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Airborne LiDAR

Airborne Light Detection and Ranging (LiDAR) is an active remote sensing technique that utilizes infrared light to create high-resolution elevation models of the Earth’s surface. Within the AMZ cycle, LiDAR can be applied during the exploratory and mapping phases of archaeological field survey. Additionally, LiDAR can also be used to monitor known archaeological sites. Under ideal conditions, LiDAR can map archaeological features on land surfaces, even when these features are hidden beneath vegetation.

What?

How does it work?

An airborne LiDAR sensor emits a laser beam toward the ground (figure 1). When this beam strikes an object, such as a tree branch, part of the beam is reflected back to the sensor. The remaining part of the beam continues to travel and can hit various objects multiple times until it encounters something it cannot penetrate, usually the ground itself. The distance between the sensor and each reflection can then be calculated, as the sensor‘s location is known and the time taken for the laser beam to travel out and return (part of the beam) has been measured.

Figure 1: The principle behind Airborne LiDAR (source: https://cherishproject.eu/en/tool-kit/airborne-laser-scanning/).

Further processing of the data results in two elevation models (Digital Elevation Models or DEMs): a Digital Terrain Model (DTM), which represents the bare ground (excluding buildings, etc.), and a Digital Surface Model (DSM), which displays all data, including buildings and vegetation (figure 2). All height variations, including those created by human activity (such as dikes, quarries, and burial mounds), are visible in these models.

Figure 2: A schematic example of a Digital Surface Model and a Digital Terrain Model (source: https://3dmetrica.it/dtm-dsm-dem/).
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In-depth explanation X

After collecting the LiDAR data, each reflection is combined with positional data, resulting in what is known as a point cloud. This point cloud consists of individual points, each with 3D coordinates corresponding to every reflection. Typically, three processing steps follow: classification (or filtering), interpolation, and visualization (figure 3).

Figure 3: General workflow for the acquisition, processing, and dissemination of Airborne LiDAR data (source: ARCfieldLAB).

In the first processing step, classification/filtering, all collected points are converted into a digital terrain model (DTM) and a digital surface model (DSM), which includes the ground surface as well as all vegetation and buildings above it. For archaeologists, the DTM is generally the most useful. Typically, two types of reflections are significant in this process: the so-called first and last reflections. In an open area, such as a field, the first and last reflections are often the same because the laser beam simply strikes the ground surface. However, in forested areas, the first reflection usually represents the tops of the trees, while the last reflection may be the ground surface, a tree trunk, or low vegetation. To create a DTM, all reflections except for the last one are typically removed. The remaining reflections are then categorized into ground and non-ground points, and any further errors are corrected.

However, at the end of the classification step, the data still consists of discrete points. To enhance the legibility of the data, these points are interpolated into a raster image during the second processing step. Interpolating the data also helps fill any gaps where measurement data may be missing. The final product of this step is a raster file in which each cell has a height value. The size of each cell determines the resolution of the raster file.

In the final processing step, visualization, the raster file is converted into a grayscale or color image that displays the differences in height values. There are many different visualization techniques available (see Kokalj & Hesse, 2017 for a comprehensive description of the most commonly used visualization methods).

What do you need?

To collect LiDAR data from the air, an aircraft, helicopter, or drone equipped with a sensor is required. This sensor emits infrared laser beams in a specific pattern and measures the returning reflections. In addition to the travel time, the sensor also measures the so-called intensity, which indicates the strength of the reflection and is partly dependent on the material causing the reflection. For example, asphalt has a low intensity, while snow has a high intensity. Besides the sensor, the aircraft is equipped with a GPS to measure its position and an inertial measurement unit (IMU) to measure the pitch and roll of the aircraft.

Various software packages are available for processing LiDAR data, such as LASTools. Nowadays, such tools are often incorporated into GIS software. For visualizing LiDAR data, the Relief Visualization Toolbox (RVT) is generally used (Kokalj & Hesse, 2017).

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In-depth explanation X

Two types of LiDAR sensors are used to collect (archaeological) data: discrete-return LiDAR and full-waveform LiDAR, with the former being the most commonly used type (figure 4).

Figure 4: Principle of discrete-return and full-waveform sensors (source: Ferraz et al., 2009).

