser Scanning (ALS): Aerial (aircraft/helicopter) Terrestrial Laser Scanning (TLS): Ground-based (tripod or mobile)

ALS involves the use of aircraft, such as planes or helicopters, to scan large areas from the air, while TLS uses ground-based scanners that are either mounted on tripods or mobile platforms to capture data from a stationary position. The choice of platform significantly affects the scale and type of data collected.

Imagine gathering data about a large forest. If you were to aerially photograph the forest (ALS), you get a broad overview, capturing the entire landscape at once. On the other hand, using a tripodal laser scanner (TLS) would be like carefully inspecting each tree from the ground, analyzing details up close.

Airborne Laser Scanning (ALS): Large-scale Terrestrial Laser Scanning (TLS): Small to medium-scale

ALS is designed for extensive areas, making it ideal for collecting data over large terrains quickly. In contrast, TLS is more suited for smaller and more specific locations, allowing for high-detail scans of individual structures or features.

Consider a sprawling outdoor music festival. Using a helicopter to take images of the entire festival grounds (ALS) lets you see overall attendance and layout, whereas using a tripod to scan the stage area (TLS) captures the intricate details of the performers and equipment.

Airborne Laser Scanning (ALS): Moderate to High Terrestrial Laser Scanning (TLS): Very High

The accuracy of ALS is generally classified as moderate to high, depending on various factors like altitude and weather conditions. TLS, however, provides very high accuracy since it operates at ground level and can capture finer details and dimensions of objects.

Imagine measuring the height of a building. If you used a drone (ALS) to estimate the height, you'd get a good guess but might not be exact. In contrast, measuring with a tape measure at the base of the building (TLS) allows for precise measurements down to the last inch.

Airborne Laser Scanning (ALS): Medium Terrestrial Laser Scanning (TLS): High to Very High

Point density refers to how many data points are collected in a given area. ALS typically has a medium point density because it captures data from a higher altitude over larger areas. TLS yields high to very high point densities, providing extensive detail as it scans from much closer up on the ground.

Think of point density like pixels in a photo. An aerial shot of a city (ALS) might provide a good picture but lacks the fine details seen in a close-up of a single building facade (TLS), where every stone and brick is clearly defined.

Airborne Laser Scanning (ALS): Topography, Forest mapping Terrestrial Laser Scanning (TLS): Structural scanning, BIM

ALS is primarily used for mapping large areas such as topography and forest structure, due to its ability to cover extensive regions quickly. TLS, however, is more specialized for detailed scanning of structures like buildings and bridges, which require precise measurements for applications like Building Information Modeling (BIM).

If you're planning to build a new highway, ALS would help you examine the landscape and overall terrain features from above. After choosing a route, using TLS would allow for precise groundwork measurements to ensure bridges and tunnels fit perfectly into the plans.

Airborne Laser Scanning (ALS): Less in open areas Terrestrial Laser Scanning (TLS): High in complex structures

Occlusion refers to situations where the scanner cannot see certain areas due to obstacles. ALS experiences fewer occlusion issues over open areas, while TLS struggles in areas with complex structures, where other objects may block the scanner's line of sight.

Picture trying to see your friend at a concert from the back of the crowd (ALS) versus standing at the front and trying to see someone behind a large speaker (TLS). The aerial view lets you see everything in the open, but ground-level scanning can easily miss details hidden behind obstacles.

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