Discrete-return sensors (also known as conventional sensors) emit laser pulses and record the intensity and timing of a limited number of returned pulses that exceed a certain intensity threshold. Most sensors typically record a maximum of four different return signals from a single pulse. Therefore, the majority of the received signal is not captured by these sensors.

In contrast, full-waveform sensors record the entire returned pulse by measuring the intensity of the return signal at frequent and regular intervals (usually at 1 ns intervals) and converting the signal into a digital data stream. From this data stream, the individual return signals are extracted. The characteristics of each return, such as echo width and amplitude, can provide additional information about the reflecting surface, contributing to a more reliable classification of the returns.

Can be used with..

Airborne LiDAR can be collected using airplanes, helicopters, or drones. The type of platform used affects two factors: the area that can be surveyed per flight and the point density. Point density (or average ground point density) indicates the (average) number of laser pulses that actually reach the ground per square meter. Point density is influenced by the platform’s altitude and speed, pulse rate (how many beams are emitted per second), angle of inclination, and the number of times the area is measured (e.g., due to overlap). Ground point density is also affected by the land use of the surveyed area. While LiDAR can be used to map the ground surface under forest cover, many of the emitted beams may not reach the ground but instead hit other objects like tree trunks or branches, which is less of an issue in other types of terrain.

For surveying large areas with lower point density, airplanes are most suitable. Helicopters can fly slower and lower, enabling the measurement of smaller areas with higher point density. Helicopters are also better suited for data collection in forested or heavily vegetated areas. Finally, drones can be used to survey small areas with very high point density.

Archaeological Applications

Place in the Dutch archaeological heritage management process

Airborne LiDAR, like most other archaeological remote sensing methods, can be applied during the exploratory and mapping phases of an archaeological inventory (IVO-O). The collected and analyzed images provide a set of anomalies (figure 5), accompanied by descriptions and interpretations. These can be used to better characterize above ground archaeological features and provide valuable information for potential further prospection or excavation research.

Figure 5: An example of archaeological structures visible on LiDAR data: burial mounds in the vicinity of Castle Drakensteyn, in Laage Vuursche (national monument number 45252) (source: AHN4).
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In-depth explanation X

In addition to detecting anomalies, LiDAR can also be used to monitor known archaeological sites. By comparing LiDAR data collected at different times, changes can be recorded. Examples of detectable changes include the digging of ditches or pits, and the leveling or raising of ground levels.

What types of archaeological materials/landscapes

Airborne LiDAR can be applied in any setting where height differences are likely to be detectable between archaeological features and the natural landscape. It is important to note that only structures that manifest as elevation differences relative to the surrounding terrain can be detected with this technique.

Limitations/uncertainties

Both the season and weather conditions influence the success of a LiDAR survey, especially in densely vegetated areas. The ideal time to collect LiDAR data in forested areas is during winter, after the snow has melted and before trees have new leaves.

Agricultural land use also greatly affects the success rate, as it often has a leveling effect, making elevation differences on the ground surface less prominent.

LiDAR is not suitable for underwater archaeology, as water strongly reflects infrared radiation. However, using so-called “green LiDAR,” it is possible to map the seabed up to a few meters below the water’s surface.

References/further reading

AHN. Actueel Hoogtebestand Nederland. https://www.ahn.nl/

Crutchly, S. & Crow, P. (2010). The Light Fantastic: Using airborne lidar in archaeological survey. English Heritage.

Kokalj, Ž. & Hesse, R. (2017). Airborne Laser Scanning Raster Data Visualization: A Guide to Good Practice. Založba ZRC.

Rensink, E., Theunissen, L., Feiken, R., Bourgeois, J., Deforce, K., van Doesburg, J., Emaus, R., van der Heiden, M., de Jong-Lambregts, N., Karagiannis, N., de Kort, J. W., Liagre, E., van Londen, H., Meylemans, E., Orbons, J., Stichelbaut, B., Terlouw, B., Timmermans, G., Waagen, J., & van Zijverden, W. (2022). Vanuit de lucht zie je meer. Remote sensing in de Nederlandse archeologie. Nederlandse Archeologische Rapporten (NAR) 80.